WETLANDS, Vol. 26, No. 4, December 2006, pp. 1151–1160
’ 2006, The Society of Wetland Scientists
EVALUATION OF WOODY PLANT RESTORATION IN THE MIDDLE
RIO GRANDE: TEN YEARS AFTER
John P. Taylor1,4, Loren M. Smith2,5, and David A. Haukos3
1
U.S. Fish and Wildlife Service
P.O. Box 1246
Socorro, New Mexico, USA 87801
2
Wildlife and Fisheries Management Institute
Texas Tech University
Lubbock, Texas, USA 79409-2125
E-mail: l.m.smith@ttu.edu
3
U. S. Fish and Wildlife Service
Department of Natural Resources Management
Texas Tech University
Lubbock, Texas, USA 79409-2125
4
deceased.
5
corresponding author
Abstract: Native riparian cottonwood (Populus deltoides) dominated habitats are declining throughout
the southwestern United States and being replaced by the exotic salt cedar (Tamarix ramosissima, T.
chinensis) dominated systems. Restoration efforts have often involved removing salt cedar and simulating
or restoring overbank flooding timed with seed rain of cottonwood and willow (Salix spp.). Success is
then often assessed by examining seedling densities of the native species and their known competitive
superiority over salt cedar at this stage. We measured plots in the Middle Rio Grande Valley, New
Mexico, USA that had been cleared of salt cedar a decade earlier, compared the initial seedling
community to the established community, and determined variables important in predicting
establishment of cottonwood. During the decade since establishment, the proportion of cottonwood
and seep willow (Baccharis glutinosa) increased, while salt cedar and coyote willow (S. exigua) decreased.
Proportion of community foliar volume changed little for salt cedar (17.0 to 17.6%), while it increased for
cottonwood (25.5 to 63.3%). Cottonwood dominated (.70%) the overstory. Initially established woody
stem density (all species) was the only predictor of the percent of cottonwood canopy cover a decade later
as declining coyote willow allowed for greater cottonwood growth. No abiotic variables were important
in predictions of subsequent cottonwood community establishment, probably because these variables
were similar among sites in this active floodplain. High initial cottonwood and salt cedar seedling
densities were related to lower cottonwood foliar volume and smaller individual tree diameters a decade
later. Low initial densities of cottonwood allowed individuals to grow rapidly to a greater height,
dominating the canopy. Restoration biologists should therefore consider areas that have low initial
cottonwood seedling densities as potentially very successful if their goal is to produce a stand of large
cottonwoods in a relatively short time.
Key Words: cottonwood, Populus, Rio Grande Valley, riparian restoration, salt cedar, Tamarix
INTRODUCTION
movement in river valleys. These anthropogenic
disturbances have promoted conversion of systems
dominated by native flora to those dominated or
co-dominated by exotic plants (Szaro 1989, Howe
and Knopf 1991). Historically, riparian zones
along these rivers were dominated by native
cottonwood (Populus spp.) and willow (Salix spp.),
but they are now dominated by salt cedar (Tamarix
Native riparian communities have been severely
degraded throughout the western United States
(e.g., Busch and Smith 1995, Shafroth et al. 2005).
The construction of dams and water withdrawals
have altered natural river hydrographs, while
construction of levees has restricted channel
1151
1152
ramosissima Ledeb., T. chinensis Lour.) (Campbell
and Dick-Peddie 1964, Graf 1978, Howe and
Knopf 1991, Busch and Smith 1995, Friedman
et al. 2005).
Seeds of cottonwood, willow, and salt cedar are
wind- and water-dispersed. These species do not
persist in the seed bank and only survive for a few
weeks following dispersal (Horton et al. 1960).
Willow and cottonwood seeds are dispersed for
a few weeks in late spring when overbank flooding of rivers historically occurred following snowmelt in adjacent mountain ranges, but salt cedar
disperses seed throughout the growing season
(Horton et al. 1960, Fenner et al. 1984, Stromberg
1997). Each of these species requires bare, moist
substrate for germination that historically was
provided as a result of overbank flooding events
(Taylor et al. 1999). However, salt cedar can
germinate any time during the growing season when
moisture and other soil conditions are suitable
(Horton et al. 1960).
Using this knowledge of seedling establishment
requirements, several studies have investigated
simulated and planned overbank flooding events as
a means of restoring native woody plants in riparian
zones in the southwestern U.S. (Friedman et al.
1995, Taylor and McDaniel 1998, Roelle and
Gladwin 1999, Taylor et al. 1999, Sprenger et al.
2001, 2002, Sher et al. 2002, Rood et al. 2003). These
studies have primarily relied upon germination data
gathered within four years after overbank flooding.
Success was measured based on the assumption that
overbank flooding treatments producing the highest
densities of cottonwood and willow seedlings were
optimal for restoring native woody plants (e.g.,
Taylor et al. 1999, Sprenger et al. 2002). Moreover,
studies examining the competitive relationships
between salt cedar and cottonwood have generally
been focused on plants within a four-year age span
and have indicated a competitive superiority of
cottonwood over salt cedar (Sher et al. 2000, 2002).
Few data exist on longer term persistence and
competitive relationships of older (.four year)
cottonwood and salt cedar, or on the relationship
of initial seedling densities to long-term restoration
of native riparian habitat in the southwestern
U.S.
We compared woody plant data collected in 1995,
after initial overbank flooding events in 1993 and
1994, to the plant community existing in 2004. First,
we compared woody plant composition, density,
height, and foliar volume of dominant taxa between
the two time periods to determine the efficacy of the
original treatments in longer term native woody
plant restoration. Next, because initial plant density
WETLANDS, Volume 26, No. 4, 2006
may influence subsequent stem diameter and height,
and therefore restoration, we explored relationships
for diameter and height vs. density for dominant
taxa. We also examined potential factors influencing
cottonwood density, canopy cover, and individual
plant volume because this species is most often
targeted in conservation plans.
METHODS
Study Area
The study was conducted at Bosque del Apache
National Wildlife Refuge (33u480, 106u530) in
Socorro County, New Mexico, USA, along 0.8 km
of the Rio Grande. Details of the study area are
described in the initial study by Taylor et al. (1999).
The Rio Grande Valley was 5.2 km wide at the
study site. The active floodplain width was restricted
to 1 km by a spoil levee built in the late 1950s on the
west side of the river in conjunction with construction of the Low Flow Conveyance Channel (Taylor
et al. 1999). Historically, river flows were characterized by an annual hydrograph typical of many river
systems in western North America (Scott et al. 1993,
Taylor et al. 1999). Diverse riparian vegetation
communities were present on the refuge prior to
irrigation diversions and flood-control developments (Bosque del Apache NWR, unpublished
data). Although cottonwood (Populus deltoides
Bartr. ex Marsh) gallery forest patches remain,
introduced salt cedar monocultures dominate large
areas of the floodplain, with little natural regeneration of native species (Taylor et al. 1999). Prior to
restoration treatments, the entire study area was
dominated by the exotic salt cedar and native seep
willow (Baccharis glutinosa (R. & P.) Pers.); 94.5%
and 5.5%, respectively, of the vegetative cover
(Taylor et al. 1999:374). Salt cedar stem density
was 27,156/ha.
Initial Site Manipulation and Field Sampling
Sixteen 50 3 50-m plots were established
immediately adjacent and perpendicular to the
river channel. Eight were randomly selected for
clearing with a D-7 bulldozer, and debris was
raked and stacked. Water was released from
upstream reservoirs on the Rio Grande north of
the refuge by the U.S. Bureau of Reclamation in
May and June 1993, 1994 to create overbank
flooding conditions in the study plots. Timing
corresponded to the historic river hydrograph.
Overbank flooding of the 16 plots occurred on 26
May 1993 at 127 cms (cubic meters per second),
Taylor et al., RIPARIAN RESTORATION SUCCESS
and inundation continued for 20 days. In 1994,
overbank flooding occurred at 128 cms from 14 to
18 May and at 143 cms from 25 to 29 May. One
of the eight cleared plots was destroyed during
creation of a wildfire line in 1996.
We collected nine soil samples from each plot in
July 1993 using a 2-cm-diameter soil bucket auger
(15-cm depth) and determined soil particle sizes
using the hydrometer method (Gee and Bauder
1986). Average proportions of sand, silt, and clay
were calculated for each plot. We measured soil
conductivity, as an index of salinity, in October 1993
across the 0.75-m soil profile (upper) and the 1.5-m
soil profile (lower) using the electromagnetic induction method (Sheets et al. 1994). Although
salinity affects germination of riparian species,
conductivity varied little among cleared plots
(upper531.8 mS/m [SE51.69] lower535.96 mS/m
[SE51.78]) and therefore seldom was selected as
an important variable in predicting initial seedling
response (Taylor et al. 1999). Therefore, to limit the
number of variables and improve inference, we did
not include salinity as a variable during subsequent
modeling. We calculated average plot elevation
(above sea level; ASL) from three random elevation
measurements within each plot using standard field
surveying techniques. Two Bureau of Reclamation
floodplain elevation cross sections were used as
reference points for the survey.
In this study, we compared data from vegetation
surveys conducted in 1995, which measured seedling
recruits from overbank flooding in 1993 and 1994,
to those in 2004, 11 to 10 years respectively, from
original recruitment. In each plot, we estimated
percent herbaceous cover (i.e., non-woody) by
averaging cover estimates in nine randomly located
1 3 1 m quadrats during late September 1994. These
small quadrat locations were not measured in 2004
because the woody vegetation had matured and thus
were not of relevant size. Herbaceous cover data
were only used in regression models (described
below) to predict long-term restoration response
and were averaged because plot was the replicate in
those analyses. To estimate density of woody plants,
we identified and counted individuals along four 1 3
30-m belt transects located parallel to and at 10-m
intervals from the main channel bank location in
1995 and 2004 within each of the seven restoration
plots. We then averaged data for all transects within
each plot, because plot was the replicate, and
converted the result to density (no./m2). Using the
end of each transect as a center point, we divided the
surrounding space into four quadrants (Cottam et
al. 1953; northeast, northwest, southeast, and
southwest). Within each quadrant (8/transect line,
1153
32/plot) the species, height (m), horizontal cover
(m2), and diameter at breast-height (DBH) of the
nearest live individual tree were recorded. We
multiplied height and horizontal area to estimate
plant (foliar) volume (m3). In 2004, we also
measured overstory of each species by estimating
the proportion of each transect above 2 m covered
by woody vegetation. The percent cover for overstory species was estimated by dividing the length of
the transect line covered by each species by the
transect length.
Data Analyses
We determined percent composition of cottonwood, coyote willow (S. exigua Nutt.; an early
successional species), seep willow, and salt cedar for
1995 and 2004 in each plot based on individuals
counted in the quadrants at each end of belt
transects. We used these data rather than density
data because they are unitless and not related to tree
size as are density data. We used the non-parametric
sign test (Hollander and Wolfe 1973) to test if
percent composition of each species increased or
decreased between 1995 and 2004. We calculated
percent change in plant density, height, and foliar
volume for each species and across species between
1995 and 2004. We used Pearson’s correlation
coefficient to determine relationships between plant
height and density for each species in 2004. Because
of relatively high measures of variation between
1995 and 2004, we also compared average values
using Cohen’s (1988) d statistic as a measure of
effect size of the difference between 1995 and 2004
mean values. We compared average plot density,
volume, and height data for cottonwood, coyote
willow, seep willow, and salt cedar between 1995
and 2004 using a paired t-test. Density estimates
were log-transformed prior to analyses (Taylor
et al. 1999).
Successful riparian forest restoration in this
region is measured by cottonwood dominance.
To explore potential factors influencing attributes of cottonwoods (i.e., density, canopy
cover, and individual plant volume) in 2004,
we used stepwise regression (P50.15 for entry
and exit) to determine important independent
variables (PROC REG, SAS 1999). Independent
variables included in the analyses were plot
elevation (m ASL), 1994 herbaceous cover (%),
percent clay, percent silt, percent sand, 1995
cottonwood density, 1995 average cottonwood
height, 1995 average cottonwood foliar volume,
1995 coyote willow density, 1995 seep willow
density, 1995 salt cedar density, and 1995 total
1154
WETLANDS, Volume 26, No. 4, 2006
Table 1. Percent composition of cottonwood, coyote willow, seep willow, and salt cedar in 1995 and 2004 in seven, 50 3
50-m plots cleared in 1993 on Bosque del Apache National Wildlife Refuge, New Mexico.
Cottonwood
Coyote willow
Seep willow
Salt cedar
Plot
1995
2004
1995
2004
1995
2004
1995
2004
4
9
11
12
13
15
16
All Plots Combined
28.1
12.5
3.2
0.0
0.0
3.1
0.0
6.5
50.0
54.2
9.7
13.3
37.5
56.3
35.5
35.6
59.4
40.6
19.4
16.1
27.3
31.3
25.0
31.4
27.3
4.2
0.0
0.0
0.0
6.3
0.0
4.5
0.0
3.1
6.5
0.0
3.0
3.1
6.3
3.1
18.2
41.7
32.3
0.0
12.5
12.5
19.4
23.7
12.5
43.8
71.0
83.9
69.7
62.5
68.8
58.7
4.5
0.0
58.1
86.7
50.0
25.0
45.2
41.1
density. We examined relationships between 1995
and 2004 cottonwood densities and 2004 average
cottonwood DBH using (1) Spearman’s rank
correlation and (2) linear regression.
RESULTS
The percent composition of cottonwood (P 5
0.008) and seep willow (P 5 0.02) increased from
1995 to 2004 (Table 1). The proportion of coyote
willow (P 5 0.008) and salt cedar (P 5 0.06),
however, decreased as a proportion of the community (Table 1).
Plant height was correlated with foliar volume for
cottonwood (r11 5 0.72, P 5 0.01) and salt cedar (r13
5 0.77, P 5 0.002). Height of seep willow was
correlated with foliar volume (r11 5 0.88, P , 0.001)
but not density (r11 5 20.10, P 5 0.76). However,
there were no relationships between plant height and
foliar volume (r10 5 0.05, P 5 0.88) or plant density
(r10 5 0.12, P 5 0.75) for coyote willow. Plant
heights of cottonwood (r11 5 20.67, P 5 0.02) and
salt cedar (r13 5 20.77, P 5 0.002) were negatively
correlated with plant densities.
Proportion of the total foliar volume of the
community changed little for salt cedar (17.0 –
17.6%) and seep willow (17.0 – 14.7%) from 1995
to 2004. However, proportion of community foliar
volume of cottonwood increased from 25.5 to
63.3%, while coyote willow decreased (40.4 –
4.3%).
Densities of coyote willow (t6 5 1.43, P 5 0.20)
and seep willow (t6 5 1.49, P 5 0.19) were not
statistically different between 1995 and 2004
(Table 2). Cottonwood (t6 5 2.13, P 5 0.08) and
salt cedar (t6 5 5.72, P 5 0.001) densities decreased
from 1995 to 2004. Total density of woody species
decreased from 1995 to 2004 (t6 5 3.55, P 5 0.012).
Plant height increased from 1995 to 2004 for
cottonwood (t3 5 13.19, P , 0.001) and salt cedar
(t5 5 9.03, P , 0.001) but not for seep (t4 5 1.98, P
5 0.12) or coyote willow (t2 5 0.19, P 5 0.87).
Similar results were noted for plant volume (cottonwood, t6 5 2.69, P 5 0.04; salt cedar, t5 5 3.19, P 5
0.002; seep willow, t4 5 2.01, P 5 0.11; coyote
willow, t2 5 0.96, P 5 0.44). However, with the
exception of coyote willow height, changes in plant
density, height, and foliar volume showed medium
to large effect sizes between 1995 and 2004
(Table 2). Across species, average density (t65
2.66, P 5 0.04), plant height (t6 5 2.61, P 5 0.04),
and foliar volume (t6 5 6.76, P , 0.001) differed
between 1995 and 2004.
In 2004, cottonwood dominated the overstory by
occupying . 70% of the overstory (Figure 1). Total
1995 density of woody species was the only predictor
of 2004 percent cottonwood overstory (r2 5 0.98,
model F1,6 5 99.29, P 5 0.01; percent cottonwood
overstory 5 51.02 + 2.87[total density]). Two
variables, total 1995 woody density (partial r2 5
0.97) and 1995 cottonwood foliar volume (partial r2
5 0.02), were identified in the model to predict 2004
cottonwood density (r2 5 0.99, model F1,6 5 75.94,
P 5 0.01; cottonwood density 5 0.206 + 0.068[total
density] 2 0.27[cottonwood volume]). Interestingly,
the only predictor of 2004 cottonwood foliar volume
was 1995 salt cedar density (r2 5 0.32, model F1,6 5
2.51, P 5 0.15; cottonwood foliar volume 5 80.64 2
25.61 [salt cedar density]; Figure 2); the influence
was negative. The 2004 overhead cover and density
of cottonwood was positively related to 1995
total density of woody species (Figure 3). No
abiotic variables were significant in cottonwood
predictions.
Densities of cottonwood in both 1995 and 2004
influenced cottonwood DBH in 2004 (Figure 4). The
1995 (r 5 20.71, P 5 0.09) and 2004 (r 5 20.78, P
5 0.05) densities were negatively correlated with
Taylor et al., RIPARIAN RESTORATION SUCCESS
1155
Table 2. Mean (SE) plant density (no./m2), height (m), and foliar volume (m3) of cottonwood, coyote willow, seep willow,
and salt cedar in 1995 and 2004 in seven, 50 3 50-m plots cleared in 1993 on Bosque del Apache National Wildlife Refuge,
New Mexico.
Species
Characteristic
Cottonwood
Density*
Height*
Volume*
Coyote willow
Density
Height
Volume
Seep Willow
Density
Height
Volume
Salt cedar
Density*
Height*
Volume*
Total woody plant density
Overall average density*
Overall average volume*
Overall average height*
Year
1995
2004
Percent Change
Effect Sizea
1.91 (0.95)
0.98 (0.27)
0.24 (0.19)
0.50 (0.15)
5.53 (0.41)
34.91 (12.89)
274
464
1444
0.78
5.36
1.44
1.05 (0.85)
1.34 (0.09)
0.38 (0.10)
0.06 (0.04)
1.49 (0.46)
2.38 (1.82)
294
11
526
0.62
0.19
0.89
0.15 (0.06)
0.78 (0.24)
0.16 (0.15)
0.30 (0.10)
1.65 (0.35)
8.11 (3.97)
100
111
4968
0.71
1.23
1.15
276
123
5969
274
273
6687
55
2.52
3.28
1.84
1.10
1.99
0.97
0.77
1.79
1.03
0.16
4.89
1.22
0.24
1.96
(0.28)
(0.12)
(0.05)
(1.72)
(0.34)
(0.06)
(0.09)
0.42
2.30
9.71
1.28
0.32
16.29
3.04
(0.07)
(0.19)
(3.00)
(0.21)
(0.05)
(4.97)
(0.41)
a
Effect size is based on Cohen’s (1988) d statistic and interpreted as ‘‘large’’ for values $ 0.80, ‘‘medium’’ for values $ 0.50 and , 0.80,
and ‘‘small’’ for values $ 0.20 and , 0.50.
* Species characteristic labeled with an asterisk indicates a difference between means.
Figure 1. Total overstory composition in 2004 of seven, 50 3 50-m plots cleared of salt cedar in 1993 on Bosque del
Apache National Wildlife Refuge, New Mexico to restore riparian woodlands.
1156
WETLANDS, Volume 26, No. 4, 2006
Figure 2. Relationship between the 1995 density of salt cedar and 2004 foliar volume (m3) of cottonwood in seven, 50 3
50-m plots cleared in 1993 on Bosque del Apache National Wildlife Refuge, New Mexico to restore riparian woodlands.
2004 cottonwood DBH. However, the slope was not
different from zero for the 1995 densities (r2 5 0.29,
model F1,6 5 2.02, P 5 0.21; cottonwood DBH 5
5.52 2 0.61[1995 density]) (Figure 5). The 2004
cottonwood density was related to the 2004 DBH
with a negative slope (r2 5 0.53, model F1,6 5 5.69,
P 5 0.06; cottonwood DBH 5 7.04 2 5.33 [2004
density]) (Figure 5).
Figure 3. Relationships between the 1995 total density of woody species (cottonwood, coyote willow, salt cedar, and seep
willow) and 2004 percent overstory of cottonwood (a) and between the 1995 total density of woody species (cottonwood,
coyote willow, salt cedar, and seep willow) and 2004 density of cottonwood (b) in seven, 50 3 50-m plots cleared in 1993 on
Bosque del Apache National Wildlife Refuge, New Mexico to restore riparian woodlands.
Taylor et al., RIPARIAN RESTORATION SUCCESS
1157
Figure 4. Patterns of 1995 and 2004 cottonwood densities and 2004 cottonwood diameter breast-height in seven, 50 3 50m plots cleared in 1993 on Bosque del Apache National Wildlife Refuge, New Mexico to restore riparian woodlands.
DISCUSSION
Based on all community measures (percent
composition, density, height, foliar volume, and
overstory coverage) restoration of native riparian
Figure 5. Relationships between 1995 (a) and 2004 (b)
cottonwood densities and 2004 cottonwood diameter
breast-height in seven, 50 3 50-m plots cleared in 1993
on Bosque del Apache National Wildlife Refuge, New
Mexico to restore riparian woodlands.
woody habitat was a success beyond the initial
establishment phase. Indeed, a decade after restoration efforts, cottonwood and seep willow comprised
nearly 60% of the community, more than five
times 1995 levels. Composition of coyote willow,
a disturbance species, had declined by 86% in nine
years, while salt cedar dropped 30%. Seep willow
density doubled over the decade since establishment,
which is indicative of their importance later in
restoration efforts because the increase in seep
willow was due to colonization after overbank
flooding events. Cottonwood was the tallest species
(.5 m) after 10 years, had the highest growth rate,
and had greatest overstory (70%) coverage. Although individual salt cedars were common (41%
composition) in the restored areas, its overstory
cover was ,10% and average height was less than
half that of cottonwood. The competitive abilities of
cottonwood over salt cedar were clearly apparent
beyond the seedling stage. These results should be
viewed within the context that maintenance river
flows occurred within the Rio Grande and drier sites
further removed from a river may show different
vegetation dynamics. Although we do not know the
exact duration, some minimal overbank flooding
likely occurred each year from 1995 to 1999 and in
2001.
The processes that were important in predicting
initial establishment of native species in cleared plots
(Taylor et al. 1999) were not important in predicting
longer term response. No initial abiotic characteristics were useful in predictions of cottonwood plant
1158
volume, canopy cover, or density a decade later.
Abiotic factors, particularly soil moisture, texture,
and elevation are important predictors of initial
establishment (Cooper et al. 1999, Taylor et al. 1999,
Shafroth et al. 2000), but once cottonwood is
established in those sites, other biotic factors such
as community composition become more valuable
to predict restoration response. It is likely that
ranges in elevation and soil characteristics were not
great enough in our plots to reflect post-establishment vegetation differences in the active river
floodplain. This is apparent in the low variability
of soil salinities observed among our plots (Taylor
et al. 1999). Although salinity is undoubtedly
important in seedling establishment, it was not
statistically selected as important when operating
within a narrow range of values (Taylor et al. 1999).
Cottonwood and salt cedar can establish in higher
salinity soils and lower ground water levels when the
soil surface is flooded to allow germination but
under higher salinity conditions cottonwood would
not likely survive as well (Sprenger et al. 2002,
Bhattacharjee et al. 2006).
We also did not measure ground-water depth,
which is important in riparian vegetation establishment and persistence (Stromberg and Patten 1990,
Rood et al. 1995, Scott et al. 1999, Shafroth et al.
2000). We do not feel, however, that this was an
important predictor due to the immediate proximity
of all plots to the river channel and the additional
overbank flooding that occurred. It is also possible
that the physical location of the plot, relative to
adjacent uncleared salt cedar plots and river
morphology, influenced overland flow, which may
have influenced subsequent vegetation response.
However, this binomial (cleared or uncleared salt
cedar) variable was difficult to include in our models
given random application of treatments (i.e., too few
replicates per variable).
The most important 1995 predictors of postestablishment cottonwood characteristics were total
woody species density and salt cedar density. As
total 1995 woody plant density increased, subsequent cottonwood density and overhead cover
increased. Much of the 1995 woody plant density
was made up of coyote willow. Coyote willow is
a relatively short-lived, early successional riparian
species. As coyote willow senesced, cottonwood
experienced a release in growth. Salt cedar density in
1995, as part of that 1995 total woody plant species,
negatively influenced 2004 cottonwood foliar volume but not density. Cottonwood is shadeintolerant (Johnson et al. 1976), and it increased in
height rather than volume (i.e., lateral branching)
when salt cedar densities were high. Thus, the initial
WETLANDS, Volume 26, No. 4, 2006
composition of all woody species is important in
subsequent riparian system response, and restoration biologists should examine the initial density and
community composition to determine if goals will be
met. Although more salt cedar plants will negatively
influence cottonwood volume, high initial densities
of coyote willow will encourage cottonwood growth
as the species disappears from the community. Data
do not suggest that coyote willow prevents germination of salt cedar, but it likely negatively
influences survival of first year salt cedar seedlings.
Therefore, low initial cottonwood seedling densities
may eventually mature into desirable communities
with beneficial transitional (e.g., coyote willow)
communities in the interim.
Densities of salt cedar and cottonwood in 2004
were negatively associated with plant height, which
was associated with foliar volume for these species.
Lower cottonwood densities would produce greater
individual tree volume in a shorter period, and this
was reflected in DBH. Sites that had the lowest
initial cottonwood seedling densities had individuals
with the greatest diameters. These individuals grow
rapidly to a greater height, dominating the canopy
relative to salt cedar. Therefore, the high mortality
of cottonwood seedlings that occurs during the first
and second years after establishment may not be
cause for concern and may actually provide thinning
that leads to stands with greater volume and larger
individual tree size (DBH). A restoration goal of
large cottonwood trees with a dominant canopy
should target lower densities of cottonwood in the
first few years within the mixture of established
native seedlings. If the goal of management is to
transition more slowly through the mid-age structure with a dense native community for certain
vertebrates, such as southwestern willow flycatcher
(Empidonax traillii Audubon), then high initial
coyote willow and cottonwood seedling densities
should be targeted.
Few studies have compared initial seedling
restoration efforts to subsequent community response. Most restoration efforts have concentrated
on seedling establishment of native riparian species
without considering the stand dynamics over time.
Others have related past events to current restoration through the use of aerial photographs speculating on potential initial establishment densities
and current success. This makes specific comparisons to our work difficult. Our study emphasizes
the importance of initial seedling densities of
important riparian species such as cottonwood in
determining the final goal of restoration (either long
term multi-canopy gallery forest or islands of faunal
habitats).
Taylor et al., RIPARIAN RESTORATION SUCCESS
If conditions favorable for the recruitment of
cottonwoods are created, such as moist, bare
substrates in late spring, cottonwoods not only are
able to dominate salt cedars during the initial
seedling phase but also continue to do so beyond
the sapling stage, thereby forming the dominant
canopy species. A dominant cottonwood canopy
should further limit the growth rate of salt cedar (in
terms of growing new stems) (Lesica and Miles
2001). As cottonwood size (DBH) and percent
overstory are related to its density, future restoration efforts should examine densities of cottonwood
that will cater to site-specific restoration goals.
As mentioned in several studies, basic mechanical
removal of salt cedar and subsequent overbank
flooding, simulated or real, timed with native seedfall, will provide long-term restoration of cottonwood riparian communities even with low (,2/m2)
initial seedling densities of cottonwood (e.g., Taylor
et al. 1999, Sher et al. 2002, Sprenger et al. 2002).
However, the stochasticity of initial establishment
events will provide a level of uncertainty regarding
the composition of the restored community. This
was apparent in the variation and independence
among the restoration plots. The timing of overbank
flooding may favor one native species over another
and the distance of native source trees influences
dispersal to flooded sites. Restoration ecologists
may be able to influence this somewhat by using
locally cut branches containing ripe seed. Branches
can be placed along the edges of receding waters
enhancing species specific germination (Sprenger et
al. 2002). By favoring a certain species in the placed
branches, composition of the community can be
manipulated. For example, the coyote willow
density in the Sprenger et al. (2002) study was
greater than in our study where they used locally cut
coyote willow branches.
ACKNOWLEDGMENTS
Funding for field work was provided by the U.S.
Fish and Wildlife Service, Bosque del Apache NWR
and the Bosque Initiative Group (C. Abeyta). We
also thank Turner Enterprises (C. Kruse, T.
Waddell) for providing financial support. We thank
N. Umbreit, C. McNerney, S. Najera, D. Leal, C.
Randall, K. Estrada, G. DelloRusso, M. Oldham, J.
Savery, and C. Lee for assistance and logistical
support. L. Smith was supported by the Caesar
Kleberg Foundation for Wildlife Conservation. We
thank G. DelloRusso, J. Bhattacharjee, A. Sher, P.
Shafroth, and two anonymous referees for comments on the manuscript. This is manuscript number
T-9-1114 of Texas Tech University.
1159
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Manuscript received 11 October 2005; revisions received 17 April
2006 and 21 September 2006; accepted 21 September 2006.