AN ABSTRACT OF THE THESIS OF
Melissa K. Fierke for the degree of Master of Science in Environmental Sciences
and Fisheries Science presented on July 3, 2002. Title: Composition, Structure,
and Biomass of Cottonwood-Dominated Gallery Forests Along a Successional
Gradient, Willamette River, Oregon.
Redacted for Privacy
Abstract approved:
J.Bo
fman
Recognizing the importance of native black cottonwood-dominated riparian
forests is especially important to preserve, protect, and manage for biodiversity in
the Willamette River Valley. Species composition, structure, and biomass along a
successional gradient from stand initiation to late succession of black cottonwood
(Populus balsamfera L. subsp. trichocarpa (T. & G.) Brayshaw) dominated
gallery forests was investigated along 145 km of the Willamette River, Oregon.
These forests were found to generally follow disturbance-initiated successional
stages; stand initiation, stem exclusion, early succession/understory re-initiation,
mid succession, and late succession.
Young stands were dominated by black cottonwood and opportunistic
herbaceous species. Understory shrub and late-successional tree species established
12 - 15 years after stand initiation. Oregon ash (Fraxinus latfolia Benth.) was the
dominant late-successional tree species. Biotic habitat variables, in contrast to
abiotic environmental variables, appeared to be the most important determinants of
herbaceous and understory species presence and abundance. Relative stand age, as
represented by average black cottonwood diameter at breast height (dbh), was the
most important measured environmental variable based on non-metric
multidimensional scaling (NMS) ordination using understory plant species
composition and abundance in sites of all ages. Abundance of reed canarygrass
(Phalaris arundinacea L.) was also an important variable across stand ages. Based
on NMS ordination of stands >20 cm average dbh, this variable was the most
highly correlated environmental variable with plant species composition and
abundance. High abundance of reed canarygrass resulted in lower values of
understory species diversity and total species richness by inhibiting establishment
of understory tree, shrub, and herbaceous species as well as late-successional tree
species.
Total above ground tree biomass was calculated for all stands and ranged
from <1 to 549 Mg/ha in stand initiation and late succession stages respectively.
Black cottonwood biomass was 36 - 100% of total above ground tree biomass (<1-
427 Mg/ha). Biomass in these forests exceeded all other published above-ground
biomass estimates for deciduous riparian forests. As cottonwood trees senesced
and pioneer tree species dominance decreased, biomass also decreased. Annual
biomass accumulation was lowest during stand initiation (0.01 Mg/halyr), peaked
when stands were between 7 and 12 years old (23.7 Mg/halyr), and decreased
thereafter as stands aged (2.9 to 7.3 Mg/ha/yr in stands >65 years old, n = 6).
Structural diversity increased during early succession as understory trees,
shrubs, and herbs established along with late-successional tree species, and created
multiple vegetative layers. This understory re-initiation occurred around 12 to 15
years after stand initiation, when the cottonwood canopy opened up. Total tree
densities were highest in early successional stands, 20,800 to 96,200 trees/ha, and
decreased in older forest stands 443 to 3710 trees/ha. Large tree (>10 cm diameter
at 1.3 meters in height) densities ranged from 180 to 1350 trees/ha. Standing dead
tree density decreased with stand age from a peak in a 4 year old stand of 9,600
snags/ha. Standing dead tree biomass and downed wood biomass increased
through time as stands aged from 0 to 12.7 Mglha and 0 to 2.1 Mg, respectively.
Abundance of invasive plant species appeared to negatively impact structural
diversity by inhibiting establishment of understory and late-successional tree
species.
This study indicated that Willaniette River cottonwood-dominated gallery
forests are structurally diverse with high carbon storage potential, but as late
successional species come to dominate, diversity and biomass decreased. Relative
stand age was important across stands relative to species composition and
abundance. However, considering only older stands, reed canarygrass was the
most strongly correlated variable with species composition and abundance.
Without intervention to control establishment and survival of reed canarygrass, and
perhaps some other invasive species, such as Himalaya blackberry (Rubus
arm eniacus L.) and English ivy (Hedera helix L.), it is likely that these species will
become more influential and that plant diversity in Willamette River riparian
forests will be negatively impacted.
In addition, stands are small in area, few remain, and pioneer forest
regeneration appears to be limited to areas where they are subject to scour and
excessive inundation during high winter flow events. As cottonwood dominance
continues to decline, late successional tree species and non-native understory
vegetation are expected to increase in dominance at the riverscape level, decreasing
overall biodiversity in the Willamette River Valley.
COMPOSITION, STRUCTURE, AND BIOMASS OF COTTONWOODDOMINATED GALLERY FORESTS ALONG A SUCCESSIONAL GRADIENT,
WILLAMETTE RIVER, OREGON
by
Melissa K. Fierke
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Presented July 3, 2002
Commencement date June 2003
Master of Science thesis of Melissa K. Fierke presented on July 3, 2002.
APPROVED:
Redacted for Privacy
Mair Professor representing Eivdironrnentai Sciences and Fisheries Science
Redacted for Privacy
Head of Department of Fisheries and Wildlife
Redacted for Privacy
Chair of the Environmental Sciences Program
Redacted for Privacy
Dean of
d.te School
I understand that my thesis will become part of the pennanent collection of Oregon
State University Libraries. My signature below authorizes release of my thesis to
any reader upon request.
Redacted for Privacy
Melissa K. Fierke, Author
ACKNOWLEDGEMENTS
I thank my husband, Brad, without whom this would not have been
possible. I thank Kaylee, my wonderful 'forest fairy'. I will always treasure our
time on the river and in the "dottondoods" - "POTR 36, FRLA 22, POTR 32,
ACMA 12".
I am grateful to J. Boone Kauffman for giving me the opportunity to pursue
this degree, for help in the field, and for guidance, encouragement, and approval. I
thank Mindy Simmons, who added immeasurably to my research, both with
assistance in the field, generosity with sharing data, and long discussions about
Willamette River riparian forests. Also, thanks to my committee members, Joe
Means and Stan Gregory, for guidance and recommendations. Thanks also to
Richard Halse for plant identification, Mark Wilson for plant community and
experimental design insights, and Bruce McCune and Brianna Lindh for data
analysis assistance.
Thanks to Dan Arp, Skip Rochefort, and Margie Haak for their roles in the
NSF GK-12 grant that not only provided funding for my time here at OSU but also
conferred to me a lifelong appreciation of the importance of K-12 teaching,
education, and outreach.
And to all other compatriots and friends, who have assisted in the field,
helped me comprehend difficult ecological (or analytical) concepts, listened to my
ideas, or provided copies of pertinent literature (especially Satomi, Russ, and Dan)
- Thanks!
TABLE OF CONTENTS
Page
Chapter 1. A Review of Cottonwood Gallery Forests
1
Introduction
2
Cottonwood (Populus spp.) Establishment and Ecology
4
Anthropogenic Problems Affecting Cottonwood Gallery Forests
7
Hydrological
Willamette Valley
Invasive Plant Species
7
8
10
Conclusions
11
Literature Cited
12
Chapter 2. Structural Dynamics and Biomass Accumulation Along a Black
Cottonwood Successional Gradient, Willamette River, Oregon
20
Abstract
21
Introduction
22
Methods
24
Study Approach
Study Area
Field Methods
Stand Age Analysis
Biomass Equations
Forest Structure Equations
Results
.
24
25
25
28
29
32
33
Succession Through Time
Biomass
Forest Structure
33
40
45
TABLE OF CONTENTS (Continued)
Page
60
Discussion
Succession Through Time
Biomass
Forest Structure
Conclusions
.. 60
61
64
.
Literature Cited
66
68
Chapter 3. Riverscape-level Patterns of Riparian Plant Diversity Along a Black
74
Cottonwood Successional Gradient, Willamette River, Oregon
...
75
Introduction
76
Methods
79
Abstract
Study Approach
Study Area
Field Methods
Analysis
Results
79
79
.. 81
83
87
Community Descriptions
Habitat Differences
Species Diversity and Composition
Analysis
Habitat Differences
87
87
90
95
95
TABLE OF CONTENTS (Continued)
Page
97
97
Species Composition
Invasive Species
Patterns In Early to Late Successional Stands
101
104
Discussion
Habitat
Species Diversity and Composition
Invasive Species
104
104
105
Conclusions
109
Literature Cited
112
Chapter 4. Conclusions and Recommendations For Future Work
..
118
Bibliography
..
123
.
136
APPENDIX: Willamette River cottonwooddominated riparian forest plant
species list.
LIST OF FIGURES
Page
Figure
1.1
2.1
Comparison depicting loss of native vegetation and channel
complexity along 55 km of the Willamette River, Harrisburg to
Corvallis, Oregon. (From Benner & Seddel 1997, with permission).
Twenty-eight sampled stand locations are indicated (*) along 145 km
study reach from the cities of Eugene to Salem, Willamette River
Valley, Oregon, U.S.A. (map source unknown).
3
26
2.2
Structural characteristics and aboveground biomass were sampled using
a nested plot design in cottonwood-dominated gallery forest stands.....27
2.3
Stand development through time along a successional gradient in black
... 34
cottonwood-dominated forests, Willamette River, Oregon. ..
2.4
Large tree importance values, based on relative frequency and relative
basal area, during black cottonwood-dominated riparian forest
succession.
2.5
2.6
2.7
2.8
41
Total aboveground live tree biomass increased through time as stands
aged.
43
Above ground tree biomass peaked in cottonwood-dominated late
successional forests.
43
Rates of biomass accumulation through time in Willamette River
cottonwood-dominated riparian forests.
44
Snag biomass and downed wood biomass increased as black
cottonwood stands aged.
45
LIST OF FIGURES (continued)
Figure
2.9
Selected structural characteristics of Willamette River cottonwooddominated riparian forests.
46
2.10
Tree densities of Willamette River black cottonwood-dominated forests 47
2.11
Reed canarygrass cover versus understory and late-successional small
tree (<10 cm dbh) densities.
48
2.12
Vertical structural diversity of vegetation layers increased through time
. 49
as stands aged.
2.13
Canopy and mid story vegetative layers versus pioneer tree species (black
cottonwood and willow species) importance value in stands past stem
50
exclusion (>12 years, n = 19). ..
.
2.14
Percent cover of understory vegetative layers change through time......52
2.15
Overstory structural composition displayed through frequency
distributions by dbh of tree species at 27 study sites.
53
2.16
Tree species importance values versus black cottonwood mean dbh.....65
3.1
Twenty-eight sampled stand locations are indicated (*) along 145 km
study reach from the cities of Eugene to Salem, Willamette River
Valley, Oregon, U.S.A. (map source unknown).Locations of study
... 80
area, Wiflamette River Valley, Oregon U.S.A.
3.1
Composition and vegetative structure were sampled using a nested plot
design in cottonwood-dominated gallery forest stands.
82
LIST OF FIGURES (continued)
Figure
3.2
3.3
Page
Understory species diversity and total species richness based on 5
age-classes.
Understory species diversity (A) and total species richness (B) based
on 5 age-classes.
84
.
92
3.4
Non-metric multidimensional scaling (NMS) ordination depicting stand
relationships based on the presence and abundance of understory plant
species.
96
3.5
Non-metric multidimensional scaling (NMS) ordination depicting stand
relationships based on the presence and abundance of understory plant
species.
. 98
3.6
Understory diversity (A) and total species richness (B) were significantly
negatively correlated with reed canarygrass cover in older stands (>19
cmdbh,n=19).
99
3.7
NMS ordination with reed canarygrass (PHAR) loaded onto Axis 2. ... 100
3.8
Non-metric multidimensional scaling (NMS) ordination depicting
stand relationships based on the presence and abundance of understory
plant species in stands past stem exclusion (average dbh >20 cm).
102
3.9
Non-metric multidimensional scaling (NMS) ordination depicting
stand relationships based on the presence and abundance of understory
plant species in stands past stem exclusion (average dbh >20 cm).
103
LIST OF FIGURES (continued)
Figure
3.10
3.11
Page
Bray-Curtis ordination of sites in species space using chi-squared
distances and selection end points to be Kropf and Marshalls.
107
Percent native species versus average black cottonwood dbh.
109
LIST OF TABLES
Table
Page
Equations used to determine components of aboveground biomass in
Black cottonwood-dominated riparian forests.
30
Structural characteristics of Willamette River cottonwood-dominated
forests.
36
2.3
Biomass and selected structural characteristics.
42
2.4
Tree biomass (Mg/ha) in selected deciduous and riparian forests.
62
3.1
Environmental variables measured and basic stand information from
28 black cottonwood-dominated riparian stands,
2.1
2.2
.
88
3.2
Plant species richness found across all 28 study sites.
90
3.3
Black cottonwood-dominated stand attributes measured for 28 stands
along the Willamette River, Oregon.
91
3.4
3.5
Dominant non-tree species in black cottonwood-dominated stands
along the Willamette River, Oregon.
..
94
Species richness, mean number, and mean percent cover of exotic
species between this study' and another study on the Dungeness and
Hoh river systems in the Pacific Northwest by DeFarrari and Naiman
(1994)2.
105
COMPOSITION, STRUCTURE, AND BIOMASS OF COTTONWOODDOMINATED GALLERY FORESTS ALONG A SUCCESSIONAL GRADIENT,
WILLAMETTE RIVER, OREGON
Chapter 1
A Review of Cottonwood Gallery Forests
Melissa K. Fierke
2
INTRODUCTION
Cottonwood-dominated gallery forests are aesthetic, environmental, and
recreational resources that provide important habitat and corridors for terrestrial
organisms as well as multiple benefits to aquatic ecosystems such as shade, organic
matter, and large wood. These riparian forests are vulnerable to anthropogenic
impacts, directly (e.g., logging, clearing, etc.) or indirectly (e.g., flood control,
stream bank hardening, invasive species introductions, etc.). Numerous studies of
cottonwood succession in riparian ecosystems have been conducted in the Great
Plains of the U.S.A. and Canada (Ware and Penfound 1949; Hosner and Minckler
1963; Wilson 1970; Keammerer et al. 1975; Shaw 1976). Most concentrated on
changes in overstory plant composition, with only a few investigating community
dynamics, and how change occurs during succession (Johnson et al. 1976; Boggs
and Weaver 1994). Little information has been published on cottonwood-
dominated riparian ecosystems in the Pacific Northwest (Dykaar and Wigington
2000).
As is true with many western river systems, gallery forests along the
Willamette River have been reduced to narrow discontinuous remnants due to
human intervention (Frenkel et al. 1984; Johnson et al. 1976; Rood and HeinzeMime, 1989; Rood et al. 1995; Benner and Sedell 1997; Dykaar and Wigington
2000). Recent studies throughout North America, including the Willamette Valley,
describe how current management strategies have impacted riparian forests (Benner
and Sedell 1997; Hulse et al. 2002) (Figure 1.1). Land and water uses often result
in a more homogenous riparian community dominated by shade-tolerant tree
species that replace senescing riparian-obligate trees (Johnson et al. 1976; Rood
and Heinze-Milne 1989; Rood et al. 1995; Benner and Sedell 1997; Barnes 1997;
3
Gutowsky 2000). Black cottonwood (Populus balsamifera L, subsp. trichocarpa
(T. and G.) Brayshaw 1965) dominates many bottomland riparian forests of the
Pacific Northwest. It is considered the largest American poplar and the largest
native hardwood in North America (DeBell 1990). Dykaar and Wigington (2000)
expressed concern upon finding few stands of young black cottonwoods along the
Willamette River and questioned whether current rates of establishment and growth
of black cottonwood are adequate to sustain even today's modest nparian area.
Bottomland
HardwoodS
Prairie and
fern
clearings
Oak
openings
1852-54
1986
Figure 1.1. Comparison depicting loss of native vegetation and channel
complexity along 55 km of the Willamette River, Harrisburg to
Corvallis, Oregon. (From Benner and Seddel 1997, with permission).
4
COTTONWOOD (POP UL US SPP.) ESTABLISHMENT AND ECOLOGY
The native range of black cottonwood extends from southern Alaska into
British Columbia, Washington, Oregon, and Northern California with scattered
populations extending south into northern Baja, California. Black cottonwoods are
present on both sides of the Cascade Range extending east to the western slopes of
the Rocky Mountains into western Montana, Wyoming, and Idaho with scattered
populations found in southeastern Alberta, Canada, Utah, Nevada, and North
Dakota (DeBell 1990). Black cottonwoods grow in arid to humid climates, with
best growth occurring in more mesic climates on deep alluvial soils of the Pacific
Northwest (DeBell 1990).
Black cottonwood has the capacity to both sexually and vegetatively
reproduce. Riparian cottonwoods produce small, numerous wind-dispersed seeds,
which are released from May through July (Bessey 1904; Hosner and Minckler
1963; DeBell 1990; Cooper et al. 1999). Percent germination of seeds is almost
complete up to the 4th day, and then gradually declines to zero at -25 days (Ware
and Penfound 1949). Seed germination following deposition on suitable substrate is
rapid; and can be as short as 12 hours (Ware and Penfound 1949). Vegetative
reproduction is accomplished via cladoptsis and sprouting. Cladoptsis, the
physiological abscission of lateral twigs and branches, was originally hypothesized
to be a low-value alternative reproductive strategy (Galloway and Worrell 1979).
A more recent study concluded that cladoptsis is probably not a major form of
reproduction and is more likely to be a form of self-pruning for plasticity during
wind storms with re-allocation of resources to more productive branches (DeWitt
and Reid 1992).
Rood et al. (1994) found that 52% of young riparian
cottonwoods along four rivers in Southern Alberta were derived from seeds.
Sprouting from roots and entrained shoots accounted for 48% of young trees (30%
5
and 18% respectively). Cladoptsis was confirmed on only 2 of 690 young
cottonwoods excavated. These findings support the relative importance of each
reproductive mechanism, though no research was found documenting if origin
affects subsequent growth and survival, or if this is widespread in Populus species
or only within the studied river system.
Ecological requirements for seedling establishment are bare moist
substrates, full sun, and continuous moist conditions for the first week of growth
(Moss 1938; Hosner and Minckler 1963; Everitt 1968; Johnson et al. 1976; Walker
et al. 1986; Cooper et al. 1999). This species has adapted to and therefore depends
upon historical flood regimes for establishment. Mahoney and Rood (1998)
defined a "recruitment box" for riparian cottonwood species, which outlines some
of the hydrologic and geomorphic conditions necessary for cottonwood
establishment and survival. Flood events must be large enough to create bare
substrate and must coincide with seed release. The declining hydrograph must be
at a rate of 2 .5 cm/day so that root growth can maintain contact with the water
table. Lastly, establishment must be above the scour zone, above which seedlings
are not subject to removal and excessive inundation during subsequent flood events
(Mahoney and Rood 1998; Cooper et al. 1999). Historically, the occurrence of
these conditions varied across the landscape, but would generally occur once every
5 to 10 years.
As a pioneer species, cottonwoods exhibit rapid initial growth on favorable
sites the first few years. This growth strategy allows cottonwood seedlings to avoid
competition with slower-growing species (Ware and Penfound 1949; Bradley and
Smith 1986). Initially, stand density can be high, but over time, stands thin as
stronger more competitive saplings out-compete weaker individuals for resources.
In the absence of disturbance, cottonwoods will gradually be replaced by shadetolerant tree species that can reproduce under their canopy (Wilson 1970; Johnson
et al. 1976; Nanson and Beach 1977).
Under natural conditions, riparian forests
6
are lost to fluvial disturbances associated with channel shifts within the floodplain.
This does not necessarily occur as an orderly progression from side to side of the
floodplain, rather meander rates are greater at the center of the meander belt, so
center stands have a higher turnover rate while stands on the outer limits can reach
advanced successional stages (Johnson et al. 1976).
Young stands of cottonwood-dominated gallery forests often have few other
woody species in the canopy (Johnson et al. 1976). Keammerer et al. (1975) found
that riparian forests dominated by Plains cottonwood (Populus deltoides Marsh.)
reach a maximum of diversity of understory vascular plants in mid-successional
stage along the Missouri River, North Dakota. Community height and structural
complexity increased to a maximum during black cottonwood-dominated stages on
Catherine Creek in Northeast Oregon and in Plains cottonwood-dominated stages
on the lower Yellowstone River, Montana (Kauffman et al. 1985; Boggs and
Weaver 1994). Surface soil environments have been found to change markedly as
cottonwood forests age, increasing in organic matter and nutrients (e.g., N, P, and
K) (Johnson et al. 1976; Boggs and Weaver 1994). Cottonwood trees typically
become more widely spaced over time, with large straight boles and little crown
branching; thus the canopy opens up and there is an increase in solar radiation.
These changes promote understory establishment, growth, and structural
development as well as facilitating germination and growth of shade-tolerant late-
successional tree species (Keammerer et al. 1975). These late-successional trees
eventually dominate, in the absence of disturbance, as they are released from
competition when short-lived pioneer tree species senesce.
7
ANTHROPOGENIC PROBLEMS AFFECTING
COTTONWOOD GALLERY FORESTS
HYDROLOGICAL
Many researchers have found that the proper management, establishment,
and protection of cottonwood gallery forest ecosystems depends upon the
maintenance of historical flood magnitudes, frequencies, durations, and recession
rates (Scott et al. 1997; Rood et al. 1998). This is beneficial not only to riparian
ecosystems, but also to aquatic ecosystems that rely on streamside plant
communities. Fluvial geomorphic and hydrologic disturbance processes affect
spatial patterns and successional development of riparian vegetation through
creation of a high diversity of microsites (Gregory et al. 1991; Cooper et al. 1999).
Significant departures from historical flood regimes, following implementation of
flood control structures, can cause changes in natural disturbance processes with
which cottonwoods evolved. The alteration or removal of natural disturbance
regimes is clearly a strong perturbation that alters ecosystem structure and function
(Bayley 1995). Flood control and reservoir management are anthropogenic
activities that disrupt riparian community formation and successional processes, as
well as altering forest composition, diversity, biomass, and structure (Reily and
Johnson 1982; Kauffman et al. 1985; Rood et al. 1989, 1990, 1995, 1998; Bradley
and Smith 1986; Nilsson et al. 1991; Johnson 1998, 2002; Barnes 1997; Scott et al.
1997; Auble and Scott 1998; Janssen et al. 2000).
Cottonwood populations evolved with local hydrologic regimes such that
seed dispersal coincides with the falling limb of the hydrograph and exposure of
recent alluvium by receding spring waters (Wilson 1970; Bradley and Smith 1986,
Cooper et al 1999). Flooding and river meandering leads to point bar formation
8
and newly formed islands, which provide suitable germinating substrates for
cottonwoods (Everitt 1968; Hupp and Osterkampl984; Auble and Scott 1998;
Dykaar and Wigington 2000). Seedling survival is also affected by floodplain
position. If seedling establishment is too high above base flow levels, then root
growth may not be able to track the receding water table (recession rate estimated
to be <2.5 cm/day) (Mahoney and Rood 1998). Conversely, if seedlings establish
too low on the floodplain or point bars, they may be subject to scour or prolonged
inundation during subsequent high-flow events (Everitt 1968; Kauffman et al.
1985; Bradley and Smith 1986; Auble and Scott 1998; Mahoney and Rood 1998;
Cooper et al. 1999). Many studies have suggested that suitable hydrologic
conditions for successful cottonwood recruitment only occur every 5-10 years
(Johnson et al. 1976; Bradley and Smith 1986; Baker 1990; Scott et al. 1997;
Mahoney and Rood 1998; Rood et al. 1998). This temporal heterogeneity is
reflected in the mosaic of even-aged tree stands in riparian cottonwood forests
along most western streams (Bradley and Smith 1986; Rood and Mahoney 1990;
Braatne et al. 1996). Flood control and subsequent hydrograph alterations have led
to an absence of cottonwood establishment due to loss of suitable substrates as well
as too quickly declining water tables (Johnson 1998; Rood et al. 1999; Dykaar and
Wigington 2000).
WILLAMETTE VALLEY
Numerous studies of have provided qualitative descriptions of Willamette
River riparian forests (Kirkwood 1902; Habeck 1961; Johannesson et al. 1971;
Franklin and Dyrness 1973; Hall 1976; Towle 1974; Frenkel et al. 1983, 1984).
9
Nash (1878) described Willamette River bottomland forests as being extensive and
difficult to traverse due to thick underbrush and large downed "Balm of Gilead"
(black cottonwood). Kirkwood (1902) inexplicably did not include black
cottonwood in his vegetation list for bottomland forests in the Willamette River
Valley. Early travel accounts and survey records describe conifers as being
frequent and a large part of the bottomland canopy (Nash 1878; Kirkwood 1902;
Johannesson et al. 1971). Today, conifers may not be as common as a result of
these trees being selectively harvested and towed "in great rafts to the paper mill in
Oregon City every year" (Nash 1878). Livestock grazing in bottomland forest was
a major use early in the century and clearing of the forest for agricultural purposes
was a gradual process that accelerated with the advent of efficient irrigation, a
diversification of crops, and flood control in the mid 1940's (Towle 1982). Urban
and industrial (e.g., gravel mining) land uses in the floodplain have led to further
deterioration of gallery forests along the Willamette River (Frenkel et al. 1983,
1984). Willamette River studies suggest that river-floodplain linkages have also
been altered/reduced through extensive snagging, diking, bank revetments, and
drainage improvements for urban and agricultural uses, as well as by reductions in
flooding (Benner and Sedell 1997, Sedell and Frogatt 1984). Decline in channel
shoreline, loss of large wood and riparian forests, and altered floodplain
interactions have affected channel structure, retention capability, forest vegetation
composition and structure, and sources and sinks of organic matter along the river
(Sedell and Frogatt 1984; Benner and Sedell 1997; Gutowsky 2000). These altered
ecosystem interactions are the result of the ecological disconnection between the
Willamette River and its floodplain.
10
iNVASIVE PLANT SPECIES
Riparian ecosystems are particularly susceptible to invasion by plant species
due to the river functioning as a connective corridor during flood events, frequent
disturbances, an abundance of heterogeneous habitats within the riparian
ecosystem, and proximity of human activities. Many exotic plant species occur
within the Willamette River system. Some of these originate from past agricultural
introductions (e.g., hops, Humulus lupulus L.) and others have escaped from
homestead cultivation (e.g., lemon balm, Melissa officinalis L.). Several
introduced species are considered particularly invasive species with the ability to
displace native plant species on a large-scale basis. These species include reed
canarygrass (Phalaris arundinacea L.), Himalaya blackberry (Rubus armenicus L.
[= discolor Weihe and Nees.]), and English ivy (Hedera helix L.).
Reed canarygrass is widespread across the continental United States
(Apfelbaum and Sams 1987). It is generally considered a non-native species,
however, there is an ongoing debate in this arena as many contend that the current
species is much more aggressive than any native Phalaris species. The most
probable explanation is that this may be attributable to genetic introgression from
introduced cultivars into native genotypes (Merigliano and Lesica 1998;
Galatowitsch et al 1999). The aggressive nature of this species has been attributed
as a major causative factor in the decline of native plant species in many North
American wetland and riparian ecosystems (Lesica 1997; Barnes 1999).
Very little literature is available on Himalayan blackberry and English ivy,
and only one article was found specifically relating to riparian ecosystems (Pyle
1995). Himalayan blackberry originates from Europe and is widely recognized as
an invader of disturbed and waste areas throughout the world (Amor 1974). It is
currently considered one of the most invasive woody plant species, and since its
introduction into northern California in the late 1800's, it has become naturalized
11
and common in the Pacific Northwest (Hitchcock and Cronquist 1973; Rejmanek
and Richardson 1996). English ivy also originated from Europe and is considered
detrimental to both canopy tree species and inhibitive to understory species
establishment (Freshwater 1991). English ivy is extensively used, both in rural and
urban areas, as a low maintenance ground cover. It has escaped and naturalized in
a wide variety of habitats, including riparian areas (Pyle 1995).
CONCLUSIONS
There are many mechanisms influencing the establishment and
development of riparian forests. Anthropogenic modifications have led to a decline
in the quantity and quality of rip arian habitats and have compromised ecological
integrity of rip arian ecosystems by severing linkages with associated aquatic
habitats. These modifications have also led to extensive changes in associated
riparian vegetation and in plant diversity (Johnson 2002). The Willamette River
has undergone extensive modifications, such as channelization, bank hardening,
and flood control, all of which have contributed to its being an ecologically altered
river system. Further impacts to Willamette River riparian plant communities have
been clearing for agriculture, urban, and industrial uses, logging, and introductions
of invasive plant species.
To date, there have been no quantitative studies of Willamette River
riparian forests, documenting species composition and associations, establishment
and structural development, and biomass accumulationlchanges through time. A
study of this nature would provide invaluable information necessary for future
restoration and management efforts to re-instate natural hydrologic processes and
restore some semblance of historical ecological integrity in this river system.
12
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Habeck, J.R. 1961. The original vegetation of the Mid-Willamette Valley, Oregon.
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16
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17
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18
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19
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20
Chapter 2
Structural Dynamics and Biomass Accumulation
Along a Black Cottonwood Successional Gradient,
Willamette River, Oregon
Melissa K. Fierke
21
ABSTRACT
Cottonwood gallery riparian forests are important contributors to ecological
biodiversity in the Pacific Northwest and in North American. Structure and
biomass of black cottonwood (Populus balsarnif'ra L. suhsp. tric/locarpa (T. and
G.) Brayshaw) dominated gallery forests was investigated along a successional
gradient from stand initiation to old growth using a chronosequence approach along
145 km of the Willamette River, Oregon.
Total above ground tree biomass was calculated for all stands and ranged
from 1 to 548 Mg/ha in stand initiation and late succession stages respectively.
Black cottonwood biomass was 36 - 100% of total above ground tree biomass (1-
427 Mg/ha). As cottonwood trees senesced and pioneer tree species dominance
decreased, biomass also decreased. Biomass in these forests exceeded all other
published above-ground biomass estimates for deciduous riparian forests. Annual
biomass accumulation was lowest during stand initiation (0.01 Mg/hatyr), peaked
when stands were between 7 and 12 years old (23.7 Mg/halyr), and decreased
thereafter as stands aged (2.9 to 7.3 Mg/ha/yr in stands >65 years old, n = 6).
Structural diversity increased during early succession as understory trees,
shrubs, and herbs established along with late-successional tree species, and created
multiple vegetative layers.
This understory re-initiation occurred when the
cottonwood canopy opened up around 12 to 15 years after stand initiation. Total
tree densities were highest in early successional stands, 20,800 to 96,200 trees/ha,
and decreased in older forest stands 443 to 3,710 trees/ha. Large tree (>10 cm
diameter at 1.3 meters in height) densities ranged from 180 to 1350 trees/ha.
Standing dead tree density decreased with stand age from a maximum in a 4 year
old stand of 9,600 snags/ha.
Standing dead tree biomass and downed wood
biomass increased through time as stands aged from 0 to 12.7 Mg/ha and 0 to 2.1
22
Mg, respectively.
Abundance of invasive plant species appeared to negatively
impact structural diversity by inhibiting establishment of understory and latesuccessional tree species.
This study indicated that mature Willamette River black cottonwooddominated gallery forests are structurally diverse with high carbon storage
potential, but as late-successional tree species (e.g., Oregon ash, Fraxinus latifolia,
Benth.) come to dominate, diversity and biomass decreased. In addition, stands are
few in number and are small in area. As regeneration is currently limited along this
river system, cottonwood dominance will continue to decline in these riparian
forests. Late-successional tree species and non-native understory vegetation are
expected to increase in dominance at the riverscape level.
INTRODUCTION
Throughout North American, cottonwood-dominated gallery forests are
threatened by anthropogenic influences such as clearing for agriculture, flood
control and bank stabilization structures, which reduce or negate natural
disturbance regimes along many river systems (Johnson et al. 1976; Rood and
Heinze-Milne, 1989; Rood et al. 1995). Land and water uses can result in a more
homogenous rip arian community dominated by shade-tolerant tree species or plant
communities dominated by invasive plant species that replace senescing riparianobligate trees (Johnson et al. 1976; Rood and Heinze-Milne 1989; Rood et al. 1995;
Barnes 1997). As a result, riparian areas as diverse, patchy habitats representing all
successional and structural stages are declining throughout the west, becoming
more homogenous and dominated by late successional tree species (Johnson et al.
1976; Rood et al. 1995; Barnes 1997; Gutowsky 2000).
23
Cottonwoods are considered keystone species in many ripanan ecosystems,
and as a riparian tree species they play important roles in maintaining ecosystem
biodiversity, both terrestrial and aquatic (Gregory et al. 1991). Black cottonwood
Populus balsamifera L. subsp. trichocarpa (T. and G.) Brayshaw) dominates
many bottomland riparian forests of the Pacific Northwest. It is the largest native
hardwood in North America and reaches maximum sizes on nutrient-rich alluvial
soils of the Pacific Northwest (DeBell 1990).
As is true with many western river systems, gallery forests along the
Willamette River, have been reduced to narrow discontinuous renmants due to
human interventions (Frenkel et al. 1984; Benner and Sedell 1997; Dykaar and
Wigington 2000). Dykaar and Wigington (2000) found few stands of young black
cottonwoods along the Willamette River and suggested current rates of
establishment and growth of black cottonwood were inadequate to sustain the
current nparian area.
A myriad of definitions exist in the literature for forest structure. Smith
(1986) described structure as variation in species and age classes, arrangement of
species into different vegetative layers, and distribution of individuals among
diameter classes. No mention was made in this description of presence of downed
wood and snags, which are important to vertical and horizontal structural diversity
attributes of forests stands. Also, canopy gaps were not considered in this
description, which is a component of horizontal forest structure. Most papers
reviewed present basal area andlor diameter-class distributions in their structural
analysis, with only a few discussing more than one aspect of structural diversity
(Pabst and Spies 1999; Hedman and Van Lear 1995).
The global objectives of this study were to study successional change and
stand development in riparian forests. Specific objectives were to determine
structural diversity through time, quantify species contributions to stand
development, quantify aboveground biomass in different successional phases, and
24
determine rates of biomass accumulation in relatively intact cottonwood-dominated
riparian forests. Results from this research could be used as reference information
to facilitate riparian management and restoration, to establish desired natural
riparian conditions, and to provide a background and basis for additional riparian
research.
METHODS
STUDY APPROACH
This study used a chronosequence approach to describe succession in black
cottonwood riparian forests. A chronosequence utilizes a series of stands,
which are of different ages, but which supposedly were or will be similar in
appearance when at the same age (Oliver 1981), in essence a substitution of space
for time. Sampled stands were all located on the main stem Willamette River, thus
holding several important site characteristics constant (e.g., climate, disturbance
processes, elevation, slope, aspect, etc.). Geomorphic position and soils varied
between different age-classes as flood deposition of alluvial soils on riparian
floodplains occur over time, thus contributing to soil development and terrace
formation (Dykaar and Wigington 2000; Johnson et al. 1976; Boggs and Weaver
2000). The chronosequence method of study is a valid approach to infer basic
patterns of successional change and is assumed to be comparable to repeated
measures of permanent plots (Foster and Tilman 2000).
25
STUDY AREA
The study area comprised a 145-km length of the Willamette River from the
cities of Eugene to Salem, in western Oregon (Figure 2.1). From an initial
reconnaissance, 34 stands were found that met site criteria: black cottonwoods
present as a canopy layer, relatively intact (no obvious within stand human
disturbance), large enough for a plot without excessive edge effects, and access
permission obtainable from landowner. Due to scarcity of young stands, all 14
sites identified between 0.5 to 35 cm average diameter at 1.3 m height (dbh) were
used. Older sites, 14 out of 20, with average dbh >35 cm, were chosen based on
spatial position along the river, so as to have a relatively even dispersal along the
study reach, and to encompass a reasonable distribution of all size/age classes.
FIELD METHODS
Structural characteristics and biomass along a successional gradient were
quantified in 28 stands using a nested plot design (Figure 2.2). At each site, species
and dbh was determined for all trees and standing dead trees >10 cm dbh within a
30 X 100 m macroplot (0.33 ha). Plots were established, arbitrarily without
preconceived bias, and were considered the sample unit. In addition to dbh, height
was measured on all snags and on a subset of trees >10 cm dbh.
Trees <10 cm average dbh and length and top/end diameters of downed
wood, >7.5 cm diameter, were measured in 4 X 50 m subplots. Cover, to the
nearest whole percent, of understory species (e.g., shrubs, herbs, grasses) was
estimated in twenty 50 X 50 cm nested microplots. Microplots were randomly
placed within macroplots. These values were averaged to macroplot value.
Microplots were also used to quantify seedling density and to measure vertical
26
Harrisburg
Eugene
Figure 2.1. Twenty-eight sampled stand locations are indicated (+) along 145 km
study reach from the cities of Eugene to Salem, Willamette River Valley, Oregon,
U.S.A. (map source unknown).
>15 cm dbh stands
4
lOOm
a
a
ci
30m
4
50m
ci
a
ci
a
cnvcn,
ci
a
o;io;"
ci
0
ci
/%
<15 cm dbh stands
50m
2m
ci
ri
I
a
4m
I,
25m
Figure 2.2. Structural characteristics and aboveground biomass were sampled using a nested plot design in
cottonwood-dominated gallery forest stands. in stands with average dhh> 15 cm dbh all live and dead trees
>10 cm dbh were measured in 30 X 100 in plot. Trees <10 cm dbh and downed wood were measured in 4 X 50 m
subplot. In stands dominated by trees < 15 cm dbh, all trees, snags and downed wood were measured in 4 X 50 in
plot. In dense stands a subsample of tree and snag measurements were taken in 2 X 25 rn plot. Cover of herbs,
grasses, sedges, and shrubs were measured in twenty randomly placed 50 X 50 cm microplots. Seedling density
and structural diversity (presence/absence of vegetative layers) was also measured in these microplots.
28
structural diversity. Vertical structural diversity was defined by the presence or
absence of six vegetative synusia (herbaceous, low shrub, high shrub, low tree,
mid-tree, and canopy) within the microplot as projected vertically from groundlevel through the canopy layer.
In early seral stands (n = 9), all trees were measured within 4 X 50 m
subplots. This was a necessary modification as all stands of these size-classes (0.5
- 15 cm dbh) were too small in area for 30 X 100 rn plots. Microplot placement in
these size-classes were also within the 4 X 50 m subplot. Trees were measured in 2
X 25 m plots in extremely dense stem exclusion stands, 1-2 cm average dbh (n =
3).
STAND AGE ANALYSIS
Tree corings were extracted from at least two, and up to six, trees within all
sampled stands with trees <50 cm dbh (n = 20). Sites with trees >50 cm dbh
were generally not cored as a large incremental borer was unavailable. Cores from
young stands were relatively easy to age; however, those from older stands
were progressively harder to age precisely. Trees from several sites exhibited
indistinct rings and extensive spongy areas, which rendered rings virtually
indistinguishable and complicated analysis. Stand age for these sites was
necessarily estimated using a combination of dendrochronology and aerial photo
interpretation.
A review of aerial photos was conducted to confirm stand ages from tree
corings, and provided a window of establishment for stands <65 years old.
Willamette River aerial photos were available for the following years: 1936, 1948,
1952, 1960, 1963, 1968, 1970, 1978, 1979, 1982, 1986, 1990, 1993, 1994, and
1996. Six of the sampled stands established prior to 1936.
29
BIOMASS EQUATIONS
Allometric diameter-height relationships were calculated for black
cottonwood, as obtaining heights on every tree was not possible due to time
constraints and difficulty in discerning tree heights in dense stands. Height and
diameter were measured for 3 to 5 trees, >10 cm dbh, at all sites. These data were
then combined with data provided by M. Simmons (unpublished data on
Willamette River closed canopy cottonwood forests) for a more robust data set
(n = 116). Height-diameter relationships were determined using standard
regression techniques (Table 2.1).
Biomass of all trees was determined using published biomass equations
except for black cottonwoods >30 cm dbh and <5 cm dbh (Table 2.1). Species
substitutions were necessary as there were no published biomass equations for
Oregon ash, willow spp. and some understory tree species. Black cottonwood
equations were implemented for willow species.
Biomass equations for black cottonwoods >30 cm dbh were developed using
published volume equations, black cottonwood wood density, and calculated
component biomass relationships. Volume was calculated for boles using the
height equation from this study and British Columbia volume equation for black
cottonwoods (1976). Volume was then multiplied by average black cottonwood
wood density, or specific gravity (0.33 dry g/fresh cm3) (United States Forest
Products Laboratory 1974) to get bole biomass. BiomasE of other components
(e.g., foliage, branches, and bark) was calculated to be 67% of bole biomass using
mean biomass component relationships from Standish et al. (1986) biomass
equations. This percent was multiplied by bole biomass and then added to bole
biomass to obtain total above ground black cottonwood tree biomass.
Table 2.1. Equations used to determine components of aboveground tree biomass in black cottonwood-dominated
riparian forests.
Parameter
Height: Populus balsam fera >40 cm dbh'
Height: Populus ba1samfera <40 cm dbh'
Bole volume: Populus balsamfera >30 cm dbh2
Biomass: Populus balsamfera >30 cm dbh'
Biomass: Populus balsamifera 5-30cm dbh3
Biomass: Populus ba1samfera <5 cm dbh1
Biomass: Fraxinus latifolia Crataegus
douglasii Rhamnus pushiaia*4
Biomass: Acer macrophyllum5
Equation
3.081 * D°6363
2.8 136 * D°7179
l0" (-4.648 + 1.735 * log (D) + 1.356 * log (H))
((By * SO) * 0.67) + (BV * SG)
(7.4 + 156.4 * (D2* H))!
(160.06 * D2 + 8.48 * D)/ 106
n
r2
116
0.90
0.97
39
347
30
20
0.95
0.97
18
N/A
18
0.15
to
0.99
(0.0293 * D2866)/
(0.0716 *D26l7)/ iO3
22
26
(exp'' (3.294 + 2.063 * in (Db)) + exp" (2.26 1 +
2.874* in (Db)) + exp" (2.792 + 1.869 * in (Db)) /106
(exp" (2.417 + 2.040 * In (Db) + exp"(3.719 +
2.989 * In (Db))/106
(lr(H/3)*(r12 + r,r2 + r22))* SG
18
0.94
0.99
0.78
0.84
0.82
0.89
(0.0163 * D235)/103
(expA (-3.765 + 1.617*ln (D)) + exp" (-4.236 +
2.430 * ln(D))+ expt'(-2.116 + 1.092 *ln(D))
+ exp'' (-3.493 + 2.723 * in (D)) + exp A (-4.574
+ 2.574 *in (D)))/ iO3
Biomass: Quercus garryana*6
Biomass: Prunus spp.
Biomass: Cornus sericae*S
Biomass: Corylus corn uta var. calfornica,
Oemlaria cerasformis*, Sam bucus racemosa*S
Biomass: Snags and downed wood'
10.6
18
Definitions for symbols used to represent various components in above equations: D = diameter at breast height (cm); Db diameter at base
(cm); H = height (m); BV = bole volume (m3); SG = Specific gravity (dry g/fresh cm3) r1 and r2 are diameters (cm). Biomass
expressed in Mg. * Indicates a species substitution equation. Sources and species substitutions are indicated by superscript numbers
following the description of each parameter: 1 this study; 2 British Columbia Forest Service 1976; 3 = Standish et al. 1986; 4 = Perala
and Alban 1994, Fraxinus americana; 5 = Grier and Logan 1978; 6 = Bridge 1979, Quercus alba; 7 = Brenneman et aL 1978, Prunus
serontina; 8 = Gholz et al. 1979, Cornus sto1onfera.
31
Allometric dbh-biomass relationships were determined using standard
regression techniques for 0-5 cm dbh black cottonwoods. Five trees were harvested
from each of 4 sites (n = 20) with dbh ranging from 0.1 3.9 cm. Trees were
selected from each site to represent a continuum of sizes within each of those
stands. These trees were then oven-dried to a constant mass and weighed.
Downed wood and snag biomass was determined by first calculating
volume (m3) using frustum of a cone equation:
V = lr(H/3)*(r12 + rlr2 + r22)
where H is bole height/length (m),
rj is radius at base (m), and r2 is radius of top
(m). A taper equation was derived, based on downed wood measured in this study,
to estimate
T2
for snags:
r2 = r - CD
where change in diameter CD (change in diameter) [= 1 .074H + 0.84 18, r2 =
0.683]. Biomass was determined by multiplying volume by average downed wood
density.
Average downed wood density (0.326787 dry g/fresh cm3) was determined
by immersion measurement of 40 woods samples (Krahmer and Van Vliet 1983).
Wood samples were collected at random from 10 field sites. Downed wood was
not reported in Mg/ha due to measurement error in methodology (full lengths were
measured, so biomass data are for all trees that crossed subplot and so would
overestimate biomass). As such, values can be compared to one another, but can
not be reported on an area basis.
Understory shrub and herbaceous vegetation, as well as forest floor litter
biomass, was not included in estimates of total aboveground biomass. In a pilot
32
study involving four study sites, biomass was measured in 11-50 X 50 cm
microplots, and ranged from 0.93-1.48 Mg/ha. This number is comparable to
another understory biomass study of Populus stands (0.9-1.7 Mg/ha) (Bray and
Dudkiewicz 1963). Biomass of this component was small in comparison to live
and dead tree biomass, <0.5% of total aboveground biomass, and so was not
included in estimates of total aboveground biomass.
Average annual biomass accumulation for each sampled site was calculated
by dividing total aboveground tree biomass by stand age.
FOREST STRUCTURE EQUATIONS
Vertical structural diversity was estimated by the Shannon-Wiener formula
(Shannon and Weaver 1949),
H' = -E
log p
where p1 is the relative percentage of presence of vegetative synusia within
microplots in each macroplot. This index considers both the number of layers
present and the evenness of the presence of multiple layers. Antilog of H' was
calculated, making the estimated diversity value comparable to the original
maximum measurement (e.g., l0'' of 6.0 indicates that all 6 vegetative layers were
present throughout the macroplot).
Relative dominance of tree species within each sampled stand were
determined by calculating importance values. Importance value was based on
relative frequency and relative basal area of large trees (>10 cm dbh) within a
macroplot,
33
IV = (F/F + B/B)/2 X 100
were F is frequency of a tree species, F is frequency of all trees, B is basal area of
a tree species, and B is total tree basal area. This relativization technique was
implemented in order to account for large differences in structural characteristics
between species within stands. Pioneer species importance value in sampled stands
should be a superior indicator of successional stage (i.e. high pioneer importance
value indicates young stands while lower pioneer importance value generally
indicates older stands).
Analysis of variance (ANOVA) was used to test differences among sites.
Duncan's Multiple Range test was used to test differences between pairs of group
means. It is considered effective at detecting differences between means when real
differences exist (Montgomery 1997). Regression analysis was used to evaluate
the significance of relationships between variables.
RESULTS
SUCCESSION THROUGH TIME
Sampled stands ranged from 1 to >65 years old. Stand development
appeared to generally follow hypothesized major disturbance-initiated successional
stages (Oliver 1981); stand initiation, stem exclusion, early succession, mid
succession, and late succession (Figure 2.3).
Stand initiation of Willamette River black cottonwood-dominated riparian
forests began with pioneer tree species (black cottonwood and willow, Salix spp.)
seedling establishment on recently deposited moist alluvial substrate. All stand
Stand Initiation Stem Exclusion
Herbaceous/Grass
Early Succession
Cottonwood
Willow
Late Succession
Ash
0
Understory tree
Shrub
Figure 2.3. Stand development through time along a successional gradient in black cottonwood-dominated forests,
Willamette River, Oregon. Pioneer tree species (black cottonwood and willow) established on bare, moist alluvial
substrate, along with opportunistic weedy herbaceous species (e.g., knotweed, mayweed). Stem exclusion was
characterized by individual trees competing for available resources, and due to light limitations, few iinderstory
plant species are present. Early succession was distinguished by the cottonwood overstory opening up, allowing
understory tree (e.g. Indian plum, beaked hazel), shrub (e.g. salmonberry, trailing blackberry), and herbaceous/
grass/sedge species (e.g. youth-on-age, Deweys sedge), and late-successional tree species (e.g. Oregon ash, bigleaf
maple) to germinate and establish. Mid succession was exemplified by high vertical structural diversity with
multiple vegetative layers. Late succession was a continuation of early successional processes with late-successional
tree species becoming more dominant and continued gennination and establishment of shade-tolerant understory and
late-successional tree species.
35
initiation sites occurred on gravel bars, with dominant vegetation being pioneer
trees species and opportunistic herbaceous species (e.g knotweed, Polygonum spp.,
mayweed, Anthemis cotula L.). The six youngest stands are likely subject to
scouring and excessive inundation during high winter flow events.
Stand development proceeded through a stem exclusion phase where
individual trees competed for sun and nutrients. Pioneer tree densities ranged from
20,800 to 96,200 trees/ha during early stem exclusion (n = 3, 1-2 cm average dbh,
4-5 year old stands) and standing dead tree (snag) densities reached a maximum
(2,400 to 9,200 trees/ha) as weaker individuals died. Few weedy herbaceous
species persisted through this stage.
Early succession generally occurred 12 years after stand establishment and
was evidenced by understory re-initiation. Understory reinitiation is when
understory trees, shrubs, herbs, and shade-tolerant late-successional tree species
germinate and establish. Stands in this stage were indicated by the presence of
small trees (<10 cm dbh) of understory tree species (e.g., Indian plum, Oemlaria
cerasformis (Hook. and Am.) Landon, beaked hazel, Corylus cornuta Marsh.
Subsp. calfornica (D.C.) E. Murray) and late successional tree species (e.g.,
Oregon ash, Fraxinus latfolia Benth., big-leaf maple, Acer macrophyllumPursh)
(Table 2.2). Late-successional trees are tree species that are able to germinate and
established in shade and at some point later in the successional process will become
dominant overstory tree species.
Mid and late succession stages were a continuation of early successional
patterns with late-successional tree species increasing in dominance. There wasn't
an exact age or black cottonwood size-class at which stands became mid or late
successional, rather it was more of a continual gradation of stand development.
36
Table 2.2. Structural characteristics of Willamette River cottonwood-dominated
forests. Stands are presented based on stand age, youngest to oldest. Tree species
are listed if more than 5 trees were recorded within the stand. For density values,
large trees are >10 cm dbh and small trees are <10 cm dbh. Mean dbh, basal area,
and importance value (IV) are based on large trees.
Site/Species
Age*
(yrs)
Crystal Lake
2
Populus balsarnfera
Hilemani
Populus balsamifera
Salix spp.
Snagboat2
Populus balsamifera
Hileman2
Populus balsamfera
Salixspp.
Blue Ruin
Fraxinuslatfolia
Oemlaria cerasformis
0.30
0.09
77
33
0.76
0.51
---
10800
2800
0.47
0.09
82
1.40
1.2
---
27400
68800
5.1
9.0
32
68
1.5
--
20800
4.4
100
1.9
1.5
---
72800
3000
25.7
74
26
9.9
4.8
1000
2150
2050
30.2
4.0
94
15.3
1500
250
36.0
100
19.8
15.9
15.9
667
33.3
26.7
22.8
0.72
0.58
2.12
92
4
18
4
5
5
0.61
7
6
12
Populusbalsamfera
Populus balsamfera
Crataegusdouglasii
Salix lasiandra
9100
2800
3
Salix spp.
Skiles2
IV
100
0.60
0.62
Populusbalsamfera
Skilesi
(m2/ha)
2
Populus ba1samfera
Salixspp.
Santiam
Basal Area
0.3
Salix spp.
Snagboatl
Populus balsamfera
Density
Mean dbh
(trees/ha)
(cm)
Large Small
12
2.0
2.0
250
150
450
3
37
Table 2.2 Continued.
Site/Species
Love Lane
Age*
(yrs)
Basal Area
(m2/ha)
Iv
41.5
0.58
94
21.2
15.9
903
77
60
11.2
1300
1450
35.7
91
23.1
16.8
17.9
4.9
500
50
200
24.6
2.12
81
1.25
6
29.9
393
29.4
96
31.6
357
90
26.7
23.3
31.0
80
3
6
15
Populus balsamfera
Marshalls
Mean dbh
Large Small
(cm)
121
Populus balsamfera
Salix lasiandra
EB
Density
(trees/ha)
16
Populus balsamfera
Fraxinus latfolia
Acer macrophyllum
Sambucus racemosa
Kropf
Populus ba1samfera
221
Christianson
Populus balsamifera
Fraxinus latifolia
Acer macrophyllum
Prunus spp.
Sambucus racemosa
Wigrich
Populus balsainifera
Fraxinus latifolia
Salix lasiandra
Corn us stolonifera
Sambucus racemosa
Harkens Lake
Populus balsam ifera
Fraxinus latifolia
Cornus stolonifera
Riverside
Populus balsam ifera
Fraxinus latifolia
Crataegus douglasii
23
14.3
15.9
18.1
1.7
83
43
11
400
700
1.55
ii
0.59
0.65
3
12.5
13.4
3.6
27
62
39.8
4.5
70
27
23.3
46
50
4
3
350
241
58.3
22.5
36.3
44
297
34
450
10
350
950
1.4
1.4
361
53.4
18.8
4.1
164
144
1000
402
53.8
17.9
14.2
93
323
27
700
550
9.1
0.44
38
Table 2.2 Continued.
Site/Species
Crowson
Age*
(yrs)
48.5
16.7
14.3
32.1
Oemlaria cerasformis
Acer macrophyllum
Acer macrophyllum
Quercusgarryanna
Corylus cornuta
93
24.6
260
150
133
363
150
40
26.7
1.41
6
30.0
47
14.0
51
22.0
40
45
4
5
10.5
3.3
8
0.36
3
43.0
0.30
3.39
79
250
53.4
12.4
26.1
2.2
3.5
180
23
60
60.3
21.2
28.3
140
104
54
50
5
15
1050
200
43.9
63
4.3
20
4.4
13
200
950
500
2.5
Oemlaria cerasformis
Sambucus racemosa
Fraxinuslatfolia
84
602
Acer macrophyllum
Corylus cornuta
Populus balsamfera
100
150
32.5
0.39
0.33
601
Populus balsamfera
Fraxinus latfolia
Wells
160
16.7
20
16.7
62.1
45.1
18.5
31.5
12.9
4.7
Acer macrophyllum
Crataegus douglasii
Qemlaria cerasformis
Cornusstolonfera
Independence Bar
Populus balsamfera
Fraxinus la4folia
IV
53
Populus balsamfera
Fraxinuslatifo/ja
Irish Bend
(m2lha)
50'
Populus balsamfera
Fraxinus latifolia
Half Moon
Basal Area
451
Populus ba1samfera
Fraxinus latfolia
Crataegusdouglasii
Salix lasiandra
Strange
Density
(trees/ha)
Mean dbh
Large Small
(cm)
3.4
3.0
>652
53.5
17.0
40.1
23.1
2.0
100
166
13
17
--
350
50
2150
26.6
4.2
2.0
0.76
56
34
5
4
39
Table 2.2 Continued.
Site/Species
Luckil
Populus balsamifera
Fraxinus latfolia
Acer macrophyllum
Gory/us cornuta
Sambucus racemosa
Beacon
Age*
(yrs)
Populusbalsamifera
Fraxinus latfolia
Corylus cornuta
Sambucus racemosa
Cornusstolonfera
Lucki2
Populus balsamfera
59.8
30.2
43.4
3.3
3.3
Basal Area
(m2/ha)
IV
94
84
30
28.4
7.4
4.6
56
28
38.8
3.4
3.7
61
12
2700
750
>652
61.1
20.0
25.0
120
97
67
200
20
15
1600
2.8
>652
110
150
448
51
280
400
200
350
300
16.0
49
89.3
31.0
36.3
1.0
57
70
57
350
600
3
6.3
7.7
52
25
23
15.8
8.2
3.7
38
43
14
69.4
24.2
2.2
1.2
3.2
>652
Fraxinuslatfolia
Acer macrophyllum
Cornus sto1onfera
Bowers Rock
Mean dbh
(cm)
Large Small
>652
Popu/usbalsamfera
Fraxinuslatifolia
Acer macrophyllum
Corylus corn uta
Willamette Park
Density
(trees/ha)
500
>652
Populus balsamfera
Fraxinus latfolia
59.1
54
24.8
147
200
Acer macrophyllum
18.2
44
100
Oemlaria cerasformis
1.5
600
*Age is based on 2 or more cottonwood tree corings.
Age is based on aerial photo interpretation and tree cores (aging of some tree cores was inexact).
2
on aerial photos only.
40
Tree species importance values changed through time (Figure 2.4). Pioneer,
shade-intolerant, tree species (black cottonwood and willow species) dominate
during early seral stages. During early succession, shade-tolerant understory (e.g.,
hawthorne, Indian plum, beaked hazel) and late-successional tree species (Oregon
ash and bigleaf maple) germinated and established. As stands age, these shadetolerant late-successional tree species increase in importance, until they become
dominant tree species.
BIOMASS
Total aboveground live tree biomass was significantly correlated with forest
age (Table 2.3, Figure 2.5). Total aboveground live tree biomass increased through
time and ranged from <1 in stand initiation sites to 549 Mg/ha in late successional
sites.
Large tree (>10 cm dbh) biomass ranged from 107 to 548 Mg/ha. Black
cottonwood biomass accounted for 51 to 100% of total aboveground live tree
biomass. Small tree (<10 cm dbh) biomass in early seral stands (n = 6) ranged
from <1 to 52 Mg/ha. Understory and late-successional small tree (<10 cm dbh)
biomass generally accounted for <2% of total aboveground live tree biomass and
ranged from <1 to 9 Mg/ha.
Biomass reached a maximum average of 429 Mg/ha, in black cottonwood-
dominated mid successional sites (Figure 2.6). Sites dominated by latesuccessional tree species (IV = 41.7), were lower in average biomass (254 Mg/ha)
than late-successional sites.
41
Early succession
Stand initiation and stem exclusion
4
88
100
2
Di
Pioneer
tree species
03
Understory
tree species
Late-successional
tree species
3
38
56
Mid succession
Late succession
Figure 2.4. Large tree importance values, based on relative frequency and relative
basal area, during black cottonwood-dominated nparian forest succession. Pioneer
tree species include black cottonwood and willow. Late-successional tree species
are Oregon ash and bigleaf maple. Understory tree species are hawthorne
(Crataegus douglasii), Indian plum, beaked hazelnut, creek dogwood (Corn us
sericae), and cascara (Rhamnuspurshiana).
42
Table 2.3. Biomass and selected structural characteristics. Total tree biomass
includes large and small trees.
Site
Crystal Lake
Hileman 1
Snagboat 1
Total Live
Tree
Biomass
(Mg/ha)
0
1
1
Snagboat2
Santiam
Hileman 2
BlueRuin
28
9
52
160
EB
Skiles 1
Skiles 2
Love Lane
Marshall
183
Kropf
Wigrich
Christianson.
Harkens Lake
Riverside
Crowson
Strange
Half Moon Bend
Irish Bend
Independence Bar
Wells Island
Luckiamute 1
Beacon
Willamette Park
Luckiamute2
BowersRock
0
0.1
9200
2400
5.4
2.3
0.0
0.2
6100
350
900
60
123.3
30
63.3
113.2
0.0
3.0
0.1
0.2
0.0
0.5
0.3
13.3
80
0
2.2
Biomass
Accum.
(Mg/halyr)
Snags!
0.01
0.5
0.33
0
0
0
0
7
1.8
10.7
22.8
141
12.2
14.7
11.8
284
23.7
203
238
215
277
360
263
265
365
206
375
464
269
359
405
549
530
221
12.7
10.8
9.0
11.3
9.0
6.6
5.9
7.3
3.9
10.4
7.7
3.6
4.8
5.4
7.3
7.2
2.9
177
0
Downed
Wood
Biomass
(Mg)
0.0
0.0
ha
Snag
Biomass
(Mg/ha)
0.1
23.3
23.3
3.3
23.3
50
20
0
20
16.7
13.3
16.7
26.7
4.1
0.2
1.5
0.6
2.1
0.5
0.5
0.5
3.5
0
1.5
11.4
1.0
3.2
5.7
0.0
0.6
2.1
0.8
1.2
6.0
1.5
0
1.8
1.2
5.7
1.0
2.0
12.7
7.8
12.5
1.4
0.4
1.6
0.1
43
600
500
r2
400
.
.
r2 = 0.758
S
p=<o.0001
.
300
E
.2
to
.
07545
.
.
.
200
100
0
5
10
20
15
25
30
35
40
45
50
55
60 >65
Average Stand Age (years)
Figure 2.5. Total aboveground live tree biomass increased through time as stands
aged.
600
stand initiation
stem exclusion
early succession
mid succession
Late succession
500
S
in
In
A,B;n= 4
B,C,D; n=
300
A;n= 4
CD;n=3
E
200
D: n = 9
B,C;n
5
100
100.0
94.7
83.8
68.1
58.2
41.7
Average Pioneer IV
Figure 2.6. Aboveground tree biomass peaked in cottonwood-dominated late
successional forests. As pioneer tree species (e.g., black cottonwoods and
willow species) became less dominant, biomass decreased. Bars are standard
errors. Approximate successional stage is indicated across top of graph. Groups
with different letters are significantly different (a 0.05).
44
Annual biomass accumulation (ABA) ranged from a low of 0.01 Mg/ha/yr
in stand initiation sites to a maximum in early succession of 23.7 Mg/ha/yr. Trends
through time were that ABA increased sharply from stand initiation into stem
exclusion and subsequently decreased through time (Figure 2.7).
25
.
.
.
.
4
.
4
$
0
0
.
.
.
*
.
//
5
10
15
20
25
30
35
40
45
50
55
60 > 65
Stand Age (years)
Figure 2.7. Rates of biomass accumulation through time in Willamette River
cottonwood-dominated riparian forests.
Downed wood biomass and standing dead tree (snag) biomass increased as
stands aged (Figure 2.8). Snag densities and dead wood biomass varied greatly
between stands and within successional stages. Downed wood biomass trends were
similar to snag biomass trends, and it is likely that downed wood biomass would
equal or exceed snag biomass in some riparian stands if estimated on an area basis.
45
13
0
$
.
11
10
C,
Snags
- 0.327
9
p = 0.0018
0
Dow ned w ood
= 0.2995
p = 0.0022
7
.
E
0
0
0
5
10
15
20
25
30
35
40
45
Stand Age (years)
Figure 2.8. Snag biomass and downed wood biomass increased as black cottonwood
stands aged.
FOREST STRUCTURE
Study stands ranged from small dense pioneer forests with small average
dbh (1.6 cm), low basal area (15.4 m2/ha), and high pioneer tree species importance
values (100%), to late successional stands with large old cottonwoods dominated
by late-successional tree species (Figure 2.9). Basal area and average cottonwood
dbh increased through time and ranged from <1 to 60.7 m2/ha and 0.3 to 89.3 cm
dbh.
46
100.0
e
- 90.0
>(
C>
.
80.0
r2 = 07317
- C 60.0
p <0 .0001
50.0
C
o
4
.
40.0
b
30.0
II
.
90.0
80.0
= 0.898
p <0.0001
70.0
.c
0
0
0
C
0
0
.
60.0
4
4
$
. .
4
50.0
.
40.0
30.0
20.0
10.0
0.0
.2
60
E
50
If
N
4
4
..
4
4
4
4
IE
r2 = 0.678
p <0.0001
!b0
0
0
5
10
15
20
25
30
35
40
11
45
50
55
60 > 65
Average Stand Age (years)
Figure 2.9. Selected structural characteristics of Willamette River
cottonwood-dominated riparian forests. Note differences in yaxis labels.
Pioneer species (black cottonwood and willow species) decreased in
dominance through time as reflected by decreased importance values (A).
Cottonwood mean dbh increased through time (B), as did basal area (C).
47
Pioneer (black cottonwood and willow) seedling (<1.3 m height) densities
were high in stand initiation sites (n - 3), ranging from 2,000 to 106,000
seedlings/ha. Understory and late-successional seedling densities were variable in
stands> 7 years old (n = 22) ranging from 0 to 32,000 seedlings/ha. Indian plum,
an understory tree species, exhibited highest seedling constancy as it occurred in 6
out of 19 stands past stem exclusion.
Maximum small tree density (<10 cm dbh, height> 1.3 m) occurred in early
stem exclusion stands (4 to 5 years old, n = 3), and ranged from 20,800 to 96,200
trees/ha (Figure 2.10). Minimum small tree densities occurred as stands aged,
ranging from 50 to 3450 trees/ha in late successional stands. Large tree (> 10 cm
dbh) density ranged from 733 to 1,350 trees/ha in 7 to 15 year old stands (n
5).
Large tree density decreased in older stands ranging from 180 to 633 trees/ha.
1600
22000
20000
4--
18000
1400
Small trees
Large trees
1200
16000
U)
14000
1000
= 12000
r
800
10000
600
8000
U)
6000
400
4000
200
2000
0
2
3
5
7
12
15
22
24
40
50
60
>65
>65
>65
Stand Age (years)
Figure 2.10. Tree densities of Willamette River black cottonwood-dominated
forests. Large trees are >10 cm dbh. Small trees are <10 cm dbh. Two peaks
occurred off the graph at 96,200 and 75,800 small trees/ha in 2 dense early stem
exclusion stands, 4-5 years old.
.!.
48
Understory small tree densities ranged from 0 to 3450 trees/ha from mid
succession to late succession. Late-successional tree species densities ranged from
0 to 1450 trees/ha in these stands. Variability between similar stands was quite
high. Abundance of reed canarygrass appeared to account for much of the variation
in the presence of small trees (Figure 2.11).
3500
.0
.
3000
E 2500
C.)
Q
2000
1500
.
r2 = 0.7012
p = 0.0001
1000
500
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Phalaris % Cover
Figure 2.11. Reed canarygrass cover versus understory and late-successional
small tree (<10 cm dbh) densities. High reed canarygrass abundance appeared
to inhibit establishment of these species.
Vertical structural diversity increased through time (Figure 2.12). Few
synusia are present during early seral stages, stand initiation and stem exclusion.
Once a stand reached early succession, understory re-initiation occurred with
establishment of understory tree, shrub, and herbaceous species, as well as late-
successional tree species. Over time, multiple vegetative layers developed and
structural diversity increased.
49
6
1.5
0
5
10
15
20
25
30
35
40
45
50
55
60
>65
Stand Age (years)
Figure 2.12. Vertical structural diversity increased through time as stands aged.
Diversity calculations were based on presence/absence of 6 vegetative layers;
herbaceous, low shrub/vine, high shrub, understory tree, midstory tree, and canopy.
Considering only stands beyond stem exclusion, canopy cover as
represented by presence of the black cottonwood canopy layer, reached a maximum
in early succession when pioneer tree species were most important (Figure 2.13).
Canopy cover decreased thereafter as late-successional tree species became more
dominant Late-successional trees occupied the mid story layer, which increased in
presence as stands proceeded into late succession and pioneer species became less
dominant.
50
.
100
80
.
C,
70
U)
60
0.
0
0
50
30
100
90
80
70
U
.
.
.
60
.
U)
a-
50
.
0
I.
2
.
.
30
20 -
r2 = 0.772
10
.
0
100
97
95
90
81
81
78
71
63
61
56
56
51
51
47
46
40
38
37
Pioneer Tree Species Importance Value
Figure 2.13. Canopy and mid story vegetative layers versus pioneer tree species
(black cottonwood and willow species) importance value in stands past stem
exclusion (>12 years, n = 19). The taller black cottonwood canopy layer
presence decreased as black cottonwoods senesced and died. Mid-story layer
presence increased as stands aged and late-successional species established and
occupied this synusia.
51
Understory shrub and herbaceous species cover varied through succession
(Figure 2.14). Cover of herbaceous plants was high, though variable, in stand
initiation sites (average of 42%), decreased dramatically during stem exclusion
(average of 5%), and remained low as stands aged and succession proceeded.
Grass cover increased and peaked in early succession (30%), and was quite variable
within age-classes. This variability was primarily driven by high cover of reed
canary grass in a few stands (up to 76%). Low and high shrub percent cover
minored each other through stem exclusion (>15 cm average dbh). At this time,
high shrub cover continued to increase at a higher rate to a maximum of 37%,
while low shrub cover increased at a lower rate to a maximum of 20% in mid
succession.
Frequency distributions of overstory trees show tree structural variability
(Figure 2.15). Tree species occupying different synusia, low and mid tree
vegetative layers, established once the cottonwood canopy opened up. This lead to
variation in size and density distributions among tree species as stands aged. In
stand initiation sites and stem exclusion sites, pioneer tree species (black
cottonwood and willow species) are dominant and tree densities are high (though
variable depending upon initial establishment densities). As stands age and
succession proceeds, shade tolerant understory and late-successional tree species
germinated and established. As stands proceeded into late succession, cottonwoods
senesced and late-successional tree species became more dominant.
52
Stand initiation
Late succession
Early succession
Stem Exclusion
Mid succession
80
Herbaceous
70
Grass/Sedge
60
o 50
0
C)
>
0
Q 30
20
10
0
60
50
S
Low Shrub
j.
High Shrub
0
0
c
0
C)
30
20
10
0
2
8
18
49
>65
Average Stand Age (years)
Figure 2.14. Percent cover of understory vegetative layers change through time.
Bars are standard error. Approximate successional stage is indicated above graph.
140
140
Hileman I
(2
years)
4--- black cottonwood
100
Snagboat I
iF
years)
(3
120
4-- black cottonwood
100
Sitka willow
Sitka willow
80
U
C-
60
U.
40
20
0
05
1.25
0.75
1
DBH (cm)
1.25
1.5
1.75
2
2.25
DBH (cm)
140
Snagboat 2
(4 years)
Hilemari 2
(5 years)
120
4--- black cottonwood
Sitka willow
4---- black cottonwood
100
1....Sitka willow
>.
80
U
60
U-
40
20
05
05
1.5
2.5
DBH (cm)
Figure 2.15 Continued.
3.5
4.5
2.5
40
Santiam
35
Blue Ruin
40
(7 years)
(5 years)
35
30
4--- black cottonwood
a,
shining willow
25
20
C
L15
C.
C.
0)
4-- black cottonwood
30
>U 25
u
20
15
10
10
0
05
1.5
2.5
2
3
0
4
3.5
10
5
DBH (cm)
40
Skiles I
35
(12 years)
30
30
EB
40
(15 years)
35
30
4--- black cottonwood
4black cottonwood
>.
25
U
C
C.
25
DAH (cm)
25
a,
20
15
hawthorn
C
20
a,
a)
20
creek dogwood
C.
-
15
shining willow
10
5
10
15
20
DBH (cm)
Figure 2.15 Continued.
25
30
35
5
10
20
25
DBH (cm)
30
35
40
Skiles 2
150
(12 years)
125
a.
125
4--black cottonwood
4--- black cottonwood
UOregon ash
100
0
Love Lane
150
(12 years)
shining willow
U
C
hawthorn
75
IX
100
A shining willow
0
U-
50
50
25
25
10
15
20
25
0
35
30
40
45
50
10
DBH (cm)
150
Marshall Landing
15
20
25
4---- black cottonwood
50
55
60
4--- black cottonwood
125
U Oregon ash
100
45
(22 years)
125
>
U-- Oregon ash
---k- alder
big leaf maple
U
C
U
35
40
DBH (cm)
Kropf
150
(16 years)
30
)K
75
walnut
C-
75
U
50
U-
50
25
25
0
0
10
15
20
25
30
35
DBH (cm)
Figure 2.15 Continued.
40
45
50
55
60
10
15
20
25
30
35
DBH (cm)
40
45
50
55
60
Christianson Boat Landing
70
Wigrich "IsIand
70
(23 years)
(24 years)
60
60
--- black cottonwood
*-- Oregon ash
50
bigeaf maple
>.
40
A---- shining willow
3---- hawthorn
30
hawthorn
U-
40
0,
cascara
w 3Q
-- black cottonwood
R-- Oregon ash
>.
-X cherry
w
50
U.
20
20
10
10
0
0
20
10
40
30
50
60
70
80
10
20
30
40
DBH (cm)
60
70
80
90
100
Riverside Landing
(36 ye3rs)
(40 years)
60
60
4-- black cottonwood
S---Oregon ash
50
70
DBH (cm)
Harken's Lake
70
50
4--- black cottonwood
creek dogwood
c0 40
shining willow
------- hawthorn
::
20
10
10
0
10
20
30
40
50
60
DBH (cm)
Figure 2.15 Continued.
70
80
90
100
10
20
30
40
50
60
DBH (cm)
70
80
90
100
70
Crowson
70
Strange Fishing Hole
(45 years)
60
(-50 years)
60
4--black cottonwood
U-- Oregon ash
-'-- hawthorn
a--- shining willow
50
>'
U
C
w
30
4--black cottonwood
50
U-- Oregon ash
>'
40
hawthorn
w
C.
E 30
U.
U-
20
20
10
10
0
10
20
30
40
50
DBH (cm)
60
80
70
90
10
20
30
40
50
60
70
80
90
100
110
DBH (cm)
70
1/2 Moon Bend Landing
50
>.
U
C 40
0'
)
4--- black cottonwood
60
U--- Oregon ash
50
bigleaf maple
>,
hawthorn
C
40
C,
U-
CC,
locust
20
(60 years)
black cottonwood
U--- Oregon ash
bigleaf maple
U
*--- walnut
30
Irish Bend Park
70
(53 years)
60
hawthorn
(
30 L
U-
--
beaked hazel
-
- Indian plum
20
10
10
0
0
10
20
30
Figure 2.15 Continued.
40
50
DBH (cm)
60
70
80
10
20
30
40
50
60
70
DBH (cm)
80
90
100
110
Independence Bar Landing
40
35
(>65 years)
35
4--- black cottonwood
30
25
bigleaf maple
)(
20
.3,
beaked hazel
15
---Oregon ash
25
C
hawthorn
w
b-- black cottonwood
30 -
----Oregon ash
C
.3)
Luckiamute I
40 -
(60 years)
big leaf maple
20
* hawthorn
*---beaked hazel
.3)
u
10
15
10
5
5
0
10
20
30
40
50
---:::
0
60
70
80
90
100
110
10
20
30
40
50
DBFI (cm)
Wells Island
40
25
100
110
o 25
C
-*- Oregon white oak
*- hawthorn
20
.3)
15
--Oregon ash
>
bigleaf maple
C
u-
90
4--- black cottonwood
30
Oregon ash
0
80
(>65 years)
35
+-- black cottonwood
30
.3,
70
Willamette Park
40
(> 65 years)
35
60
DBH (cm)
0)
C.
0)
20
15
10
10
5
5
0
0
10
20
30
40
50
60
DBH (cm)
Figure 2.15 Continued.
70
80
90
100
110
10
20
30
40
50
60
DBH (cm)
70
80
90
100
110
Beacon Landing
40
C
0,
C.
4-- black cottonwood
4--- black cottonwood
-s-- Oregon ash
30
c.
(>65 years)
35
35
>
Luckiamute 2
40
(>65 years)
25
big leaf maple
20
cherry
15
*-- beaked haze'
a,
30
bigleaf maple
c
U
C-
20
U
,.
--- cascara
10
UOregon ash
25
15
10
5
-
5
o'
0
10
20
30
40
50
60
70
80
90
100
110
10
20
30
50
40
DBH (cm)
60
70
80
90
100
110
DBH (cm)
Bower's Rock
40
(>65 years)
35
4--black cottonwood
--- Oregon ash
30
bigleaf maple
' 25
C
0)
--- hawthorn
20
-+--- sugar maple
0)
15
4--- cascara
10
5
.--
0
10
20
30
40
50
60
70
80
90
-.
100
110
DBH (cm)
Figure 2.15. Overstory structural composition displayed through frequency distributions by dbh of tree species at
27 study sites. Tree species >10 cm dbh included. The earliest stand initiation site, Crystal Lake, was not
included as only I tree >1.3 m in height was encountered in the study plot.
60
DISCUSSION
SUCCESSION THROUGH TIME
Patterns of plant succession in Willamette Valley black cottonwooddominated riparian forests generally follow the successional patterns described by
Oliver (1981) for Pacific Northwest upland forests. Stand initiation sites occurred
on gravel bars with pioneer species and weedy herbaceous species dominating.
Black cottonwood was generally the dominant tree species, though one stand was
dominated by willow. Stand age at which stem exclusion occurred varied, most
likely depending on initial establishment densities, or subsequent stem removal by
disturbance events.
Understory re-initiation or early succession appeared to occur around 15
years after stand establishment, though there were older stands that did not exhibit
traits associated with this stage. Succession in these stands appeared to be affected
by high cover of reed canarygrass, an invasive grass species that has the capability
of establishing in monospecific stands that can prevent establishment of other
vegetation (Apfelbaum and Sams 1987). Establishment of native understory trees,
shrubs and herbaceous plants, as well as late-successional tree species was
decreased at high densities of reed canarygrass. This is apparent when comparing
frequency distributions of tree species (Figure 2.15). Maximum reed canarygrass
cover was encountered in Kropf (76%), Love Lane (46%), and Crowson (29%).
Comparing understory and late-successional tree densities between these three
sites, and those of similar ages, it appears that establishment of these species was
decreased.
Late successional stands also exhibited variable composition and structure.
Geomorphic position appeared to play a part in black cottonwood growth and
survival through time. The mean diameters in several stands located on high
61
terraces were comparable to much younger stands on lower terraces (e.g., Bowers
Rock and Wigrich, Figure 2.15). This suggested that limiting factors were present
which inhibited black cottonwood growth and possibly survival. Initial
establishment density might also play a part in this as both scenarios are subject to
limiting environmental factors (e.g., water availability, light, nutrients).
There could be multiple causative factors contributing to observed
variability in composition and structure of forest stands (e.g., geomorphology,
historic vs. current flow regimes, seed source). A limitation of chronosequence
studies are the difficulties of accounting for historical factors, ones that can not be
ascertained, and so can not be held equal across stages. This uncertainty is
partially accounted for by replication within stages, and as such should result in
valid inferences for general successional patterns at the riverscape level.
BIOMASS
Biomass ranged from <1 to 549 Mg/ha in Willamette River black
cottonwood-dominated forests. Compared to other deciduous forest communities
these forests have high carbon storage potential (Table 2.4). At 549 Mg/ha, late
successional forests have more than twice that of other measured cottonwood
riparian forests, 147-193 Mg/ha (Boggs arid Weaver 1994), and conifer-dominated
riparian forests in Northeastern Oregon, 203-26 1 Mg/ha (Case 1995). These values
are comparable to old growth coniferous riparian forest stands in the Cascade
Mountains of Oregon, 589-667 Mg/ha (Grier and Logan 1977).
Biomass reached a maximum when cottonwood IV values were from 50-
70%. This is attributable to black cottonwoods attaining considerable volume
while still being dense enough to be considered the dominant tree species. As
shade-tolerant late-successional tree species, Oregon ash and big-leaf maple,
62
Table 2.4. Tree biomass (Mg/ha) in selected deciduous and riparian forests.
Source
Bray and Dedkiewicz 1963
N. Minnesota, U.S.
Community
Biomass
Aspen and cottonwood (dense)
Aspen (ages 39, both)
58
203
Zavitkovski and Stevens 1970
W. Oregon, U.S.
Red alder
(age 60)
320
Peterson et al. 1970
Alberta, Canada
Aspen
(mature)
77-290
Mountain alder
(age 22)
125
Plains cottonwood
(pole to mature)
147-193
Douglas fir
(old growth)
589-667
Grand fir riparian
(mature)
203-261
Ovington 1956
Great Britain
Boggs and Weaver 1994
Montana, U.S.
Grier and Logan 1977
Cascades, Oregon
Case 1995
NE Oregon, U.S.
This study
W. Oregon, U.S.
Black cottonwood
(pole to mature, age 12 - >75)
141-549
became more dominant, total tree biomass decreased. These late-successional tree
species do not achieve the physical dimensions that are possible for black
cottonwood (Owston 1990; Minore and Zasada 1990; DeBell 1990).
Rates of total aboveground tree biomass accumulation ranged from 0.3 to
23.7 Mg/ha. Accumulation rates were highest in early successional forests and
were higher than rates in tropical secondary forests (3-19 Mg/ha) (Hughes 1999).
63
Rates of accumulation in late successional forests (2.9 to 7.3 Mg/ha) was higher
than those found in old growth Cascade coniferous riparian forests (1.8 to 2.6
Mg/ha) (Grier and Logan 1977), though accumulation rates may decrease as these
forests proceed into later successional phases and cottonwoods become less
dominant.
Dead wood biomass increased through time. This is attributable to
increased cottonwood volume as stands age and large cottonwoods senesce and
fall. High variability between stands of similar successional stage was most likely
attributable to initial establishment densities and subsequent minor disturbance
events (e.g., high winds, beaver activity).
Standing dead trees are important in forest ecosystems as they become an
intricate part of food webs (DeBell et al. 1997). In Willamette River riparian
forests, standing dead trees are food sources for terrestrial insects and are used as
nesting platforms for birds (e.g., blue heron, osprey, eagles). Accumulation of
downed wood is important in terrestrial and adjacent aquatic ecosystems (Harmon
et al. 1986; Sedell and Frogatt 1984). It provides food and shelter to riparian
organisms, creates a mosaic of microhabitats for vegetation establishment by
increasing soil nutrient values with addition of fixed carbon and more biologically
available soil nutrients. Downed wood also contributes to riverine fluvial
processes, as well as causing accumulation of sediments during over-bank flow
events (Sedell and Frogatt 1984). In aquatic ecosystems, downed wood is
acknowledged as an integral part of aquatic ecosystems (e.g., nutrient source, and
provides shelter, and facilitates formation of microhabitats) (Gregory et al 1991).
64
FOREST STRUCTURE
Maximum tree densities (517 to 96,200 trees/ha) and basal areas (24.3 to
0.7 m2/ha for trees >10 cm dbh) found in this study are greater than those found in
other cottonwood riparian forests. Johnson et al (1976) found Plains cottonwood
(Populus deltoides) density ranged from 92.9 to 1,105 trees/ha and basal area of
trees >10.2 cm dbh froml5.8 to 56.9 m2/ha along the Missouri River, North
Dakota. Pioneer species seedlings did not occur in forests past stand initiation, >3
years in age, confirming shade-intolerance documented in these species.
Understory and late-successional tree species were establishing in most stands past
stem exclusion. A few saplings of black cottonwood were encountered at two sites,
however, all were associated with stumps of downed trees, and did not appear to be
healthy individuals capable of long-term survival.
Density of standing dead trees peaked during stem exclusion as fitter
individuals out compete others for available resources (e.g., water, nutrients and
light). Average dbh for snags in these dense stem exclusion stands were <1 cm
dbh. Light was the likely limiting resource early in succession, when initial
establishment densities were high.
There were gaps and clusters in the distributions of stand size-classes of the
28 sampled sites. Two tight groupings of sites occur with average dbh of'53 cm
dbh (n=4) arid -.60 cm dbh (n6). Diameter distributions did not completely
correlate with stand age. Geomorphic position appeared to play a part in reduced
cottonwood dbh. This most likely is attributable to water availability and other
unmeasured environmental conditions.
Importance Values are integrative measures of species importance based on
relative density and relative basal area. In general, they paralleled density and
basal area values, but differences occurred in stands where species differed
significantly in density or size. Using importance values of late-successional
65
species (e.g., Oregon ash and big-leaf maple) and pioneer species (e.g., black
cottonwood and willow) we can see late-successional tree species become
dominant when cottonwoods reach approximately 70 cm dbh (Figure 2.16). These
relatively shade-tolerant late-successional tree species are able to establish and
increase in biomass and density under cottonwood canopy, provided an intervening
disturbance does not occur, thus increasing in dominance over time. Cottonwoods,
however, are shade intolerant and are unable to germinate and establish beneath
their own canopy, and so decrease in importance as stands age.
100.,
.
.
I
r2 = 0.7278
I
0
0
10
20
30
40
50
60
70
80
90
Black Cottonwood Mean DBH (cm)
Figure 2.16. Tee species importance values versus black cottonwood mean
dbh. Late-successional tree species become dominant as stand reach
approximately 70 cm dbh in Willamette River riparian forests.
66
CONCLUSIONS
Stand age and successional processes are important influences in
determining structural diversity and aboveground tree biomass in Willamette River
black cottonwood-dominated forests. These forests are only a fraction of their
historical magnitude, primarily due to clearing for agriculture (Towle 1982), and
from this study, it appears that cottonwood-dominated stands are not regenerating
in a manner to even sustain the current aerial extent of riparian forests. This
phenomenon was investigated using peak streamfiow events for the Willamette
River at the Albany gaging station from 1862 to 2001 (USGS 2001). Peak flow
events have significantly decreased in the past 139 years (r2 = .25 15, p = <0.0001).
In 1862, the largest flood event on record occurred at 340,000 cubic feet per second
(cfs). Since that time, there have been 11 floods > 200,000 cfs all of which
occurred prior to 1947. Since 1947, only 4 floods >100,000 cfs have occurred.
These dramatic decreases in peak flows are significant relative to turnover and
renewal of floodplain forests.
This study also suggests that the presence and abundance of invasive plant
species are causing changes in successional patterns that may be irreversible at a
riverscape level. Reed canarygrass may be significantly affecting structural
development by inhibiting understory re-initiation and late-successional tree
establishment. These effects may be far-reaching, changing current successional
trajectories from historic late-successional tree species towards an open grass and
shrub-dominated system.
Recognizing the importance of riparian forests is especially important in
light of mandates to preserve, protect, and manage for biodiversity. Effective
management and monitoring strategies need to encompass all vegetative layers and
identify biotic and abiotic conditions that maintain high levels of species diversity.
67
Research into feasibility and implementation of management practices, as well as
alternative controls of invasive species is highly recommended. Over time, if no
effort is made in this area, black cottonwood will likely decline in importance as a
dominant component of these riparian forests, thereby greatly affecting structural
and biological diversity in Willamette River riparian forests.
68
LITERATURE CITED
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canarygrass. Nat. Areas J 7:69-74.
Barnes, W.J. 1997. Vegetation dynamics on the floodplain of the lower
Chippewa River in Wisconsin. J. Torr. Bot. Soc. 124(2): 189-197
Benner, P.A. and J.R. Sedell. 1997. Upper Willamette River Landscape: A
Historic Perspective. Chapter 2 in A. Laenen and D.A. Dunnette, Eds.
River Quality: Dynamics and Restoration. Lewis Publishers, New York.
Boggs, K. and T. Weaver. 1994. Changes in vegetation and nutrient pools
during riparian succession. Wetlands. 14:98-109.
Bray, J.R. and L.A. Dudkiewicz. 1963. The composition, biomass and
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Bridge, J.A. 1979. Fuelwood production of mixed hardwoods on mesic sites in
Rhode Island. M.S. Thesis. Univ. of RI, Kingston, RI. 72 pp. As
presented in M.T. Ter-Mikaelian and M.D. Korzukhin. 1997. Biomass
equations for sixty-five North American tree species. For Ecol. and Man.
97: 1-24.
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British Columbia Forest Service. 1976. Whole stem cubic metre volume
equations and tables centimetre diameter class merchantable volume
factors. Inventory Division, Dept. of Forests.
Case, R.L. 1995. The ecology of riparian ecosystems of Northeast Oregon:
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headwater Upper Grande Ronde ecosystems. Masters thesis. Oregon
State Univ.
Debell, D.S. 1990. Populus trichocarpa Ton. and Gray: Black cottonwood. In
R.M. Burns and B.H. Honkala (Eds.). Silvics of North American
Hardwoods. Vol. 12. USDA handbook. pp. 570-576.
DeBell, D.S., R.O. Curtis, C.A. Harrington, J.C. Tappeiner. 1997. Shaping
stand development through silvicultural practices. In K.A. Kohm and J. F.
Franklin (Eds) Creating a Forestry for the 2lS Century. Island Press,
Washington D.C. pp. 141-150
Dykaar, B. B. and P.J. Wigington, Jr. 2000. Floodplain formation and cottonwood
colonization patterns on the Willamette River, Oregon, USA. Envir. Man.
25:87-104.
Foster, B.L. and D. Tilman. 2000. Dynamic and static views of succession:
testing the descriptive power of the chronosequence approach. Plant
Ecol. 146(1):1-10.
Frenkel, R.E., S.N. Wickramaratne, and E.F. Heinitz. 1984. Vegetation and land
cover change in the Willamette River greenway in Benton and Linn
counties, Oregon: 1972-1981. Assoc. ofPacfIc Coast Geog. Yearbook.
46:64-77.
Gholz, H.L., C.C. Grier, A.G. Campell, A.T. Brown. 1979. Equations of
estimating biomass and leaf area of plants in the Pacific Northwest. For.
Res. Lab, Oregon State Univ., Corvallis, Oregon. Res. Pap. 41.
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Gregory, S.V., F.J. Swanson, W.A. McKee, and K.W. Cummins. 1991. An
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74
Chapter 3
Riverscape-!evel Patterns of Riparian Plant Diversity
Along a Black Cottonwood Successional Gradient,
Willamette River, Oregon
Melissa K. Fierke
75
ABSTRACT
Recognizing the importance of native black cottonwood-dominated riparian
forests is important to preserve, protect, and manage for biodiversity in the
Willamette River Valley, Oregon. Species composition along a successional
gradient from stand initiation to late succession of black cottonwood (Populus
balsamfera L. subsp. trichocarpa (T. and G.) Brayshaw) dominated gallery forests
was investigated along 145 km of the Willamette River, Oregon. Young stands
were dominated by pioneer tree species (black cottonwood and willow species) and
opportunistic herbaceous species. Understory trees, shrubs, and herbaceous species
as well as late-successional tree species established 12-15 years after stand
initiation. Oregon ash (Fraxinus latfolia Benth.) was the dominant latesuccessional tree species. Biotic or vegetative habitat variables, in contrast to
abiotic environmental variables, were the most highly correlated measured
environmental variables with understory species presence and abundance. Relative
stand age, as represented by average black cottonwood diameter at breast height
(dbh), was the most strongly correlated environmental variable based on non-metric
multidimensional scaling (NMS) ordination using sites of all ages. Abundance of
reed canarygrass (Phalaris arundinacea L.) was also strongly correlated with plant
composition and abundance. Based on ordination of older stands (>20 cm average
dbh) reed canarygrass cover was the most highly correlated measured
environmental variable with plant species presence and abundance. High
abundance of reed canarygrass resulted in lower values of understory species
diversity and total species richness by inhibiting establishment of understory tree,
shrub, herbaceous species and late-successional tree species. Without intervention
to control the establishment and survival of reed canarygrass, and perhaps some
other invasive species, such as Himalaya blackberry (Rubus arm eniacus L.) and
English ivy (Hedera helix L.), it is conceivable that these species will become more
76
influential with adverse effects ensuing for overall biodiversity at the riverscape
level.
INTRODUCTION
Few studies have quantified the composition and structure of black
cottonwood-dominated gallery forests in the Pacific Northwest (Kauffhian et al.
1985). This is surprising considering riparian areas are diverse dynamic portions of
the landscape that contribute to both terrestrial and aquatic ecosystems (Naiman et
al 1988, Gregory et al. 1991). In addition, this area produces the largest individuals
of black cottonwood (Populus balsamfera L. subsp. trichocarpa (T. and G.)
Brayshaw), which are considered the largest native hardwood in North America
(DeBell 1990).
Riparian gallery forests along the Willamette River, Oregon, were
historically extensive, ranging from 1-6 km in width, but are currently fragmented
and small due to human development and agriculture (Towle 1982; Frenkel et al.
1983, 1984; Benner and Seddell 1997). Dykaar and Wiggington (2000) found a
paucity of young stands regenerating along the Willamette River, suggesting that
even the limited current aerial extent of the gallery forests are not being sustained.
Others have documented the overall decline in the establishment of Populus
riparian forests in the western U.S.A. (Johnson et al. 1976; Rood and Heinze-Milne
1988; Fierke and Kauffman 2002). Limited regeneration has been attributed to
anthropogenic changes in river hydrology (e.g., dams, channelization, bank
hardening, etc.). These changes limit or alter natural riverine processes (e.g.,
meandering patterns and rates, reduction in peak flow events, historic rates of flood
recession, etc.).
77
In conjunction with the overall decline in riparian forests, there is growing
concern as to the invasibility of riparian systems by non-native or aggressive plant
species (DeFerrari and Naiman 1994; Planty-Tabacchi et aL 1996). Invasibility is
partially attributable to the river functioning as a connective corridor during flood
events, to frequent disturbances, to an abundance of heterogeneous habitats within
the riparian ecosystem, and to the proximity of human activities, which often result
in introductions of invasive species. Two species of particular concern in riparian
forests of the Northwest are reed canarygrass (Phalaris arundinacea L.) and
Himalaya blackberry (Rubus armeniacus L.). Reed canarygrass is an aggressive
invasive grass that has been planted in the Northwest for forage and hay production
as well as for stream bank stabilization (Apfelbaum and Sams, 1987). The nativity
of this species is debated, with the most likely scenario being introgression of
aggressive phenotypes into native populations (Merigliano and Lesica 1998).
Himalaya blackberry was introduced into northern California and has since become
naturalized and common in the Pacific northwest (Hitchcock and Cronquist 1973).
Both of these species have displaced native riparian species within riparian plant
communities (Amor 1974; Lesica 1997; Barnes 1999). English ivy (Hedera helix
L.), another exotic species invading riparian forests, is detrimental to trees, and
inhibits understory establishment (Pyle 1995; Freshwater 1991).
Recent studies have demonstrated that the most probable abiotic
environmental variables influencing plant species presence and abundance (e.g.,
pH, soil composition, nutrients, etc.) were not sufficient to explain variation in
species composition found within riparian plant communities (Malanson and Butler
1991; Dollar et al. 1992; Lyon and Sagar 1998). Malanson (1993, p. 81) stated
"stochastic processes of dispersal and founder effects may be important. ." as an
.
explanation for vegetational differences among forest stands in early successional
stages. An example of the relationships of biotic variables with abiotic processes
78
would be the timing of a flood event and seed dispersal of a pioneer species. The
chance timing of the disturbance event would allow a particular species dispersing
at that time a greater opportunity to establish on a site that otherwise would be
suitable for any number of other pioneering, opportunistic species that dispersed
later. This founder effect may then negatively influence establishment and
abundance of other species with differently-timed dispersals. These stochastic
processes with their associated biotic or vegetative factors are credible as
influential factors of richness and diversity in riparian areas. Further, the
aggressive nature, including growth, reproduction, and other inhibitive strategies,
of increasingly common and abundant invasive plant species may also play
important roles in subsequent establishment of native riparian vegetation.
The global objectives of this study were to study successional change and
stand development in riparian forests. Specific objectives were to quantitatively
describe species composition and structural changes of black cottonwood-
dominated forests from initial establishment through late succession. Attempts
were also made to ascertain the current status of some particularly invasive species
of concern and their effects on native plant species. Further, we endeavored to
isolate and assess environmental factors, both biotic and abiotic, controlling
patterns of herb and woody species presence and abundance. Research questions
are suggested and management implications of results are also discussed.
79
METHODS
STUDY APPROACH
Compositional dynamics were quantified utilizing a chronosequence
approach. A chronosequence is a series of stands, which are of different ages, but
which supposedly were or will be similar in appearance when at the same age
(Oliver 1981). This study is, in essence a substitution of space for time.
Willamette River black cottonwood-dominated riparian forests followed general
disturbance-initiated successional stages; stand initiation, stem exclusion, early
successionlunderstory re-initiation, mid succession and late succession (Oliver
1981; Fierke and Kauffman 2002). Sampled stands were all located on the main
stem Willamette River, thus having similar site characteristics (e.g., climate,
disturbance mechanisms, elevation, slope, aspect, etc.). This method of study is a
valid approach to infer basic patterns of successional change and is comparable to
repeated measures of permanent plots (Foster and Tilman 2000).
STUDY AREA
The study area comprised a 145-km length of the Willamette River from the
cities of Eugene to Salem, in western Oregon (Figure 3.1). From an initial
reconnaissance, 34 stands were found that met site criteria; relatively intact (no
obvious within stand human disturbance), large enough for a plot without excessive
edge effects, and access permission obtainable from landowner. Due to the scarcity
of young stands, all 14 sites identified between 0.5 to 35 cm average diameter at
1.3 height (dbh) were used. Mid to late seral sites, 14 out of 20, with average dbb
80
>35 cm, were chosen based on spatial position along the river, so as to have a
relatively even distribution along the study reach, and to ascertain a reasonable
distribution between size classes.
Salem
Harrisburg
Eugene
Figure 3.1. Twenty-eight sampled stand locations are indicated (4-) along 145 km
study reach from the cities of Eugene to Salem, Willamette River Valley, Oregon,
U.S.A..
81
FIELD METHODS
Species composition and vegetative structure were quantified in 28 stands
utilizing a nested plot design (Figure 3.2). Plots were established, arbitrarily
without preconceived bias, and are considered the sample unit. At each site dbh
was measured for all trees and snags >10 cm dbh within a 30 X 100 macroplot (i.e.
0.33 ha). Genus and species of all vegetation within the macroplot were recorded
to compile a subjective species list. Vascular plant nomenclature follows Gilkey
and Dennis (2001) and Hitchcock and Cronquist (1973).
Trees <10 cm average dbh and length and top/end diameters of downed
wood, >7.5 cm diamer, were measured in 4 X 50 m subplots. Cover, to the nearest
whole percent, of understory species (e.g., shrubs, herbs, grasses) was measured in
twenty 50 X 50 cm microplots. Microplots were randomly placed within
macroplots. These values were averaged to macroplot value.
In stand initiation and stem exclusion sites (n = 9), all trees were measured
within 4 X 50 m subplots. This was a necessary modification as all stands of these
size-classes (0.5 - 15 cm dbh) were too small in area for 30 X 100 m plots.
Microplot placement in these size-classes were also within 4 X 50 m subplot.
Trees were measured in 2 X 25 m plots in extremely dense stem exclusion stands,
1-2 cm average dbh (n = 3).
Geomorphic position, riverine position, and meander position, were defined
based on fluvial processes as outlined by Leopold et al (1964). Geomorphic
positions were gravel bar, low area/old meander channel, low (<3 m above the low
water level), medium (3-6 m) and high (>6 m) terraces. Riverine positions were
confluence, single channel form, and multiple channel form. Meander positions
were inside a meander curve, outside a meander curve, or along a straight section of
the river. Longitudinal position (river km) was estimated based on Willamette river
maps (Smith and Associates 1995) and 1996 aerial photos. Stand area was
>15 cm dbh stands
I
lOOm
a
ci
I
a
0
ci
ci
Ui
50m
.4
30 m
ci
ci
ci
0
V
ci
0
ci
ci
<15 cm dbh stands
50m
2m
0
n
4
ci
0
p.-
ci
0
ci
ci
4m
n
25m
Figure 3.2. Composition and vegetative structure were sampled using a nested plot design in cottonwood-dominated
gallery forest stands. in stands with average dbh> 15 cm dbh all trees >10cm dhh were measured in 30 X 100 in
plot. Trees <10 cm dbh and downed wood were measured in 4 X 50 in subplot. In stands dominated by trees < 15
cm dbh, all trees, standing dead trees, and downed wood were measured in 4 X 50 in plot. In dense stands a
subsample of tree and standing dead tree measurements were taken in 2 X 25 rn plot. Cover of herbs, grasses,
sedges, and shrubs were measured in twenty randomly placed 50 X 50 cm microplots. Seedling density and
structural diversity (presence/absence of vegetative layers) was also measured in these micropiots. Genus and
species of all vegetation within the plots were recorded to compile a subjective species list.
83
calculated using digitized aerial photos and GIS. Some common environmental
measurements were not measured as sites were at approximately equal elevations
and in general were not subject to slope. Soils were different, but as with other
fluvial river systems, are correlated with geomorphic position and stand age
(Dykaar and Wigington 2000; Johnson et al 1976; Boggs and Weaver 1994).
ANALYSIS
Understory plant diversity was estimated by the Shannon-Wiener formula
(Shannon and Weaver 1949),
if = - p log p1
where p, is the relative cover of each species. This index considers both the
number of species present and the evenness of species cover. Antilog of H' was
calculated, converting the value back to the species level.
Jacknife calculations (Burnham and Overton 1979) were performed to
predict understory species richness in these riparian forest stands based on the
number represented in microplot samples (Coiwell and Coddington 1994).
Understory species richness was calculated using second-order jackknife equation;
S
Sobs + ((L(2n-3)/n) - ((M(n-2)2)/(n(n-l))
in which S* is understory species richess, Sobs is the number of species occurring
in sample plots, L is the number of species occurring in only one sample plot, n is
the number of sample plots, and M is the number of species occurring in only two
84
sample plots. If variable M was not encountered within a stand, first-order
jackknife equation was used;
S = S0 + (L(n-1)/n)
These calculations have been evaluated in forest vegetation studies and have
proven useful estimators of true species richness (Palmer 1990, 1991; Colwell and
Coddington 1994). The jackknife estimates for species richness were compared to
the subjective species lists compiled for each macroplot. If S* (jackknife value)
was higher, then it is plausible that species were missed in subjective lists due to
observer inconsistency, yet, if the subjective species list was higher then the
jackknife value was certainly underestimating richness. The higher of these two
values was added to tree species richness to obtain total species richness for each
sample site.
Species data was summarized in three ways, alpha, beta, and gamma
diversity (Whittaker 1975). Alpha diversity is defined as the average species
richness per plot. Gamma diversity is the total number of species sampled in all
plots. Beta diversity is the degree of heterogeneity in species composition between
sample plots, and is represented by the ratio of total number of species (gamma)
divided by average species richness (alpha).
Relative dominance of understory species within each sampled stand were
determined by calculating importance values. Importance value (IV) was based on
relative frequency and relative cover within a macroplot,
IV = (F5/F + C5/C)/2 X 100
were F is frequency of an understory species, F is frequency of all understory
species, C is cover of a species, and C is total cover of understory species in the
85
macroplot. This relativization technique was implemented in order to account for
large differences in cover between species within and between stands. Constancy
of each species was also determined by calculating the percent of stands in which
each species occurred.
Analysis of variance (ANOVA) and regression analysis were used to test
differences among sites and to evaluate the significance of relationships between
habitat variables and species composition. Most habitat variables were derived
from vegetation measurements; mean cottonwood dbh, total large tree biomass,
downed wood biomass, vertical structural diversity, percent cover of reed
canarygrass. Abiotic environmental variables included were geomorphic position,
longitudinal position, river position, and meander position.
Ordination techniques are commonly used in community studies to
graphically summarize complex relationships and to describe the strongest patterns
in species composition (B. McCune, personal communication). Non-metric
Multidimensional Scaling (NMS) is an ordination technique that avoids some of the
more difficult to meet assumptions of other ordination methods (e.g., normality,
linear relationships among variables), and performs well even with high beta
diversity (Clarke 1993; McCune 1994). 'Vegetation and habitat variables were
analyzed using PCORD multivariate analysis package (McCune and Mefford
1999).
Sampling sites were ordinated according to similarity in species
composition and abundance using a raw understory percent cover data matrix of 28
sites X 95 species. Data transformations and relativizations were not implemented
as they did not result in dramatic improvements in interpretation of subsequent
analyses. Outliers were identified by examining a frequency distribution of
average Sorensen distance between each sample unit and other sample units in
space. Sites close together in species space were most similar in species
composition and abundance. Only one sample unit was identified as an outlier, the
86
youngest stand, however, because it was only a very weak outlier (average distance
2.32 standard deviations larger than the mean), it was retained in all analyses. A
similar analysis of species in sample space revealed four uncommon species to be
weak outliers (2.03-2.24). All outliers were retained and non-metric
multdimensional scaling (NMS) was chosen as the analysis method of choice. A
further exploratory analysis, using Euclidean and chi-squared distance measures,
was also used, in combination with Bray-Curtis ordination (Bray and Curtis, 1957),
to determine species whose presence and abundances were associated with
identified outlier and borderline outlier sample sites.
NMS ordinations provided a graphical depiction of community
relationships and habitat variables, based on presence and abundance of understory
plant species. Autopilot mode with the slow and thorough option in PCORD used
Sorenson distance measure and the best of 40 runs with real data along with 50 runs
with randomized data for a Monte Carlo test of significance. With this method,
sample sites were ordinated according to similarity in species composition. A
matrix of habitat variables was then overlain on resulting ordinations using joint
plots. Overlays were based on correlations of environmental variables and
individual species with axes of the community ordination. Relative significance of
relationships to each axis was indicated by the length and direction of lines that
represent individual variables and species. Ordinations were rigidly rotated to load
the strongest environmental variable on Axis 1, maximizing the proportion of
variance explained by that axis.
87
RESULTS
COMMUNITY DESCRIPTIONS
Habitat Differences
There was considerable variation in the physical location of early early
successional sites compared to late-successional sites (Table 3.1). Stand age was
significantly correlated with stand area, geomorphic, longitudinal position (river
km), and riverine positions (p = <0.0001, 0.00 16, 0.00 12, 0.0087). Stand age was
not correlated with meander position. Geomorphic positions, and associated soil
deve'opment, ranged from gravel bars for all early successional sites to the majority
of late-successional sites being located on medium and high terraces. Sites were
adjacent to a variety of river positions, including tributary junctions, multiple
channels, and single channels. Meander positions varied from inside meander
curves to outside meander curves and straight channels.
The average diameter of black cottonwood ranged from 0.30 to 89.3 cm
(Fierke and Kauffiiian 2002). Total aboveground tree biomass ranged from <1
Mg/ha in stand initiation sites to 549 Mg/ha in late successional sites. Downed
wood biomass increased through time and was highest in older stands and lowest in
younger stands. Vertical structural diversity increased through time as multiple
vegetative layers developed.
Table 3.1. Environmental variables and basic stand information from 28 black cottonwood-dominated riparian
stands.
Meander
Position
Straight
Stand
Area (ha)4
213.4
River
Position
Multiple
-M.05
GPS Location
440 33.147N, 123° 15.082W
Gravel bar
277.9
Multiple
Straight
0.05
44° 08.7 16N, 123° 07.479W
3
Gravel bar
231.9
Multiple
Inside
-0.05
440 25.823N, 123° 13.596W
Snagboat 2
4
Gravel bar
231.7
Multiple
Inside
--0.1
44° 25.652N, 123° 13.515W
Santiam
5
Gravel bar
172.2
Single
Straight
--0.1
44° 45.182N, 123° 08.801W
Hileman 2
5
Gravel bar
278.1
Multiple
Straight
--0.1
44° 08.716N, 123° 07.479W
Blue Ruin
7
Low terrace
265.5
Multiple
Inside
0.885
44° 13.409N, 123° 08.796W
Skiles I
12
Low terrace
247.8
Single
Inside
0.21
44° 19.985N, 123° 14.825W
Love Lane
12
Low terrace
261.5
Multiple
Outside
0.696
440 15.284N, 123° 10.899W
Skiles 2
12
Med terrace
247.5
Single
Inside
0.554
44° 20.237N, 123° 14.395W
EB
15
Low terrace
257.8
Single
Straight
--0.17
44° 16.877N, 123° 10.835W
Marshalls
16
Med terrace
271.2
Multiple
Straight
23.3
44° 11.664N, 123° 19.064W
Kropf
22
Low terrace
269.1
Single
Straight
3.12
440 13.247N, 123° 09.439W
Wigrich
23
Low terrace
158.0
Single
Straight
3.0
440 49.592N, 123° 09.047W
Christianson
24
Low area
271.7
Multiple
Straight
3.56
440 11.051N, 123°08.462W
River
2
Geomorphic
Position2
Gravel bar
Hileman 1
2
Snagboat 1
Site
Crystal Lake
Age'
km3
Table 3.1 Continued.
245.9
River
Position
Single
Meander
Position
Straight
Stand
Area (ha)4
3.13
GPS Location
440 20.655N, 123° 14.079W
Med terrace
201.7
Single
Inside
9.02
44° 36.128N, 123° 11.656W
45
Med terrace
249.8
Single
Inside
2.37
44° 19.868N, 123° 13.467W
Strange
-50
Med terrace
147.2
Multiple
Straight
3.00
44° 53.175N, 123° 08.520W
Half-Moon Bend
53
High terrace
203.6
Single
Inside
4.04
44° 35.565N, 123° 11.157W
Irish Bend
60
Med terrace
243.0
Single
Inside
4.93
44° 21.682N, 123° 13.221W
md. Bar
-60
High terrace
156.4
Single
Inside
14.1
44° 49.744N, 123° 10.049W
Wells Island
>65
Low & Med
terrace
170.6
Multiple
Straight
10.3
44° 46.706N, 123° 08.700W
Luckiamute 1
>65
Low terrace
174.1
Confluence
Straight
72.5
44° 44.782N, 123° 08.553W
Beacon
>65
Med terrace
277.9
Single
Outside
4.04
44° 08.804N, 123° 07.908W
Willamette Park
>65
Low terrace
214.4
Single
Inside
7.68
44° 32.881N, 123° 14.704W
Luckiamute 2
>65
Low area
174.6
Confluence
Inside
20.3
44° 44.406N, 123° 08.429W
Bowers Rock
>65
Med terrace
196.3
Single
Inside
49.7
44° 38.202N, 123° 09.956W
River
36
Geomorphic
Position2
Low terrace
Riverside
-40
Crowson
Site
Harkens Lake
Age'
km3
tAges based on tree cores and aerial photo interpretation. 2Geomorphic position definitions: Low area was generally an old meander channel or
an abandoned channel, low terrace was defined as a terrace <3 m above 2001 low flows; med terrace, 3-6m, and high terrace, >6m. 2River km
was estimated from Willamette River Recreation Guide (Smith 1995). Stand area calculated using digitized aerial photos and GIS, those with
are estimated from site measurements.
90
Species Diversity and Composition
Beta diversity was high at 4.8 for these riparian forests, indicating that
species richness was variable across stand ages (Table 3.2; Table 3.3). Mean
species richness (alpha) was 33.3 species per stand and 151 total species (gamma)
were encountered. Understory species diversity (1 O") and total species richness
were high in stand initiation sites (n = 3, Figure 3.3). This was followed by a
period of lower richness in stem exclusion sites (n = 6). A subsequent increase
followed with a leveling off in both diversity and richness as understory
reinitiation occurred and stands aged after early succession (n
19).
Stand initiation sites (1 - 3 years) were dominated by pioneering tree
species (black cottonwood and three willow species; shining willow, Salix lucida
Muhi. [= S. lasiandra Benth.]; dusky willow, S. melanopsis Nutt., and Sitka
willow, S. sitchensis Bong.), and opportunistic herbaceous species, such as
knotweed (Polygonum lapathzfolium L.), horseweed (Conyza canadensis (L.)
Cronq.), and mayweed (Anthemis cotula L.) (Table 3.4). These early seral
herbaceous species generally did not persist through stem exclusion into
understory reinitiation (> 12 years). Reed canarygrass (Phalaris arundinacea L.)
and Himalaya blackberry (Rubus arm eniacus L.) reached maximum dominance
values in stem exclusion sites (4 - 12 years). Trailing blackberry (Rubus ursinus
Table 3.2. Plant species richness found across all 28 study sites. Beta
diversity (f3) was measured as the total number of species (p., gamma)
divided by the average number of species (c alpha) (Whittaker 1975).
(sd)
28 sites
33.3 (9.7)
4.8
151
91
Table 3.3. Black cottonwood-dominated stand attributes measured for 28 stands
along the Willamette River, Oregon.
Age*
Site
Crystal Lake
Hileman 1
Snagboat 1
Snagboat2
Santiam
Hiieman2
Blue Ruin
(yrs)
2
2
3
4
5
5
Shies 2
7
12
12
Love Lane
12'
Skiles 1
EB
Marshall
Kropf
Christiansons
Wigrich
Harkens Lake
Riverside
Crowson
Strange
Half Moon
Irish Bend
Indep. Bar
Wells Island
Luckiamute 1
Beacon
WiIlam.Park
Luckiamute2
Bowers Rock
15
16
22'
23
24'
36'
402
451
50'
53
60'
602
>652
>652
>652
>652
>652
>652
Cottonwood
ave. dbh
(cm)
Species
Richness
Understory
Diversity
(10")
% Non
native
0.30
0.60
0.76
30
53
11.1
5.1
33
12.8
70
56
57
1.4
1.5
1.9
22
32
32
24
20
4.2
61
3.7
3.3
65
3.4
35
53
37
25
59
19
32
19
27
9.9
15.3
19.5
21.2
11.2
23.1
29.1
31.6
58.3
53.4
53.8
48.5
62.1
45.1
53.4
60.3
53.5
59.8
61.1
69.4
89.3
59.1
61
28
22
38
24
41
42
36
36
25
23
27
28
40
51
32
34
36
31
32
4.1
6.6
4.3
3.4
13.7
2.2
7.1
8.6
9.9
3.9
4.7
4.4
5.9
7.5
5.0
8.8
7.2
7.3
7.9
4.6
3.6
44
17
23
20
26
32
8
Reed
canary
cover
0
12.7
8.9
2.2
44.6
13.2
29.8
3.0
3.55
45.55
9.5
3.75
76.05
5.25
8.1
0
6
29.3
19.1
19.6
6.15
18
0
24
3.75
0.15
14
18
17
15
16
0
2.7
0.4
0.95
*Age is based on 2 or more black cottonwood tree cores from multiple-sized trees that were chosen to
represent a continuum of tree sizes; 1based on aerial photo interpretation and tree cores (aging of some
tree cores was inexact); 2based on aerial photos only.
92
Stand initiation
A.
0
'-
Mid succession
Stem exclusion
16
Late succession
Early succession
14
>'
12
03
>
D 10
0
03
6
03
a)
Cu
a)
B.
2
60
U)
U)
C,
55
.50
C
45
0
03
40
35
cu
30
C)
>
< 25
20
2
8
21
48
75
Average Age (wars)
Figure 3.3. Understory species diversity (A) and total species richness (B) based
on five age-classes. Approximate successional stage is shown at the top of the
graphs. Bars are standard errors. Species diversity calculated based on percent
cover of understory species.
93
Cham. and Schlecht) and stinging nettle (Urtica dioica L.) increased in abundance
and importance during stem exclusion and reached maxium values during
understory reinitiation. Snowberry (Symphoricarpos albus (L.) Blake) and
salmonberry (Rubus spectabilis Pursh.) increased in dominance in mid succession
and late succession.
Non-tree cover of stand initiation and stem exclusion stands was variable
and ranged from 19 to 98% and 5 to 80%, respectively (Table 3.4). The high
variation in stand initiation sites was likely attributable to age and soil
development as the youngest stands
(2
years) were located on coarse substrate
with only minimal soil development, thus limiting plant establishment. High
variability in stem exclusion sites was due to initial establishment densities of
pioneer tree species. Sites with low tree densities had higher percent cover of
understory species, while dense stands obstructed light penetration and thus
inhibited understory growth and cover. Understory cover was less variable in
early through late successional stands.
Nineteen tree species were encountered, 1 canopy, 9 midstory and 9
understory species (Appendix A). Black cottonwood was the dominant tree species
in 21 out of 28 sites. Sitka willow was dominant in one stem exclusion site, and
Oregon ash (Fraxinus lafolia Benth.) in five late successional sites (Fierke and
Kauffman 2002). Oregon ash was the most common late-successional tree species
establishing in older sites,
18
out of 19 sites. Big-leaf maple (Acer macrophyllum
Pursh), the other late-successional tree species was found at
12
out of 19 sites.
Beaked hazelnut (Coiylus cornuta Marsh. subsp calfornica (DC) E. Murray), red
elderberry (Sam bucus racemosa L.), hawthorn (Crataegus douglasii Lindl.), Indian
plum (Oemleria cerasformis (Hook. and Am.) Landon), and creek dogwood
(Cornus sericea L. {= C. stolonifera]) were common understory tree species.
One hundred and fifty-two total species were encountered across all 28
sites; 78 native, 60 non-native, 13 unknown (Appendix A). Non-native species
94
Table 3.4. Dominant non-tree species in black cottonwood-dominated stands along
the Willamette River, Oregon. Approximate successional stages and number of
stands are given in parentheses.
Stand age and species
1-3 years (stand initiation, n 3):
Polygonum lapathfolium
Conyza canadensis
Anthem is cotula
Hypericum perforatum
Agrostis spp.
4-12 years (stem exclusion, n = 6):
Cover
(%)'
Herb
Herb
Herb
Herb
Grass
Native
Native
Non
Non
Both
18
17
16
13
12
Grass
High shrub
Low shrub
Grass
Vine
Non
Non
Native
Both
Non
38
Low shrub
Grass
High shrub
High shrub
Herb
Native
Non
Native
Non
Non
26
23
Low shrub
High shrub
Herb
High shrub
High shrub
Native
Native
Non
Non
Native
24
High shrub
Low shrub
High shrub
High shrub
Sedge
Native
Native
Native
Native
Native
25
21
7.2
3.3
3.2
Ave.
1V2
40 (31)
12
12
8.0
4.4
79 (15)
Phalaris arundinacea
Symphoricarpos albus
Rubus armeniacus
Urtica dioica
40-60 years (mid succession, n = 6):
Rubus ursinus
Symphoricarpos albus
Urtica dioica
Rubus armeniacus
Rubus spectabilis
>75 years (late succession, n 6):
Symphoricarpos albus
Rubus ursinus
Rubus spectabilis
Corylus cornuta
Carex deweyana
Native
Status
52 (56)
Phalaris arundinacea
Rubus armeniacus
Rubus ursinus
Agrostis spp.
Solanum dulcamara
12-36 years (early succession, n = 7):
Rubus ursinus
Life
form
13
6.8
5.6
77 (17)
23
9.1
5.5
5.4
60 (6)
'Average cover is based on age-classes with standard error in parentheses. 2lmportance value (IV)
is based on frequency of occurrence in microplots and on relative percent cover of each species
within macroplots in each sampled stand.
95
occurred in stands across the successional gradient. Three non-native species were
found at sites of all stand ages, reed canarygrass (93% constancy), Himalaya
blackberry (89% constancy), and bittersweet nightshade (Solanum dulcamara L.),
(82% constancy). In fact, these species were some of the most dominant
understory species found within the research stands based on importance value
(Table 3.4).
ANALYSIS
Habitat Differences
Using the raw understory percent cover data matrix of 28 sites X 95 species,
NMS autopilot chose a 2-dimensional ordination (Figure 3.4). Stress (15.45) was
statistically significantly reduced as compared to randomized data (Monte Carlo
test) and the final ordination was stable (<0.00001). After rigid rotation of 52° to
load the strongest environmental variable onto Axis 1, 80.6% of community
variation was explained, 60.7% by Axis 1, and 19.9% by Axis 2.
An overlay of environmental variables, biotic and abiotic, revealed that
relative stand age, as represented by mean cottonwood dbh (CWDBH) was
strongly correlated with Axis 1 (r = 0.85). Tree biomass (Trbiom), another stand
age correlated variable, was also significant, r = 0.83. Downed wood (dwndwd)
was less strongly correlated, but still significantly so with r
0.45. Vertical
structural diversity (Strdiv), another biotic variable, was also strongly correlated
with Axis 1 (r = 0.88). This variable was also correlated with stand age (i.e. stand
development and understory re-initiation of multiple vegetative layers
leads to higher structural diversity). The only significantly correlated abiotic
96
Sg2
A
CIok
Ibd
Lkit
A
M
A
A
A
Lki2
A
A
dgnc
Chn
TrBom
A
A
CWDBH
dwndwd
K1k2
Ibr
A
GeoPos
Strdi
A
A
II,kl
A
RCGcov
A
Ski4d
Axis I
Figure 3.4. Non-metric multidimensional scaling (NMS) ordination depicting stand
relationships based on the presence and abundance of understory plant species. A
joint plot overlay of environmental variables shows correlations of variables with
axes. Symbols are sites in species space with sites closer to one another having
similar plant associations. RCGcov = reed canarygrass percent cover, Phypos =
physiographic position, Barea = Basal area of cottonwoods, TrBiom total tree
biomass, CWDBH = cottonwood mean dbh, Strdiv = structural diversity (1O").
97
variable (r
0.71) with Axis 1, was physiographic position (Phypos). Longitudinal
position, river position, meander position, and stand area were not significantly
correlated (r < 0.30).
Axis 2 appeared to be correlated with species associations and abundances.
Reed canarygrass cover was the most strongly associated variable with Axis 2 (r
0.59) Himalaya blackberry cover was also entered as an explanatory variable,
however, it was not significantly correlated with either axis (r < 0.30).
Species Composition
A species joint plot overlay revealed several species that were highly
correlated with Axis 1 (Figure 3.5). In particular, snowberry (SYAL), was
strongly correlated with Axis 1 (r = 0.70). Trailing blackberry (RUTJR), another
common native species, presence and percent cover also exhibited a similar trend.
The cluster of species on the far left of the overlay includes sticktight, (Bidens
frondosa L.) (BIFR), and several other opportunistic herbaceous species, that
occurred on gravel bars along the river. Sticktight was the most strongly
negatively correlated species with Axis 1 (r = -0.50) while the other species
presence and abundance appeared to be partially responsible for the separation of
early successional sites along Axis 2 (e.g., Clake and Hilel).
Invasive Species
Reed canarygrass (PHAR), an invasive grass species, was highly correlated
with Axis 2 (r = -0.706). This is a species of some concern and in high abundance,
was correlated with low levels of species richness and understory species diversity
(Figure 3.6).
-
98
S,g2
A
A
Ibd
A
Lki1
A
Kbkt
A
s"co,'
A
PLLA
ANCO
HYR.'
EPGL
ECC
S,.gl
A
Lk
A
ST
MiI2
A
A
A
Skikl
A
A
Poas 1
Figure 3.5. Non-metric multidimensional scaling (NMS) ordination depicting
stand relationships based on the presence and abundance of understory plant
species. A joint plot overlay of species shows correlations of species with axes.
Symbols are sites in species space with sites closer to one another having similar
plant associations. Symphoricarpus albus (SYAL) is strongly positively correlated
with Axis 1, the stand age gradient, as is Rubus ursinus (RUUR). Reed
canarygrass (PHAR) is most strongly correlated with Axis 2. The cluster of
species to the upper left of center are early seral species responsible for the
separation of stand initiation and early stem exclusion sites.
99
.
14
A.
12
C
10
4
C)
0.3974
p = 0.0038
r2
.
.
2
B.
65
60
55
50
45
= 0.286
p = 0.0183
40
35
30
25
20
0
10
20
30
40
50
60
70
80
Reed Canarygrass Percent Cover
Figure 3.6. Understory diversity (A) and total species richness (B) were significantly
negatively correlated with reed canarygrass cover in older stands (>19 cm dbh, n 19).
Rigid rotation of the ordination (Figure 3.5) to load reed canarygrass
(PHAR) onto one axis revealed three native high shrub species and one sedge
species that were strongly negatively correlated with the reed canarygrass gradient
100
(Figure 3.7). These included Pacific ninebark (Physocarpus capitatus (Pursh)
Maxim., PHCA, r = -0.48), salmonberry (RUSP, r = -0.54), snowberry (SYAL, r
-0.51) and Dewey's sedge (Carex dewyanna Schw., CADE, r = -0.54). Elderberry
(SARA, r = -0.40), Indian plum (OECE, r = -0.44). Five other native herbaceous
species were also significantly negatively correlated (r = -0.38 to -0.30) but were
not included for purposes of clarity. Common tansy (Tanacetum vulgare L.,
TAVU) was significantly positively correlated with reed canarygrass in early seral
stands.
Ib,,d
a
a
a
acm,
a
a
a
a
Axis 1
Figure 3.7. NMS ordination with reed canarygrass (PHAR) loaded onto Axis
2. There was a significant negative relationship between abundance of reed
canarygrass and several native plant species in stand re-initiation sites,
including Pacific ninebark (PHCA), Deweys sedge (CADE), salmonberry
(RUSP), and snowberry (SYAL).
101
Patterns In Early to Late Successional Stands
In addition, I attempted to discern patterns of species presence and
abundance in early to late successional stands only (mean dbh >19 cm, n
19).
Kropf, the site with highest reed canarygrass cover (76%), was removed due to its
extreme outlier status for this analysis. After rigid rotation of the resulting 2dimensional NMS ordination to load the strongest environmental variable onto
Axis 1, 81% of variation was explained, with Axis 1 explaining 50% and Axis 2,
31% (Figure 3.8). Reed canarygrass cover was the strongest correlated
environmental variable (r = 0.81). Tree biomass and cottonwood dbh (r = -0.46, r
-0.52) were other significant environmental variables. All other environmental
variables (e.g., physiographic position, downed wood) were not significantly
correlated (r < 0.30).
A species joint plot overlay revealed several species that were highly
correlated with both Axis 1 and Axis 2 (Figure 3.9). Reed canarygrass (PHAR, r =
0.81) and Himalaya blackberry (RUAR, r = 0.59) were significantly positively
correlated with Axis 1. Pacific ninebark (PHCA, r = 0.81), youth-on-age (Tolmiea
menziesii (Pursh) T. and G., TOME, r
and elderberry (SARA, r
0.67), an herbaceous understory species,
0.56), were positively correlated with Axis 2, while
snowberry (SYAL, r = -0.41) and trailing blackberry (RUUR, r -0.46) were
negatively associated. As with the previous ordination, snowberry was associated
with cottonwood dbh (relative stand age) as indicated by the direction of lines
representing significant relationships. Pacific ninebark and reed canarygrass
tended to be associated more strongly with younger stands, though there were
exceptions.
102
Mh,
A
nI
a
a
Ch,j5
a
igr
A
TrBiom
RCGcv
LIn
A
A
A Sk2
A
Ibr
a
a
L..i2
A
A
A
Axis I
Figure 3.8. Non-metric multidimensional scaling (NMS) ordination depicting stand
relationships based on the presence and abundance of understory plant species in
stands past stem exclusion (average dbh >20 cm). A joint plot overlay of
environmental variables (A) indicated reed canarygrass cover (RCGcov) was the
strongest correlated variable with Axis 1. Other correlated variables were total tree
biomass (TrBiom) and cottonwood mean dbh (CWDBH).
103
MI
lbnd
PHCA
wp
vIj
Axis 1
Figure 3.9. Non-metric multidimensional scaling (NMS) ordination depicting stand
relationships based on the presence and abundance of understory plant species in
stands past stem exclusion (average dbh >20 cm). A joint plot overlay of species
shows correlations of species with axes. Symbols are sites in species space with
sites closer to one another having similar plant associations. Reed canarygrass
(PHAR) was the most strongly correlated species with Axis 1. Himalaya
blackberry was positively correlated with reed canary grass.
104
DISCUSSION
HABITAT
Older stands along the Willamette River tended to be larger in area, on
higher terraces, and located on the downstream end of the study reach (Table 3.1).
Stand initiation sites and early stem exclusion sites (dbh <10 cm dbh; n = 7) were
located on gravel bars adjacent to multiple channel riverine positions. All of these
young sites are likely subject to prolonged inundation and scouring by high winter
water events, except one site, Blue Ruin, which is located adjacent to a channel
abandoned -5 years before this study. There were no young stands associated
with medium or high terraces where many of the late-successional stands were
located (n = 9). This phenomenon was investigated using peak streamfiow events
for the Willamette River at the Albany gaging station from 1862 to 2001 (USGS
2001). Peak flow events have significantly decreased in the past 139 years (r2 =
.25 15, p = <0.0001). In 1862, the largest flood event on record occurred at
340,000 cubic feet per second (cfs). Since that time, there have been 11 floods>
200,000 cfs all of which occurred prior to 1947. Since 1947, only 4 floods
>100,000 cfs have occurred. These dramatic decreases in peak flows are
significant relative to floodplain forest turnover and renewal.
SPECIRS DIVERSITY AND COMPOSITION
There were distinct compositional and abundance changes in herbaceous
species along the successional gradient from stand initiation to late succession.
Opportunistic species present on gravel bars did not persist through time, though
they were minimally present through stem exclusion. Species establishing during
105
early succession/understory re-initiation did not always exhibit increasing trends
through time, and many species presence/absence appeared to be uncorrelated with
stand age or environmental variables and may be more directly attributable to
founder effects and stochastic disturbance events associated with seed dispersal,
seed sources, and chance events.
The pattern of high diversity/species richness in stand initiation sites has
been documented by few studies in the literature (Halpern and Spies, 1995).
However, this pattern is not surprising based on successional stages of understory
reinitiation and late-successional tree seedling establishment as the cottonwood
canopy opens up. For example, Kauffman (1982) found gravel bar plant
communities to be the most diverse and the second highest in species richness of 9
different riparian plant community types measured in NE Oregon.
In contrast to findings of Planty-Tabbachi et al. (1995) also within the
Willamette River system, there was no pattern of increasing richness
longitudinally, using regression analysis on all size classes, older age classes, and
even-age classes, in these stands. Also, there were no significant correlations
between species diversity or species richness with physiographic, riverine, or
meander positions.
iNVASIVE SPECIES
Compared to another study of Willamette River Valley riparian vegetation
(Planty-Tabbachi et al. 1995), average percent non-native species was slightly
higher in this study, 31.5% (range = 7.7-70%) versus 26.4%. This study's findings
were also similar to a multi-scale assessment of exotic plants by DeFerrari and
Naiman (1994) on the Olympic Peninsula, Washington (Table 3.5).
106
Table 3.5. Species richness, mean number, and mean percent cover of exotic
species between this study1 and another study on the Dungeness and Hoh river
systems in the Pacific Northwest by DeFerrari and Naiman (1994)2.
Species Richness
Willamette River'
Dungeness River2
Hoh River2
Mean number
Willamette River'
Dungeness River2
Hoh River2
Mean % cover
Willamette River'
Dungeness River2
Hoh River2
Gravel bars
Mature
Rinarian forests
35 (n = 7)
32 (n = 10)
24 (n = 10)
17 (n 19)
13 (n 10)
4 (n = 10)
7.1
16.7
12.8
10
2.7
0.9
70
49.2
29.1
4.8
2.5
1.3
From this study, it appeared that reed canarygrass was the most important
variable associated with understory species presence and abundance in older
coftonwood-dominated stands along the Willamette River. As an example, two
sites, Marshall and Kropf (Table 3.3), are close in average dbh (23.1 and 29.1 cm),
close longitudinally (within 2 1cm), on the same approximate geomorphic position
(medium terrace), and on the same side of the river. Reed canarygrass percent
cover was 3.75% at Marshall and 76.1% at Kropf. Species richness was 37 and 22
at each site respectively, while species diversity was 13.7 and 2.2. This could be
partially attributable to inhibition of understory and late-successional tree species
germination and establishment (Fierke and Kauffman 2002). Bray-Curtis
ordination, after selecting end points to be Marshall and Kropf, emphasized that
107
reed canarygrass cover was the environmental variable primarily responsible for
differences in species composition and abundance between these two sites (r =
0.915) (Figure 3.10).
A
c'J
tg
11=' CWDB-I
STRDJV
A
'irel3io
GeoPos
Stdarea
A
A
RCGr
Cr
A
rowo
Is
Is
A
Is
opt
A
A
MarIha
Lie
Is
A
A
)4V1
A
si
Is
Axis 1
Figure 3.10. Bray-Curtis ordination of sites in species space using chi-squared
distances and selecting end points to be Kropf and Marshalls. Reed
canarygrass cover is strongly correlated with Axis 1, while variables associated
stand age were significantly correlated with Axis 2 (r> 0.30).
108
Himalaya blackberry is a common invasive high shrub species and appeared
to be of equal importance as salmonberry in some sampled stands. Salmonberry is
a charismatic species symbolizing many of the riparian areas in western Oregon.
From data collected in this study, it is conceivable that salmonberry could be
displaced over time by Himalaya blackberry within the Willamette River riparian
system.
Another invasive species of concern in the Willamette River Valley was
English ivy. This species was found in the two northern most sites, just south of
Salem, Oregon, and has the potential to be the most destructive of current invasive
species as it is detrimental to overstory canopy as well inhibiting understory
establishment (Freshwater 1991).
Overall presence and abundance of non-native species was not as great in
older stands (Figure 3.11). This may be an actual trend that holds over time,
however, it is more probable that several of the invasive species may not have been
introduced, or at least were not as abundant when late successional stands
underwent stand reinitiation. These invasive species are also likely less capable of
invading into established stands without an intervening disturbance. In the case of
reed canarygrass, introgression of invasive characteristics into native genotypes
would require time. Therefore, aggressive phenotypes would be expected to
become more widespread with time.
There were environmental variables measured that were correlated with
stand age (e.g., geomorphic position, stand area, riverine position, etc.) that could
contribute to an increased number of non-natives in young stands. Early seral sites
likely experience more severe and frequent disturbances than older sites, and as
such would have higher numbers of opportunistic (and often non-native) species.
These were explored, with geomorphic position and stand area appearing correlated
with percent non-native species, but neither were as strongly correlated as stand
age.
109
0
10
20
30
40
50
60
70
80
90
Aierage 08H (cm)
Figure 3.11. Percent native species versus average black cottonwood dbh.
Percent native species is based on species richness.
CONCLUSIONS
Riparian ecosystems are often composed of a variety of plant communities
of differing successional stages. Disturbance is a necessary component of these
ecosystems to maintain pioneer population levels and to maintain the mosaic of age
classes. Observations from this study indicate that black cottonwood stands are not
establishing in zones conducive to their survival (above the scour zone and
excessive inundation) along the Willamette River. Due to the scarcity of sites
meeting site criterion, many riparian forests are no longer cottonwood-dominated
or are too small or anthropogenically impacted to be considered intact stands.
110
In these spatially dynamic riparian forests, many species coexist by
occupying different successional stages, but even within stand establishment sites,
distinct differences can be detected in plant composition and abundance. These
differences appear to be partially attributable to stochastic processes of dispersal
and founder effects. It also appears that these differences can have long-term
impacts, especially considering the increasing abundance of several invasive
species. These species have the capability of forming dense homogenous cover,
inhibiting germination and establishment of native plant species, and possibly
initiating an alternate successional pathway; away from historical replacement of
pioneer trees species with late-successional species.
Since black cottonwood riparian forests follow generally accepted
successional stages, stand age and successional processes are important in
determining plant species composition through time. Chance events and life history
strategies also contribute to presence and abundance of both early successional
opportunistic species and understory tree, shrub, and herb and late-successional tree
species. Establishment and survival may well be a matter of which species arrives
first, or which species seed dispersal coincides with stochastic disturbance events.
The presence and abundance of invasive plant species are causing changes
in successional patterns in these riparian forests that may be irreversible at a
riverscape level. From this study, it appears that invasive species are affecting plant
composition, species richness, and understory diversity of some stands - to the point
that this influence may be greater than that of stand age. Negative consequences are
likely to ensue for native species (i.e., relegation to a relic species state or even
eventual local extirpation due to seed source limitations over a long time period).
As in other riverine systems, flood management is primarily responsible for
the lack of disturbance and the subsequent lower regeneration of young cottonwood
stands (Rood et al. 1999; Friedman et al. 1998). Clearing of bottomland floodplain
forests for agriculture, urban, and industrial uses has resulted in the forests being
111
fragmented and small in area (Frenkel et al. 1983, 1984). None of these negative
influences are likely to be alleviated within the near future. As a result, black
cottonwoods and other pioneer tree species will continue to decline at riverscape
levels over time as they are replaced by late successional species.
Implications for management of these forests include implementing a
simulated disturbance event that brings about conditions necessary for stand
establishment on suitable substrate. Meeting life history requirements of black
cottonwood would be a monumental undertaking, especially considering water table
recession rates. An alternative would be to actively remove current cottonwoods
and allow regeneration to proceed from stumps. This is offered as a viable
alternative (though with many reservations) as upon further investigation into the
historical context of one research site, Marshall, it was found to have been
harvested 16 years ago. This site was relatively species rich and had the highest
overall understory diversity (Table 3.3). It is surrounded by minimally impacted
riparian forests and is located on a relatively high terrace, both of which could
account for limited abundance of invasive species.
Recognizing the importance of native black cottonwood-dominated riparian
forests is especially important to preserve, protect, and manage for biodiversity in
the Willamette River Valley. Effective management and monitoring strategies need
to encompass all vegetative layers and identify biotic and abiotic conditions that
maintain high levels of species diversity. Research into feasibility and
implementation of management practices, as well as alternative controls of invasive
species is highly recommended. Over time, if no changes in riverine forest
management are implemented. Abundance of black cottonwood will continue to
decline as will the biological diversity of the Willamette Valley.
112
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Chapter 4
Conclusions and Recommendations for Future Work
Melissa K. Fierke
119
This study quantified successional processes in black cottonwood-
dominated riparian forests along the Willamette River, Oregon. These riparian
forests were diverse forests exhibiting marked changes in composition, structure,
and biomass through succession and followed generally accepted disturbance-
initiated successional patterns. Stand initiation occurred on recently deposited
alluvial material and proceeded through stem exclusion as trees grew and competed
for light and other available resources. This stage was followed by understory reinitiation and subsequently proceeded through mid and late successional stages
where late-successional tree species (e.g., Oregon ash and big-leaf maple) became
more dominant. These shade-tolerant, late-successional tree species were able to
reproduce under cottonwood canopy and thus will become dominant tree species at
the stand level, unless an excessive disturbance resets succession.
Structural diversity increased through time as understory herbaceous and
shrub and shade-tolerant tree species established during early succession!
understory re-initiation. As forests proceed into late succession, however, vertical
structural diversity will decrease slightly as cottonwoods senesce and are no longer
present as the canopy species.
Biomass increased through time until late-successional trees become
dominant during late succession. As large relic cottonwoods senesced, total tree
biomass decreased. Overall, total tree biomass for these systems is high in
comparison to other riparian and forest systems in the U.S. Historically, these
forests could have been relatively high carbon reserves. At this point in time, after
extensive riparian forest removal, these fragmented and remnant stands are a small
fraction of their former extent, though some stands located in tributary junctions
and on greenway properties serve as pearls in a picket fence at the riverscape level.
Cottonwood snags and downed wood contribute greatly to structural
diversity in these forests, and to processes in adjacent ecosystems. Terrestrial
120
animals use these dead trees extensively for shelter, nesting platforms, food
sources, etc. Cottonwood contributions to downed wood and allochthonous inputs
within adjacent aquatic ecosystems are also important. Downed wood drives
formations of point bars and other riverine features while also serving as shelter
and food for aquatic organisms. Annual inputs of deciduous leaves as a food base
for aquatic organisms are also important in aquatic food webs.
Plant diversity was high during stand initiation due to the establishment of
weedy herbaceous species along with pioneering tree species, cottonwoods and
willow. Diversity decreased during stem exclusion as cottonwoods generated a
light-limited environment, inhibiting understory plant germination and growth.
Later, diversity increased as less competitive cottonwood trees died, allowing light
penetration and understory reinitiation. Understory reinitiation is the early
successional stage where understory tree, shrub and herbaceous species, as well as
late-successional tree species germinate and establish.
Based on this research, plant composition and abundance across all
successional stages was strongly correlated with a stand age gradient. Vegetative
variables, especially abundance of invasive plant species, were also significantly
correlated, and may be more important influences than stand age on plant
composition and abundance in riparian forests beyond stem exclusion. Reed
canary grass abundance was having major impacts along this river system,
affecting both species richness, species diversity, and structural diversity. A dense
cover of reed canarygrass appeared to inhibit establishment of understory and late-
successional plant species. High cover of invasive plant species affect overall
biodiversity of plant communities, creating homogenous understory environments,
which are in contrast to historical patchy mosaics of riparian habitat types.
In addition to the above, these riparian forests are aging, with latesuccessional tree species becoming more dominant at the riverscape level as these
riparian forests age and cottonwoods senesce. Very little cottonwood regeneration
121
was occurring, causing a reduction in presence and importance of this pioneer tree
species. Historically, this riparian system, along with many others in the western
U.S., was a mosaic of age-classes, with cottonwood stands regenerating in large
patches. Currently, these riparian forests are small in area, fragmented, and
becoming more homogenous. Stand size and fragmentation is attributable to
clearing of bottomland forests for agriculture, urban, and industrial uses.
Homogeneity, i.e. inhibited regeneration of pioneer forests, is mainly attributable to
flood control modifications and river containment efforts.
This study is especially important as a reference base for future restoration
efforts, because it documents riparian forest conditions and structure from stand
initiation to old growth in the early 20th century in the Pacific Northwest. This
should be valuable as ecologists and hydrologists study natural systems in response
to changing climate and various management/restoration scenarios.
Reestablishing historical riverine processes in the Willamette River system
is probably not realistic or acceptable considering current and future human
population levels. Floods of historical magnitudes are no longer occurring and
river bank modifications are inhibiting river meandering. Only one management
option appears feasible at this time, tree and understory removal to encourage
regeneration from black cottonwood stumps. This management option is the only
one encountered that might offer a chance of maintaining cottonwood-dominated
gallery forests and associated native plant species. This option should be explored,
though we should proceed cautiously and conduct further research prior to
implementation.
Suggested objectives of Willamette River management efforts include:
encouragement of historical successional processes (e.g., compositional and
structural diversity, biomass accumulation, etc.); maintenance of black cottonwood
as an important plant species with desirable qualities that contribute to riparian and
adjacent aquatic ecosystems; maintenance of biodiversity, especially native plant
122
species; and control of invasive species. Suggested research goals include:
determination of the extent of tree and understory removal necessary to achieve
desired objectives; ideal timing of removal, considering life history traits and
germination requirements; ideal stand density necessary to achieve goals. Further
location of adjacent native plant seed sources and proximity to invasive plant
sources must be considered in any management efforts.
123
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136
APPENDIX
137
Plant species list for Willamette River cottonwood-dominated riparian
forests. 151 plant species were identified in 0.33 ha plots located in 28 stands
along 145 km of the Willamette River, Oregon. Species categorized based on
native, non-native, or unknown status. Species wetland indicator status determined
based on USDA-NRCS wetland plant classifications. Species constancy was
calculated as the percentage of sites in which each species was encountered.
Native
Wetland
Indicator
Plant Species and Authority1
Common Name'
Status'
Status2
Growth
Form
stancy
(%)
Populus balsam fera L. subsp.
trichocarpa (T. & G.) Brayshaw
Black cottonwood
Native
Fac
Overstory
100
Acer macrophyllum Pursh
Big-leaf maple
Native
FacU
Midstory
43
Acer saccharum Marsh.
Sugar maple
Non
FacLI (NO)
Midstory
4
Alnus rubra Bong.
Red alder
Native
Fac
Midstory
11
Fraxinus latfolia Benth.
Oregon ash
Native
FacW
Midstory
75
Juglans nigra L.
Black walnut
Non
FacU (NO)
Midstory
11
Prunus spp.
Cherry
Both
UK
Midstory
18
Quercus garryanna Doug!. Ex Hook.
Oregon white oak
Native
UPL
Midstory
4
Robinia pseudoacacia L.
Black locust
Non
FacU-(No)
Midstory
4
Salix lasiandra Benth
Shining willow
Native
FacW+
Midstory
43
Acer circinatum Pursh
Vine maple
Native
FacU+
Understory
11
Cornus sericea L.
(== stolonfera Michx)
Creek dogwood
Native
FacW
Understory
61
Corylus cornuta Marsh.
subsp. calfornica (D.C.) E. Murray
Western hazel
Native
FacU
Understory
57
Con-
Native
Wetland
Indicator
Constancy
Plant Species and Authority1
Common Name1
Status'
Status2
Growth
Form
Crataegus douglasii Lindl.
Western hawthorn
Native
Fac
Understory
61
Oemleria cerasformis
(Hook. & Am.) Landon
Indian plum
Native
FacU
Understory
50
Rhamnuspurshiana D.C.
Cascara
Native
Fac-
Understory
4
Sambucus racemosa L.
Red Elderberry
Native
FacU
Understory
43
Salix melanopsis Nutt.
Dusky willow
Native
OBL
Understory
11
Salix sitchensis Bong.
Sitka willow
Native
FacW
Understory
18
Holodiscus discolor (Pursh) Maxim.
Ocean spray
Native
NI
High shrub
4
Physocarpus capitatus (Pursh) Ktze.
Ninebark
Native
Fac+
High shrub
4
Ribes spp.
Gooseberry
Native
UK
High shrub
39
Rosa nutkana Presi.
Common wild rose
Native
NI
High shrub
43
Rubus armeniacus, L.
(=discolor Weihe & Nees.)
Himalaya
blackberry
Non
Fac..
High shrub
89
Rubus laciniatus Wilid.
Evergreen
blackberry
Non
FacU+
High shrub
4
Rubus parvUlorus Nutt.
Thimbleberry
Native
FacU+
High shrub
43
(%)
Native
Wetland
Indicator
Constancy
I0I
/0
Plant Species and Authority1
Common Name1
Status1
Status2
Growth
Form
Rubus spectabilis Pursh.
Salmonberry
Native
Fac
High shrub
36
Spiraea douglasii Hook.
Spirea
Native
FacW
High shrub
7
Symphoricarpos albus (L.) Blake
Snowberry
Native
FacU
High shrub
75
Berberis nervosa Pursh
Oregon-grape
Native
UK
Low shrub
7
Rubus ursinus Cham. & Schlecht
Dewberry
Native
FacU
Low shrub
79
Toxicodendron diversilobum (T. & G.)
Greene (= Rhus diversiloba T. & G.)
Poison Oak
Native
UK
Low shrub
25
Clematis ligusticfolia Nutt.
Wild clematis
Native
FacU
Vine
18
Convolvulus arvensis L.
Morning glory
Non
UK
(No)
Vine
7
Hedera helix L.
English ivy
Non
UK (No)
Vine
7
Humulus lupulus L.
Hops
Non
FacU (NI)
Vine
29
Lonicera involucrata (Rich.)
Banks ex Spreng.
Twinberry
Native
Fac
Vine
4
Marah oreganus (T. & G.) How.
Manroot
Native
UK
Vine
18
Parthenocissus quinquefolia
(L.) Planch.
Virginia creeper
Non
Fac (NO)
Vine
4
Native
Wetland
Indicator
Constancy
(%)
Plant Species and Authority'
Common Name'
Status'
Status2
Growth
Form
Solanum dulcamara L.
Bittersweet
Non
Fac
Vine
82
Unknown vines (2)
UK
UK
UK
Vine
7
Eleocharis spp.
Spike-rush
UK
UK
Rush
4
Equisetum arvense, L.
Common horsetail
Native
Fac
Rush
4
Equisetum hyemale L. subsp. affine
(Englem.) Calder & R. Taylor
Scouring rush
Native
FacW
Rush
7
Carex deweyanna Schw.
Dewey's sedge
Native
Fac+
Sedge
71
Carex obnupta Bailey
Slough sedge
Native
OBL
Sedge
50
Cyperus spp.
Flatsedge
Native
UK
Sedge
7
Iris psuedoacorus L.
Water flag
Non
OBL
Sedge-like
4
Agrostis spp.
Bentgrass
Both
UK
Grass
46
Brachypodium sylvaticurn (Huds.)
Beauv.
False Brome
Non
UK
Grass
4
Bromus spp.
Brome
Both
UK
Grass
39
Dactylis glomerata L.
Orchard grass
Non
FacU
Grass
4
Native
Wetland
Indicator
Constancy
(%)
Plant Species and Authority1
Common Name'
Status'
Status2
Growth
Form
Danthonia calfornica Boland
Oat grass
Native
FacU-
Grass
4
Echinochloa crusgalli (L.) Beauv.
Barnyard grass
Non
FacW
Grass
11
Elymus glaucus Bucki.
Blue wildrye
Native
FacU
Grass
36
Elymus (L.) Gould
Quack grass
Non
FacU
Grass
4
Festuca spp.
Fescue
Both
UK
Grass
7
Holcus lanatus L.
Velvet grass
Non
Fac
Grass
21
Leersia oryzoides (L.) Swartz
Rice cut grass
Native
OBL
Grass
11
Lolium multUlorum Lam.
Ryegrass
Non
NI
Grass
4
Panicum capillare L.
Witch grass
Native
Fac
Grass
4
Panicum occidentale Scribn.
W. witch grass
Native
FacW
Grass
4
Phalaris arundinacea L.
Reed canary grass
Non
FacW
Grass
93
Poafendleriana (Steud.) Vasey
(= longiligula)
Mutton grass
Native
UPL
Grass
4
Poapratensis L.
Kentucky bluegrass
Non
FacU+
Grass
4
Plant Species and Authority'
Common Name'
Native
Status1
Setaria viridis (L.) Beauv.
Green foxtail
Unknown grass (several spp)
Wetland
Indicator
Constancy
Status2
Growth
Form
Non
NI
Grass
4
UK
UK
UK
Grass
36
Anthemis cotula L.
Stinking chamomile
Non
FacU
Herb
11
Arctium minus L.
Common burdock
Non
NI
Herb
18
Artemisia douglasiana Besser
Douglas' mugwort
Native
FacW
Herb
14
Artemisia suksdorJii Piper
Coastal mugwort
Native
NI
Herb
4
Asarum caudatum Lindl.
Wild Ginger
Native
NI
Herb
7
Aster spp.
Aster
UK
UK
Flerb
14
Barbarea orthoceras Ledeb.
Winter cress
Native
FacW+
Herb
14
Bidensfrondosa L.
Beggar-tick
Native
FacW+
Herb
21
Chenopodium ambrosioides L.
Mexican tea
Native
Fac
Herb
11
Cirsium arvense (L.) Scop
Canada thistle
Non
FacU+
Herb
21
Claytonia sibirica L.
Miner's lettuce
Native
FacW
Herb
14
Con ium maculatum L.
Poison hemlock
Non
FacW-
Herb
4
Native
FacU
Herb
18
Conyza canadensis (L.) Cronq.
Can. horseweed
(%)
Native
Wetland
Indicator
Constancy
(%)
Plant Species and Authority'
Common Name'
Status'
Status2
Growth
Form
Daucus carota L.
Queen Anne's Lace
Non
NI
Herb
18
Dicentraformosa (Andr.) Waip.
Pac. bleeding heart
Native
FacU
Herb
4
Digitalis purpurea L.
Foxglove
Non
NI
Herb
11
Dipsacusfullonum L.
Teasel
Non
Fac?
Herb
18
Epilobium ciliatum Raf.
Willow-herb
Native
FacW-
Herb
36
Epilobium glaberrimum Barbey
Smth willow-herb
Native
FacW
Herb
7
Galium spp.
Cleaver
Both
UK
Herb
46
Gnaphalium spp.
Cudweed
UK
UK
Herb
4
Hieracium scouleri Hook.
Hawkweed
Native
NI
Herb
7
Heracleum lanatum Michx.
Cow parsnip
Native
Fac
Herb
46
Ilesperis matronalis L.
Dame's rocket
Non
NI
Herb
7
Hydrophyllum tenuipes Heller
Pacific waterleaf
Native
FacW
Herb
14
Hypericum formosum H.B.K.
W. St. John's Wort
Native
Fac
Herb
29
Hypericumperforatum L.
St. John's Wort
Non
NI
Herb
11
Hypochaeris radicata L.
Cat ear
Non
FacU (NO)
Herb
18
Plant Species and Authorily1
Common Name'
Native
Status'
Impatiens noli-tangere L.
Jewel-weed
Kickxia elantine (L.) Dumort.
Wetland
Indicator
Constancy
Status2
Growth
Form
Native
FacW
Herb
29
Sharp-point fluellin
Non
UPL
Herb
7
Lactuca serriola L.
Prickly lettuce
Non
Fac-
Herb
7
Lapsana communis L.
Nipplewort
Non
Herb
18
Leucanthemum vulgare Lam.
Oxeye daisy
Non
NI
Herb
18
Lotus corniculatus L.
Bird's-foot trefoil
Non
Fac
Herb
11
Lysimachia nummularia L.
Moneywort
Native
FacW
Herb
18
Lythrum salicaria L.
Purple loosestrife
Non
OBL
Herb
4
Matricaria discoidea DC.
Pineapple weed
Non
FacU
Herb
4
Mentha arvensis L.
Field mint
Non
Fac
Herb
4
Me nt ha pulegiurn L.
Peimyroyal
Non
OBL
Herb
18
Mentha xpiperita L.
Peppermint
Non
FacW+
Herb
4
Mentha spp.
Mint
UK
UK
Herb
4
Maianthemum slellatum (L.) Link
(=Smilacina stellata (L.) Desf.)
Small false
Solomon's seal
Native
Fac-
Herb
36
Fac
O)
(0/
/0
k
Plant Species and Authority1
Common Name'
Native
Status'
Melissa ofjIcinalis L.
Lemon balm
Melilotus a/ba Desr.
Wetland
Indicator
Constancy
Status2
Growth
Form
Non
NI
Herb
36
White sweetciover
Non
FacU
Herb
11
Nepeta cataria L.
Catnip
Non
Fac
Herb
4
Oenanthe sarmentosa Presi. ex DC.
Water parsley
Native
OBL
Herb
4
Osmorhiza berteroi D.C.
(= chilensis Hook & Am.)
Sweet cicely
Native
NI
Herb
7
Oxalis suksclorjIi Trel.
Yellow wood-sorrel
Native
NI
Herb
11
Phacelia hastata Dougi.
Silverleaf
Native
NI
Herb
4
Plantago lanceolata L.
English plantain
Non
FacU+
Herb
18
Plantago major L.
Common plantain
Non
Fac+
Herb
11
Polygonum lapathfolium L.
Willowweed
Native
FacW+
Herb
11
Polygonum spp.
Knotweed
Both
UK
Herb
11
Prune/la vulgaris L.
Self-heal
Non
FacU+
Herb
11
Ranunculus repens L.
Creeping buttercup
Non
FacW
Herb
4
Raphanus sativus L.
Wild radish
Non
NI
Herb
4
(%)
Plant Species and Authority'
Common Name'
Native
Status1
Rumex acetolsella L.
Sour dock
Rumex con glomeratus Murr.
Wetland
Indicator
Constancy
Status2
Growth
Form
Non
FacU
Herb
7
Green dock
Non
FacW
Herb
7
Rumex crispus L.
Curly dock
Non
FacW
Herb
11
Saponaria officialis L.
Bouncing Bet
Non
UPL
Herb
7
Senecio spp.
Groundsel
Both
UK
Herb
7
Sisymbrium altissimum L.
Tall mustard
Non
FacU-
Herb
4
Sonchus asper (L.) Hill
Sow thistle
Non
Fac-
Herb
7
Stachys cooleyae Heller
Giant hedge-nettle
Native
FacW
Herb
68
Tanacetumparthenium L.
Feverfew
UK
NI
Herb
7
Tanacetum vulgare L.
Tansy
Non
UPL
Herb
21
Taraxacum officinale Weber ex Wigg.
Dandelion
Non
FacU
Herb
4
Tell/ma grand/flora (Pursh)
Dougl. ex Lindl.
Fringe-cup
Native
FacU
Herb
50
Thalictrum occidentale Gray
W. Meadow-rue
Native
FacU
Herb
29
(%)
Native
Wetland
Indicator
Con-
Plant Species and Authority'
Common Name'
Status'
Status2
Growth
Form
Toimlea menziesii (Pursh) T. & G.
Youth on age
Native
Fac
Herb
36
Trfoliumpratense, L.
Red clover
Non
FacU
Herb
4
Urtica dioica L.
Stinging nettle
Non
Fac+
Herb
75
Verbascum blattaria L.
Moth mullein
Non
UPL
Herb
11
Verbascum thapsus L.
Common mullein
Non
NI
Herb
7
Xanthium stumarium L.
Cocklebur
Native
FacU
Herb
11
Unkown herbs (5)
UK
UK
UK
Herb
18
Athyriumfilix-femina (L.) Roth
Lady fern
Native
Fac
Fern
11
Polypodium glycyrrhiza D.C. Eaton
Licorice fern
Native
UK
Fern
11
Polystichum munitum (Kaulf.) Presi.
Sword fern
Native
FacU
Fern
39
stancy
(%)
1 Species nomenclature follows that of Gilkey and Dennis (2001) and Hitchcock and Cronquist (when not supplied by Gilkey and
Dennis) (1973). Common name and nativity also taken from these authorities.
2 Wetland indicator values taken from the following website: http://plants.usda.gov/cgibinitopics.cgi?earl=wetland.html and CD-Rom
accompanying the Guidebook for Hydrogeomorphic (HGM)-based Assessment of Oregon Wetland and Riparian Sites by Paul Adamus,
200 1.
INDICATOR CATEGORIES
Code
Wetland Type
Comments
OBL
Obligate Wetland
Occurs almost always (estimated probability 99%) under natural
conditions in wetlands.
FACW
Facultative Wetland
Usually occurs in wetlands (estimated probability 67%-99%), but
occasionally found in non-wetlands.
FAC
Facultative
Equally likely to occur in wetlands or non-wetlands (estimated
probability 34%-66%).
FACU
Facultative Upland
Usually occurs in non-wetlands (estimated probability 67%-99%),
but occasionally found on wetlands (estimated probability l%-33%).
UPL
Obligate Upland
Occurs in wetlands in another region, but occurs almost always
(estimated probability 99%) under natural conditions in
non-wetlands in the regions specified. If a species does not occur in
wetlands in any region, it is not on the National List.
NI
No indicator
Insufficient information was available to determine an indicator
status. This was listed on the website or I inserted if there was no
listing for the species.
NO
No occurrence
The species does not occur in that region. For the species with this
listing, I inserted the general listing for the U.S. and still listed the
NO to the right.
Table adapted from USDA plant website: http://plants.usda.gov/cgi bin/topics.cgi?ear1=wet1and.htm1