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The Development of Southwestern Riparian Gallery Forests
Ward Brady, David R. Patton, and Jay Paxson
1
2
Abstract.--Riparian gallery forests along two rivers in
the southwestern United States are described in a developmental continuum ranging from nursery bar to mature forest.
Habitats suitable for tree reproduction are recognizable by
their position relative to the active water course. These
sites are typically located in overflow channels and receive
flow only during floods. Flooding and the subsequent aggradation appear to be the major variables for the natural
sequence of development within riparian stands.
INTRODUCTION
within the forest. The developmental processes
within these forests occur over very long time
sequences, therefore an understanding of these
processes is difficult to acquire by direct
observation. However, by observing the characteristics of stands which are at different stages
of development, inferences can be drawn and
hypotheses formulated regarding the processes
which produce the observed vegetation structure.
The objective of this paper is to describe the
developmental processes hypothesized to occur in
riparian gallery forests of the Southwest.
Importance of the riparian communities for
wildlife habitat has been well described in the
literature (Bristow 1969, Carothers 1977,
Gallizioli 1965, Todd 1972). Other equally
important resource values include aesthetics,
livestock grazing, recreation, and watershed
protection. Better understanding of these
communities is important if these values are to
be preserved because various use-related
pressures such as channelization, phreatophyte
control, water impoundment, and grazing threaten
to further reduce the extent of these
communities.
Study Areas
Literature concerning patterns of temporal
and spatial development of riparian communities
is limited. Glinski (1977) discussed temporal
aspects of riparian regeneration. Other authors
have discussed classification systems (Brown
1982, Dick-Peddie and Hubbard 1977, Pase and
Layser 1977); wildlife importance (Anderson and
Ohmart 1977, Hubbard 1977, Johnson et ale 1977);
management systems (Brown et ale 1977, Davis
1977); and grazing impacts (Ames 1977, Platts
1979, Szaro and Pase 1983, Thomas et ale 1979).
Two study areas were selected which were
felt to be representative of well-developed
riparian gallery forests in the Southwest. The
Gila River Bird Habitat Area, (hereafter termed
Gila BRA) was 16 km southwest of Cliff, New
Mexico. The Winkelman study area was 7 km
southeast of Winkelman, Arizona, and was on the
San Pedro River. Both sites exhibited a
developmental continuum from seedling through
mature stands.
The Gila BHA was 1,310 m above sea level
with steep adjacent mountains over 1,830 m in
elevation. Riparian vegetation was dominated by
Fremont cottonwood (Populus fremontii Wats.),
Goodding willow (Salix gooddingii Ball), and
Arizona sycamore (Platanus wrightii Wats.).
Surrounding upland vegetation varied from
desert/shrub/grassland type on drier, more level
sites to oak-juniper-pinyon woodland on steeper
hillsides and in ephemeral drainages. Dominant
species included western honey mesquite (Prosopis
juliflora (Swartz) DC. var. torreyana Benson),
grama grasses (Bouteloua spp.), pinyon pine
(Pinus edulis Engelm.), one-seed juniper
(JUnIperus-monosperma (Engelm. Sarg.), and gray
oak (Quercus griesea Liebm.). Nomenclature
follows Kearney and Peebles (1960).
This paper reports on studies which were
initiated on the structural characteristics of
two typical riparian gallery forests in the
southwestern United States which led to a
hypothesis concerning developmental processes
1
Paper presented at the Riparian Ecosystems
and their Management: Reconciling Conflicting
Uses Conference. [Tucson, Arizona, April 16-18,
1985]2
Ward Brady is Associate Professor of
Department of Agriculture, Arizona State
University, Tempe, Arizona; David R. Patton is
Principal Wildlife Biologist, USDA Forest
Service, Arizona State University Campus, Tempe,
Arizona; and Jay Paxson is Range Conservationist,
USDA Forest Service, Pinetop, Arizona.
39
Table 1. --Composition, density, basal area, and developmental status of
selected riparian forest stands
The Winkelman study site, in contrast, was
575 m above sea level with a surrounding topography of gently rising alluvial fans which
terminated on steep mountain slopes, the highest
of which reached 1,375 m. The riparian community
was dominated by Fremont cottonwood, Goodding
willow, and salt cedar (Tamarix pentandra Pall.).
Surrounding upland vegetation was typical Upland
Sonoran Desert Shrub. Dominant species included
foothill paloverde (Circidium microphyllum
(Torr.) Rose and Jonst.) and creosotebush (Larrea
tridentata DC. Coville). Associated specie-s----included ocotillo (Fouquieria splendens Engelm.)
and Opuntia species.
Stand II
Overstory
species
Pofr
Sago
Overstory
density
(II/ha)
1
Pofr
Sago
Both the Gila and Winkelman study areas were
characterized by winter-summer precipitation and
spring-fall drought (Kincer 1922, Jurwitz 1953).
The summer precipitation was in the form of
cyclonic storms. Winter precipitation in
particular was highly variable from year to year
(Sellers 1960, Sellers and Hill 1974).
Stand type
designation
-0-0-
-0-0-
123,077
67,692
-0-0-
-0-0-
88,000
6,000
S-NB
12,308
4,615
80.98
26.73
29,231
9,231
A-NB
Pofr
Sago
15,714
-0-
61. 01
-0-
42,857
1,429
A-NB
Pofr
Sago
Tape
Prju
1,401
329
41
-0-
8.90
3.02
0.08
-0-
1,977
329
329
206
A-NB
11
Pofr
Sago
Tape
8,000
-0-0-
27.64
-0-0-
118,000
10,000
14,000
A-NB
12
Pofr
Sago
Tape
Sasa
Prju
2,965
198
-0-0-0-
90.05
9.28
-0-0-0-
10,734
-01,829
99
49
A-tlB
Pofr
Sago
Cere
770
110
-0-
119.26
15.07
-0-
640
100
50
SM
Pofr
Sago
Tape
1,220
200
-0-
216.63
16.79
-0-
1,480
1,030
50
SM
Pofr
Sago
Tape
Sasa
Frve
Prju
684
l34
75
-0-0-0-
23.59
1. 35
0.24
-0-0-0-
-0-
SM
8
2,634
25
50
75
Pofr
Plwr
Sasa
Frve
Juni
Cere
Prju
20
80
30
90
40
35.41
18.80
2.52
9.19
0.80
-0-0-
-0-0320
-072
14,200
1,560
320
200
10
100.66
15.66
0.83
-0-0-0-
10
Pofr
Sago
Acne
Each stand was sampled along its main axis
parallel to the river channel. Placement of the
first sample plot was randomly determined, as was
the intra-stand distance between sample plots.
The following data were recorded: (a) soil type
and texture; (b) distance of the stand from the
active river course; (c) mean canopy height; (d)
overstory canopy cover; (e) all overstory
individuals by species; (f) DBH of overstory >5
cm; (g) estimated understory cover by species.
Twelve stands were sampled using 72 plots.
Stands were categorized based on relative
developmental status as: nurs~ry-bar,
sub-mature, and mature. Stands 1 through 8
represent the Gila BRA, while stands 9 through 12
represent the Winkelman study area.
Regeneration
density
(ll!ha)
Pofr
Sago
METHODS
Well-defined perennial stands, lenticular to
elliptical in outline, typified the Gila BHA and
Winkelman study areas. These stands were
discernable by the limits of their canopies.
Stands were selected to represent a cross section
of stand ages as inferred by Diameter at Breast
Height (DBH) measurements. Selected stands were
required to be visually homogeneous and of
sufficient extent to accommodate a minimum of
four appropriately sized sample plots. Seedling
stands, having ~ majority of trees <5 cm DBH were
sampled using 1/2 m2 rectangular plot frames.
Hature and sub-mature stands having the majority
of overstory individuals >5 cm DBH were sampled
using a 10 m x 20 m rectangular plot.
Overstory
basal area
(m 2 /ha)
Pofr
Sago
Tape
Prju
Sasa
Cere
Frve
-0-0-
= Populus fremontii
= Salix gooddingii
= Tamarix pentandra
= Prosopis juliflora
= Sapindus saponaria
= Celtis reticulata
Plwr
Juni
2 Acne
S-NB
A-NB
M
M
= Platanus wrightii
= Juniperus spp.
'
= Acer negundo
= Seedling nursery-bar stand
= Sapling or adolescent
nursery-bar stand
SM = Sub-mature stand
M = Mature stand
= Fraxinus velutina
Table 2. --Developmental status and geographic relationship to river
course
Stand II
RESULTS
11
12
5
8
10
3
4
The youngest stands were described as either
seedling nursery-bars'(Stands 2 and 6), or
sapling nursery-bars (Stands 1, 7, 9, 11, and
12). Seedling nursery-bars were composed of
overs tory individuals <5 cm DBR. Sapling
nursery-bars, on the other hand, included
individuals larger and smaller than 5 cm DBH.
The mean DBR for individuals >5 cm varied between
8.21 and 9.17 cm (tables 1, 2, and 3).
Stand
developmental
status
Elevation
above
present
river level (m)
Tree
canopy
height
(m)
1
S-NB
S-NB
A-NB
A-NB
A-NB
A-NB
A-NB
A-NB
SM
SM
H
M
0.5
1.4
0.9
1.7
1.1
1.1
2.0
1.7
0.9
1.1
1.4
2.6
0.9
2.4
4.6
7.6
7.6
4.6
7.0
22.9
21.3
18.3
33.5
35.1
Estimate
of
survival
(%)
0.0
0.0
90-95
98
85
95
60
85-90
95
95
90-95
98
2
Baccharis
glutinosa
presence
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
3
no
yes
S-NB = seedling nursery-bar stand, A-NB = sapling or adolescent
nurse y-bar stand, SM = sub-mature stand, and t1 = mature stand.
Occular estimate of stand survival percentage following 100-200
year ~loods of November-December 1978.
Dead Baccharis glutinosa was ohserved in stand 113.
2
40
primary channels. The depth of scouring in these
oxbows may approach ground water, providing ideal
conditions for establishment of hydrophytic
species. Because of reduced water velocities,
secondary channels often appeared as vegetated
furrows, whereas the maiu streamcourse was kept
barren by scouring at times of flood.
Table 3.--Relative density of Fremont cottonwood by diameter classes
(%)
Stand /I
2
6
1
7
9
11
12
S
8
10
3
4
Stand development
status
S-NB
S-NB
A-1m
A-NB
A-NB
A-NB
A-NB
SM
SM
SM
1
M
M
~ 5 em
5+-20 em
20+-40 em
100
100
69.5
-0-030.5
26.2
42.3
5.9
22.4
52.5
44.2
55.3
12.0
11.5
-0-0-0-01.0
-0-033.6
28.1
38.6
48.0
46.2
73.B
56.6
94.1
77.6
12.9
26.1
6.1
-0-0-
> 40 em
-0-0-0-0-0-0-
As flows in the primary channel increase in
volume, these overflow channels also receive
increasing flows, however, the relative velocities are reduced by friction of overland flow,
reduction in volume, and usually by a reduction
in stream gradient. Therefore, the degree of
protection to young vegetation which any particular overflow channel provides to an existing
nursery depends largely on the vertical and
horizontal distance from the main stream which
any flood flow must traverse. Major flooding
events are potentially destructive because
overflow channels provide a natural pathway for
water flow.
-o1.0
1.6
-040.0
42.3
1 8-NB = seedling nursery-bar stand, A-NB = sapling or adolescent
nursery-bar stand, 8M = sub-mature stand, M = mature stand.
Another factor which reduced the velocity of
water in overflow channels was the presence of
vegetation and debris. Seepwi110w (Baccharis
glutinosa Pers.) may be particularly important in
this respect. Live seepwi110w was observed in
92% of all stands sampled (table 2). Seedlings
of seepwi110w were found in all nursery-bar
stands. The regularity of occurrence of this
species suggests the possibility that a
nurse-plant relationship may exist between tree
seedlings and seepwi110w. At least the presence
of seepwi110w is closely associated with the
establishment of stands. Seepwi110w averaged 24%
of total ground cover in sapling nursery-bars,
providing some protection from flooding. Reduced
water velocities also results in sediment
deposition and possible expansion of the
nursery-bar Of iller 1970). Glinski (1977)
related a flooding occurrence on Sonoita Creek in
Santa Cruz County, Arizona, where only cottonwood
and willow growing among stands of seepwi110w
survived.
These stands developed upon bars or shoals
composed of fine sediments. Little or no regeneration of Fremont cottonwood took place on other
micro-habitats. Nursery-bars exhibited a lenticular to ribbon-like shape. Vegetation was
composed of tree seedlings, associated shrub
species, forbs, and grasses. The dominant was
Fremont cottonwood.
Sub-mature stands (Stands 5, 8, and 10)
exhibited reduced regeneration of mesic riparian
(as compared with semi-riparian) species, and had
larger Fremont cottonwoods
= 21.1 cm DBH) than
nursery-bar stands (tables 1, 2, and 3). Trees
within these stands were physically mature;
however, the stands represented an intermediate
developmental stage. Less regeneration occurred
and a greater spectrum of size classes were
present. The habitat in which these stands were
found was shaded with dry, well-drained, finetextured substrates.
(x
Mature stands exhibited no regeneration of
riparian species (Stands 3, 4), and had the
largest Fremont cottonwood individuals (63.8 to
149.3 cm DBH) (table 1). Trees were physically
mature, and the stands were approaching the end
of developmental sequence. The habitat was similar to the sub-mature, except they were further
elevated above the present river channel.
Sub-Mature and Mature Stands
Observations at both study areas suggested
that mature to sub-mature stands of cottonwoodwillow may result from the coalescing of smaller
nursery-bars into a larger stand (fig. 1). The
hypothetical mechanism for this coalescing
follows: Seepwi1low and other riparian shrubs
reduce and redirect flow volumes and velocities
in and around nursery-bar stands during light or
moderate floods. Reduced flow velocities also
result in deposition of sediments, which creates
additional substrate favorable to establishment
of seedlings. When this micro-habitat development coincides with production of viable seed,
potential for abundant regeneration is created.
If this occurs over a period of several years,
numerous nursery-bars become established and grow
together appearing as a single macro-stand.
Successful stand occurrence, however, depends
largely on favorable flood volumes during the
developmental period.
DISCUSSION
Nursery-bars
Nursery-bars were located in overflow
channels on both study areas. Miller (1970)
discussed the formation of these secondary
channels within the floor plain. Crescentshaped bars were formed by the nonuniform
accretion of alluvium on the inner side of a
stream bend. High flows temporarily turned thealluvial deposit into an island with a secondary
channel behind it. Other overflow channels
appeared as abandoned meanders or oxbows of
41
SEEDLING
NURSERY - BAR
--COALESCING NURSERYBARS (SAPLING STAGE)
MATURE/SUB-MATURE
STAND
1"=50'
I"
= 1000'
INCREASING
STAND SIZE, STAND MATURITY, TREE MATURITY (DBH)
Figure 1.--Riparian development sequence showing coalescing
of seedling nursery-bars in an overflow channel into
a mature stand.
driftwood and other debris lodged among the trees
further supports the possibility that such
hydraulic activity explains the apparent elevation of stands and individuals.
Age distributions inferred from DBH
measurements (table 3) indicated that as stands
mature, the habitat became progressively less
suitable for regeneration. Little or no regeneration of mesic riparian species takes place in
stands which have developed beyond the nurserybar stage. Shading, aggradation within the stand
due to sediment deposition, shifting river
patterns, and possibly other factors combine to
provide adverse conditions for seedling
establishment.
SUMMARY
Establishment and development of southwestern riparian gallery forests appears to
follow an orderly, well-defined sequence.
Starting with the creation of favorable seedbed,
stands progress from nursery-bars to senescent
individuals as habitat is continually modified.
The riverine system plays an important role in
the survival and development of these stands.
Flooding, when light or moderate, favors establishment and development through deposition of
nutrient-rich sediments and increased soil
moisture. Successful regeneration cannot be
expected on an annual basis, since it depends
greatly on a "proper sequence of flooding," 1. e. ,
no flood large enough to be catastrophic until
any given stand is sufficiently well developed to
provide its own protection. Major flooding is
catastrophic, regardless of the developmental
stage.
Stands which have developed to the mature or
sub-mature stage seem isolated and/or pedestaled.
Field observations suggest that both aggradation
within and degradation around a particular stand
may account for this phenomenon. Wolman and
Leopold (1970) reported that natural levees may
be formed by sediment being entrapped in
streamside vegetation. They also discussed
specific examples of aggradation, ranging from
several cm to several m which occurred at times
of peak flood, calling the phenomenon overbank
deposition.
Numerous examples were noted at both study
areas where trunks of overs tory species appeared
buried. Partial excavation of several trees
revealed that the main stem of the trunk continued to a greater depth. During periods of
high stream flow, suspended sediments were
carried into these stands where they are
deposited. The process, repeated many times and
in sufficient magnitude, may help explain this
phenomenon. Examination of cutbanks adjacent to
these stands revealed obvious grading and crossbedding which supports this hypothesis. Stands
appeared to be elevated in the study areas 1.5 m
or more (in a few cases up to 3 m) above the
present river level. Additionally, masses of
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42
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43