1 running head: climate and exotics threaten cottonwoods · an exotic species is impacting native...
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Running Head: Climate and Exotics Threaten Cottonwoods 1
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Applying Climate Predictions and Spatial Modelling to Prioritizing Riparian Habitat
Restoration
A. R. Gitlin1 and T. G. Whitham, Dept. of Biological Sciences, Northern Arizona Univ., P.O. Box
5640, Flagstaff, AZ 86011-5640, USA
1Corresponding Author: Alicyn R. Gitlin; [email protected], fax (928) 523-7500 12
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Abstract. Organizations providing financing for conservation need tools to empirically
prioritize restoration projects. To maintain diverse habitat and migration corridors, organizations
need to incorporate climatic variation and competitive interactions with exotic species into long-
term management plans. We combined field studies with spatial analyses of native cottonwood
trees, (Populus fremontii, P. deltoides, P. angustifolia), their naturally occurring hybrids, and the
invasive exotic tamarisk tree (Tamarix spp.). We found six major patterns. 1. During adverse
drought conditions that are predicted to intensify in the future, the mortality of the parental
species was 3-4 times greater than their naturally occurring hybrids. 2. Sixty-two percent of the
variation in the mortality of broadleaf cottonwoods (P. fremontii, P. deltoides) was associated
with the density of tamarisk. 3. Our GARP model accurately predicted the distribution of upper-
and lower-elevation cottonwood species and their overlap was a significant predictor of hybrid
tree locations, as verified by three independent validations. 4. Broadleaf cottonwoods currently
have a greater potential niche than other cottonwoods (narrowleaf (P. angustifolia) and hybrids),
and tolerate the greatest environmental variation, but can become rare under extended extreme
drought conditions. 5. Temperature and precipitation changes will have opposing effects on the
two cottonwood species we studied. Lowland broadleaf cottonwoods will be highly vulnerable
to drier conditions, and upland narrowleaf cottonwoods will be vulnerable to temperature
increases. Populations of narrowleaf cottonwoods and hybrids will be more drought resilient, as
will the exotic tamarisk. 6. Tamarisk will increase cottonwood forest fragmentation. These
finding have 3 major implications. 1. Our method identifies riparian areas in the southwest U.S.
that are most drought sensitive and most resilient, which provides a basis for prioritizing
management. 2. Effective conservation practices may require attention to finer scales than
species-level protections, i.e., conserving genetic diversity and more drought tolerant naturally
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occurring hybrids. 3. An exotic species is impacting native riparian forests in a manner similar
to climate change. We argue that proactive management, coordinated across a large region,
should maintain climate refugia in the most resilient areas and mitigate compounding pressures
in the most vulnerable areas to preserve biodiversity through future climate fluctuations.
Key words: drought; extreme events; dominant species; riparian habitats; hybrids;
exotic species; spatial modelling; restoration; cottonwood (Populus spp.); tamarisk (Tamarix
spp.).
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INTRODUCTION 1
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Studies published in the last decade argue that anthropogenic influences on western
riparian systems are threatening cottonwoods (Populus spp.), which, as dominant species,
structure the riparian community by creating locally stable conditions for other species, and
modulate and stabilize fundamental ecosystem processes (Dayton 1972, Ellison et al. 2005,
Whitham et al. 2006). Flow alteration, water depletion, bank stabilization, water salinization,
grazing, mining, pollution, exotic species, and land development have all been shown to
fragment riparian forests and prevent seedling recruitment (Rood and Mahoney 1990, Howe and
Knopf 1991, Busch and Smith 1995, Lejeune et al. 1996, Scott et al. 1999, Scott et al. 2000,
Lytle and Merritt 2004, Rowland et al. 2004, Friedman et al. 2005, Lite and Stromberg 2005,
Pataki et al. 2005, Williams and Cooper 2005). Management actions are inhibited by water
supply shortages, land ownership and access, and budgetary restrictions, while climate change
and extreme weather events add uncertainty to conservation planning. Drought, a recurring
event throughout the west that is predicted to intensify, increases water needs for both human
welfare and conservation. In combination, all of these factors are contributing to the demise of
riparian habitats, which are classified as an endangered habitat type (Noss et al. 1995).
Numerous lines of evidence dictate that land managers in the southwest U.S. should
prepare for repeated extended droughts. The three anomolous dry periods of the past 100 years
(1893-1904, 1942-1977, 1999-present) are representative of similar events that occurred here 11
times during the preceding 350 years (Fye et al. 2003), but new research indicates a drier future,
more severe than has occurred during recorded history on this continent (Seager et al. 2007). In
its Third Assessment Report, the International Panel on Climate Change (IPCC) predicted that a
doubling of pre-industrial CO2 levels would cause a 1.5 to 4.5˚C increase in temperature (see
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Hare and Meinshausen 2006), and working groups preparing the IPCC’s Fourth Assessment
Report predict global temperatures will increase between 1.8 and 4.0˚C by 2100 (IPCC 2007a).
Increasing temperature was implicated as the cause of a 400-year “megadrought” period in the
western US between 900 and 1300 ybp during which drought intensity and duration surpassed
anything recorded in North America during the last century, and warming will intensify future
midcontinental droughts by increasing summer evapotranspiration and reducing soil moisture
(Cook et al. 2004, Cook et al. 2007). Ice, marine, and other paleoclimatic records demonstrate
that global temperatures are synchronous at 900-1100 year intervals, placing us in a period that
should mimic global temperatures during the 400-year drought, although the 20
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th century
warming trend is unlike anything seen in the last 14,000 years (Cook et al. 2004, Viau et al.
2006).
Temperature increases will aggravate drought impacts on vegetation and simultaneously
cause greater per capita anthropogenic water use (IPCC 2001, IPCC 2007b, Breshears et al.
2005). Anthropogenic water consumption impacts vegetation in a manner similar to drought,
and recent southwestern population expansions have led to unprecedented anthropogenic water
consumption (Scott et al. 2005). The signature of the current climatic and anthropogenic
groundwater drawdown may not appear for many years: decreased precipitation 50 years ago in
the upper Rio Grande watershed is only now affecting the lowland riverine water table and
surface flow (National Science Foundation 2004). The large scale impedance of water flow
caused by dams, canals, and groundwater withdrawals may cause similar long-term damage to
hydrologic systems. Therefore, the most essential sites for protecting instream flow must be
identified, and necessary conservation water rights obtained, before remaining water resources
are claimed by dueling cities and states (Reisner 1993).
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Cottonwoods exhibit life history traits that enable populations to persist through short-
term moisture fluctuations, but the gradual replacement of native riparian forests by invasive
exotic trees such as tamarisk (Tamarix spp.) will impede post drought recovery by filling the
spaces cottonwoods need to germinate (Busch and Smith 1995, Friedman et al. 2005, Birken and
Cooper 2006). Tamarisk is an aggressive invader of floodplains (Birken and Cooper 2006) and
is an indicator of high salinity and variable groundwater levels (Lite and Stromberg 2005, Pataki
et al. 2005). By inhabiting drought stressed areas, it should also act as an indicator of locations
where broadleaf cottonwoods (e.g., P. fremontii and P. deltoides) are most likely to suffer
drought mortality (Gitlin et al. 2006). If tamarisk populations are higher in areas of high
cottonwood mortality, the current drought will signify a major dominance shift in tamarisk
infested areas, and will have high biodiversity implications (Shafroth et al. 2005).
This study aims to predict the future of riparian cottonwood forests in the southwest as
water availability fluctuates or decreases and temperature increases. We combine field
observations of cottonwood population dynamics during an ongoing multi-year drought with
computer-generated spatial models of potential cottonwood niches under a variety of decreased
moisture and increased temperature regimes. Based on patterns of cottonwood mortality and
competitive interactions that we observed during the drought, we superimposed limitations on
the distributions of the species to create a comprehensive model of what cottonwood forests will
look like in the future and identify drought susceptible and drought tolerant regions. We then
infer conservation suggestions based on patterns shown by the model.
Three major hypotheses were addressed. First, different species of cottonwoods (P.
fremontii, P. deltoids, P. angustifolia) and their naturally occurring hybrid offspring would have
different levels of mortality and reproduction during drought, and tamarisk cover would be a
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significant predictor of low elevation cottonwood mortality. Second, in light of previous models
of vegetation range shifts during climate change, we predicted that drought would act to constrict
the edges of the potential niches of cottonwood species and their hybrids, and that smaller scale
factors would act within riparian forests to cause forest fragmentation. Third, higher
temperatures would cause greater range reduction than decreased precipitation without a
temperature change. Although we acknowledge that other climate scenarios such as changing
cloud cover, altered seasonality, and depleted snow pack will also affect these systems, answers
to these hypotheses are important because they combine two major interacting factors (climate
and exotics) to develop a method of prioritizing conservation actions in the American West.
METHODS
Study area and climate trends.---We recorded locations of cottonwood forests throughout the
southwestern states of Utah, Colorado, New Mexico and Arizona, USA, during the summers of
2003 and 2004 (Fig. 1). The southwestern states are mostly arid (Peel et al. 2007) with extreme
temperature ranges and high spatial and temporal precipitation variability. Elevation ranges
from < 30 m to > 4300 m above mean sea level, and is punctuated by narrow canyons and steep
mountain ranges. Yearly precipitation in the region is more aptly characterized by its range,
which varies between 8 cm/yr in dry lowlands and > 60 cm/yr in the mountains, than by its mean
of 22.86 cm/yr (Lenart 2003, and data provided by the Western Regional Climate Center,
www.wrcc.dri.edu, accessed July 2006). 19
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The geology of the region causes all overstory vegetation, including riparian trees, to
depend upon sufficient precipitation levels for survival. Geologic formations are primarily
deeply faulted sedimentary and volcanic rocks. Nearly all southwestern river reaches are
intermittent or ephemeral, primarily influent, drainages, and groundwater depths outside of
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riparian areas are usually too deep to be accessed by tree roots (Robson and Banta 1995). Lack
of precipitation, as well as anthropogenic stressors such as groundwater pumping and flow
diversions, increase the depth to alluvial groundwater and create drought stress for the native
phreatophytes, Populus and Salix spp. (Scott et al. 2005). Long-term drought and dewatering for
human needs have also been associated with a change in dominance from native trees to the
invasive exotic tamarisk (Tamarix spp.) in many areas, causing tamarisk to become second only
to cottonwood in riparian ground cover across the American West (Cleverly et al. 1997,
Friedman et al. 2005, Birken and Cooper 2006).
Cottonwood population dynamics.---Two taxonomic sections of Populus grow in the southwest,
and intersectional hybridization is common (Eckenwalder 1984a, Keim et al. 1989). Narrowleaf
cottonwoods (P. angustifolia) in section Tacamahaca dominate at higher elevations; the
broadleaf plains and Fremont cottonwoods (P. deltoides, P. fremontii) dominate at lower
elevations. The overlap zone between the two sections is often dominated by hybrids (Floate and
Whitham 1995, Keim et al. 1989, Martinsen et al. 2001). We refer to the parental species, F1 and
backcross hybrids as cross types. Because plains and Fremont cottonwoods are closely related
and morphologically similar (Eckenwalder 1984b), and the precise boundaries of their
distribution and extent of interbreeding is currently unknown, hereafter we refer to them
collectively as broadleaf cottonwoods (see also Bangert et al. 2005). Narrowleaf cottonwoods
and their hybrids both reproduce asexually (Schweitzer et al. 2002). Because broadleaf
cottonwoods require specific multiyear sequences of flow parameters for seedling recruitment,
human water use and/or altered climate patterns change flow patterns to threaten cottonwood
establishment (Mahoney and Rood 1998, Lytle and Merritt 2004).
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Stand selection.---We recorded the locations, cottonwood morphotypes, and exotic tree species
present at over 100 riparian forests, and input into our model the positions of 95 broadleaf and 52
narrowleaf cottonwood stands. We used Trimble Pathfinder Global Positioning System (GPS)
units to capture tree locations. We then chose a subset of these sites and added 11 stands of F
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type, or first generation hybrid crosses as sampling locations for our population studies.
F1 type hybrids have a distinctive leaf shape that is unlike either parent, making them
clearly recognizable (Martinsen et al. 2001). Studies that have identified trees using both
morphological traits and molecular markers confirm that the distinctive leaf morphology of the
F1 type hybrids is generally consistent with the classification of trees based upon 35 species
specific molecular markers (Floate and Whitham 1995, Martinsen et al. 2001). Though
narrowleaf trees cannot be distinguished from complex backcross hybrids in the field, trees with
the narrowleaf morphology found outside the hybrid zone tend to be “pure” (Martinsen et al.
2001). Only narrowleaf cottonwoods found in stands free of broadleaf and F1 hybrids were
included in studies comparing parent species with hybrids. All stands containing trees with
narrowleaf morphology were input into the narrowleaf spatial model.
Mortality and reproduction during drought.---To determine whether cottonwoods and their
hybrids experienced different levels of drought mortality, surveys were conducted on a subset of
stands located on and around a large (~210,000 km2) and topographically variable feature called
the Colorado Plateau. All of the stands were in low order streams because they have less human
impacts that could confound the effects of climate on tree survival. Survey sites were chosen for
their legal and physical accessibility. Accessible cottonwood stands and their associated
floodplain habitats varied in size, so the number of trees observed in each stand was held
constant at 30 trees. The first 30 standing trees encountered when walking transects
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perpendicular to the river’s edge were counted and identified based on leaf morphology.
Individuals were defined as being > 2 m tall and included all ramets that connected to the main
trunk above the ground level. Death was defined as the complete mortality of a single
individual, and a tree was considered live if there was evidence of basal resprouting from the
trunk. The taxonomic status of dead trees was based upon dried leaves, tree structure and
placement in relation to other trees. When a tree could not be clearly identified, it was not
included in the survey. In order to capture the effect of the current drought on tree stands, only
recently dead trees were counted (i.e., standing trees with intact bark and small diameter
branches present). Trees were not counted if the cause of death appeared to be from mammalian
herbivory, woodcutting or fire. The number of dead trees was compared across cross types. All
data was categorical, and compared using χ
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2 tests.
In 2003, 46 stands were chosen, including 20 narrowleaf stands, 15 broadleaf stands, and
11 stands in the hybrid zone where upper and lower elevation species are both found along with
their F1 type hybrids. As F1 type hybrids are less common, their sample size is smaller than their
parent species (narrowleaf n = 628 trees; F1 type n = 100; broadleaf n = 574). Because some
rivers have missing pure or hybrid stands, and to be sure that we weren’t counting trees in outlier
areas of their distributions, we visited three rivers with intact zones of all three cross types in
2004. Fifty trees each of narrowleaf, F1 type and broadleaf cottonwoods were counted along
each river, in stands dominated by the cross type being counted. The number of dead trees of
each cross type was compared with a χ2 test.
To determine whether tamarisk cover correlated with broadleaf cottonwood mortality
across the Colorado Plateau, we measured tamarisk cover when it was encountered. Tamarisk
cover was determined along three 50-m transects at each of 13 sites (Gitlin et al. 2006). We used
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simple linear regression to test for a correlation between tamarisk cover and broadleaf mortality.
Statistical analyses, unless noted otherwise, were performed in JMP-IN 5.1 (SAS Institute,
2003).
The ability of each cross type to reproduce during drought was observed in 2003.
Presence or absence of juvenile trees (< 2 m tall, not connected to another trunk above ground
level) within the tree cluster counted in the mortality study was noted for 40 stands (narrowleaf
stands n = 18; hybrid stands n = 11; broadleaf stands n = 11). Note that the number of stands is
unequal because many rivers do not have all three tree cross types present. The number of stands
with juvenile trees present was compared across cross types with a χ2 test.
Spatial modelling and analyses
Current distributions.---We modelled potential niches for narrowleaf and broadleaf cottonwoods
using the desktop version of the Genetic Algorithm for Rule-set Prediction (Scachetti-Pereira,
Desktop GARP, University of Kansas Center for Research 2001). GARP uses presence-only
data, has been shown to reliably predict the distributions of species capable of dispersion across
large distances, and has been successfully applied to riverine species such as freshwater fish
(Iguchi et al. 2004, McNyset 2005), zebra mussels (Dreissena polymorpha -Drake and
Bossenbroek 2004) and the invasive exotic riparian tree, Russian olive (Elaeagnus angustifolia
L. –Peterson et al. 2003). GARP compares the pixel values of known occurrence locations to a
random sample of all other pixels and attempts to create a rule to explain where the species is
found. It then runs through a specified number of iterations where it modifies the rule in an
evolutionary process, attempting to improve its accuracy. In this way, the program allows
different combinations of rules to be applied in different regions. After each step, the program
determines whether the new rule improves predictive accuracy, and thereby whether it should
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become part of the final model. Accuracy can be determined as the proportion of training points
that were not predicted (intrinsic omission error), or a percent of input points can be set aside
allowing both intrinsic and extrinsic omission errors to be determined. GARP will continue to
modify the rule for the specified number of iterations, or until the improvement from one step to
the next is less than a predetermined convergence value. The program repeats this process to
produce a user-specified number of maps. Based upon work by Stockwell and Peterson (2002),
determining that GARP produces the best fitting spatial models with a minimum of 50 training
points, and Wiley et al. (2003), who found that highly predictive models could be produced
without reserving points for extrinsic data testing, we decided to use 100% of our observation
points for model creation.
The purpose of this project was to explore the effects of meteorological drought on
cottonwood trees. Meteorological drought specifically refers to “a prolonged and abnormal
moisture deficiency” and “is dependent on the average climate of the area and on the prevailing
meteorological conditions both during and preceding the month or period in question” (Palmer
1965). Meteorological drought can be rapidly and objectively measured as a deviation from
normal precipitation, as opposed to other drought definitions such as agricultural drought or
hydrological drought, which interpret the delayed and complicated effects of meteorological
drought on land use and water availability. Because we were focused on predicting the effects of
changing temperature and precipitation, we limited the raster inputs to mean growing season
(March-October) precipitation, maximum and minimum temperature, and elevation. We
obtained 1-km resolution ANUSPLIN-interpolated rasters from the Climate Atlas of North
America - Western Region (CANA-W) (available from the Northern Arizona University
Department of Geography, Planning and Recreation at
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http://www.geog.nau.edu/global_change/climate_surfaces.html). The accuracy of this model
may be improved in the future with hydrologic drought predictions, which could be created by
linking watershed geology to the climatic sensitivity of specific streams and aquifers.
Unfortunately, high resolution groundwater data and linked precipitation and stream gauges are
lacking in much of the region.
Several similar methods of creating habitat suitability maps from GARP data have been
tested with satisfactory results (Anderson et al. 2003, Wiley et al. 2003, Iguchi et al. 2004). Like
Wiley et al. (2003), we used 100% of our input points for model creation, but we otherwise
followed the method outlined in Iguchi et al. (2004): 1) we produced 1000 output maps, each
created with 1000 potential iterations and a convergence limit of 0.01; 2) we determined the
median percentage of predicted niche area from all models with intrinsic omission errors < 5%;
3) we chose the 10 models that predicted niche areas closest to the median; 4) we summed the
final 10 maps so that each pixel gave a probability for habitat suitability by indicating the
number of models that predicted presence in each pixel. This method produces a surface of
potential niche probabilities, rather than simply outlining the current distribution.
We then masked all non-riparian areas out of the summed maps to create a more accurate
visual representation of the cottonwood distributional range, and to use as input for landscape
analyses. We performed the mask after the predictions were created, rather than using river
locations as an input to the GARP modelling process. This is because the modelling software
would inevitably identify river locations as the most accurate predictor of cottonwood location,
incorrectly predicting trees anywhere there was a river and ignoring all other inputs.
Tamarisk distribution was derived from the National Institute of Invasive Species Science
(NIISS 2006, data available at http://squall.nrel.colostate.edu/cwis438/websites/niiss/home.php, 23
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accessed June 2006) and from locations recorded during this study. Due to its rapid dispersive
capability, tamarisk probably occupies all or most of its niche in warm regions of the U.S.
(Friedman et al. 2005). Therefore, its actual distribution was used rather than creating a
predictive model.
Validation of current distributions.---Because GARP uses presence-only data and creates a
potential niche map rather than mapping realized distribution, traditional validation measures
such as the kappa statistic are not applicable (see Iguchi et al. 2004). Therefore, we applied three
validation measures: one at the scale of the entire study area, one at a regional scale, and one at
the scale of a single river. At the largest extent, we tested whether GARP accurately predicted
the upper elevational boundary of broadleaf cottonwoods and the lower elevational boundary of
narrowleaf cottonwoods by overlaying the locations of 25 F1-type hybrids on the prediction
surface and determining how many were found within the area that GARP predicted to contain
both parent species (the hybrid zone). A binomial probability test was performed with Vassar
College’s online statistical package (http://faculty.vassar.edu/lowry/VassarStats.html, accessed
August 2006) to compare the successful predictions with the proportion of pixels predicted to
occur in the overlap zone.
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A second qualitative validation was performed by driving a 500-km route in southern
Utah that crossed several riparian areas and had large elevational gradients, where no previous
data collection had occurred. We recorded cottonwood locations along the transect and
compared species locations with GARP’s niche predictions. At the scale of a single river, we
qualitatively assessed the model’s accuracy by overlaying the locations of trees of known
genotype from the Weber River in Utah on top of it (Martinsen 2001). This data was derived
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from a separate study (Martinsen 2001), and the trees were chosen independently of any location
data input into the GARP model.
Niche differentiation.---To determine whether cottonwood species, their hybrids, and tamarisk
are exploiting different niches within their overlapping distributions, we extracted mean growing
season precipitation, temperature extremes and elevation from raster grids at the pixels
corresponding to the location of each stand. We compared these parameters, first using each tree
species’ entire sample size and then with a subset limited to the hybrid zone where narrowleaf
and broadleaf cottonwood species were predicted to co-occur (Anderson et al. 2002).
Environmental parameters were compared at both scales with Tukey’s HSD means comparison
tests. To test whether one cottonwood species is dominating in areas suitable for both, we
performed a χ2 test on location (in or out of niche overlap) by cottonwood species.
Drought-altered distributions.---GARP enables the user to project the distribution of potential
niches on surfaces that differ from the input rasters. These projections are created
simultaneously with the original distribution maps, thereby ensuring that the same rules are
applied to all. To predict the potential niche distribution of broadleaf cottonwoods if drought
becomes the average condition for a number of years, we created raster datasets representing
decreased precipitation with and without increased temperatures. Since the CANA-W surfaces
that were used to represent current climate conditions contained a 30-year window of averaged
data, the drought surfaces we created are representative of a multidecadal drought period. We
first created a set of rasters simulating 50% of average growing season precipitation levels,
which we determined to be representative of the driest years of the last century (historic climate
information available from the National Climatic Data Center,
http://www.ncdc.noaa.gov/oa/climate/research/2002/dec/st002dv00pcp200212.html, accessed
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May 2007). We then created a second set of rasters simulating a decrease to 25% of average
precipitation to represent megadrought conditions so that we could identify the most drought
resilient areas on the landscape. We modelled increased temperature scenarios based on values
predicted by the IPCC under doubled CO
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2 concentrations (see Hare and Meinshausen 2006).
The moderate temperature increase scenario was 1.5˚C above average, approximating current
drought conditions and representing the lower range of temperature rise predicted by the IPCC
(Breshears et al. 2005, Hare and Meinshausen 2006, IPCC 2007a). We selected + 4˚C as the
severe warming scenario, based on the upper range of temperature rise predicted by the IPCC
(Hare and Meinshausen 2006, IPCC2007a).
Population fragmentation with and without tamarisk.---In order to predict the ability of trees to
recolonize their potential niche after drought, we needed to understand smaller-scale patch
dynamics within the cottonwood species’ distributions. Percent landscape covered, number of
patches, and distance between patches will affect gene flow, susceptibility to exotic colonization,
and seed availability. As seeds remain viable only long enough to travel < 3 km from parent
trees, we were interested in the percentage of patches that were within 6 km of another patch,
thereby enabling the patches to reconnect during the first post-drought recruitment event (Imbert
and Lefèvre 2003). Seed recruitment events generally occur less than once per decade (Lytle and
Merritt 2004), so patches further apart than 6 km would be unlikely to rapidly reconnect after
extended drought without active management. Recovery can also be inhibited by spatial
competition. If other spatially competitive trees such as tamarisk do not perish, the sunny bare
soil patches needed for cottonwood seed germination will be limited, inhibiting re-establishment
after drought.
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We analyzed our models with FRAGSTATS 3.3 build #4 (McGarigal et al., Univ. of
Massachusetts, Amherst 2002). We converted all non-riparian areas to exterior background
pixels so the total landscape area was restricted to riparian corridors. To explore the impact of
drought-induced tamarisk dominance on cottonwood forest fragmentation, we evaluated models
with and without current tamarisk locations subtracted from the cottonwood predictions. We
measured the class metrics of percent landscape area, number of patches, and connectivity under
each climate scenario. Connectivity was defined with a 6 km threshold distance.
Broadleaf dominant refugia.---After completing the other analyses outlined here, we concluded
that broadleaf cottonwoods would likely become co-dominant in many areas, and that western
broadleaf dominated forests would become rare if drought persisted long-term. Since riparian
trees are restricted to linear systems, upland migration can be limited or blocked by a dense
population of another dominant tree. To predict the spatial extent of broadleaf-dominated
cottonwood forests under the harshest conditions, we masked out areas predicted to overlap with
narrowleaf and tamarisk. We then determined the areas of remaining broadleaf habitat as the
likely distribution of broadleaf dominated stands under megadrought conditions. These are the
places we consider most important for preservation of this increasingly rare habitat type.
Finally, we created “drought vulnerability surfaces” by summing all of the potential niche
probability maps into a single file. We colored the areas that have a low likelihood of remaining
as potential niche space red to indicate a high vulnerability to extended drought, and areas that
have a high likelihood of remaining as potential niche space were colored blue. This product
presents the information created in this paper in a format that is easy for land managers to
interpret and apply toward prioritizing conservation projects.
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Mortality and reproduction during drought.---Although mortality levels of all three cross types
differed in 2003 and 2004, hybrids survived significantly better than either of their parental
species. In 2003, 4% of F1 type hybrids died, which was ~1/3 the mortality of narrowleaf and
~1/4 the mortality of broadleaf cottonwoods (Fig. 2a). The same patterns emerged in 2004; F1
type hybrid mortality remained at 4%, while we recorded 23.3% of narrowleaf and 11.3% of
broadleaf cottonwoods dead in that year (Fig. 2b).
Large-scale mortality patterns are not always reflected in individual rivers. Relative
survival of parental species varied among the sampled rivers, but hybrids showed the highest
average persistence throughout the region (Fig. 2b, c). F1 type mortality remained at 4% for both
years and never exceeded 8% for a single stand, while mortality for pure species ranged between
0 and > 50% for individual stands.
Increased tamarisk cover in individual stands predicted 62% of the variation in broadleaf
cottonwood mortality during the drought (Fig. 3). Broadleaf cottonwoods on the Colorado
Plateau died off in the areas most infested with an exotic dominant tree.
Reproduction during the drought differed between pure stands (Fig. 2d). Nearly twice as
many narrowleaf stands produced new trees during the drought than broadleaf stands. The
percentage of hybrid stands containing young trees was intermediate between the pure zones.
These data provide important performance variables for modelling cottonwood responses to
future drought and provide insight into each cross type’s ability to remain in place, which will be
important for preventing dominance shifts and reducing exotic invasion.
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Current distributions.---Across four states (Arizona, New Mexico, Utah, Colorado), 78.4% of
the region was identified as suitable habitat for broadleaf cottonwoods and 63.0% was identified
as suitable for narrowleaf cottonwoods (Fig. 4a). Both broadleaf and narrowleaf species share
overlap at their elevational boundaries to form a hybrid zone that occupies 46.1% of the region.
Because cottonwoods are riparian species, we adjusted these estimates to include only riparian
corridors and found that 6.5% of the four states was classified as suitable habitat for broadleaf
cottonwoods, 5.2% for narrowleaf, and 3.8% for their naturally occurring hybrids (Fig. 4b).
Validation of current distributions.---The shared potential niche was a significant predictor of F1
type hybrid locations (z = 2.93, P = 0.001). Of 25 F1 type hybrids, 19 were found within the
overlap area and all of the unpredicted locations were within just 7.5 km (< 8 pixels) of the
predicted hybrid zone. Although there is no evidence that hybrid germination is limited to this
overlap zone, the correlation of hybrids and the predicted overlap area indicates that the
boundaries of the parent species distributions are accurate.
Of the observations along the southern Utah transect, 93% were accurately predicted.
Thirteen broadleaf locations, eight narrowleaf locations, and four F1 locations were correctly
identified by the model. One broadleaf cottonwood and one F1 type tree were found outside their
predicted ranges, though both were < 1.2 km (< 2 pixels) from their predicted range boundaries.
A 10-km wide swath at very high elevation and populated entirely by quaking aspen (P.
tremuloides) was predicted as narrowleaf habitat, indicating that there may be some
overprediction of narrowleaf habitat at their upper elevational limit in that area.
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The GARP model also correctly predicted the locations of 1020 out of 1038 genetically
identified cross types along the Weber River in Utah. Eighteen hybrids fell < 8 km (< 8 pixels)
outside the upper limit of the predicted hybrid zone.
Niche differentiation.---Across the region, the three cottonwood cross types and tamarisk are
divided into three distinct elevational ranges, though they are positioned according to climate
parameters within their elevations (Fig. 5, left side). The lowland broadleaf cottonwoods and
tamarisk do not differ in elevational range or maximum temperature tolerance, though tamarisk
occupies the driest areas and areas with moderate minimum growing season temperatures. F1
type hybrids and narrowleaf trees are elevationally differentiated, but occur in areas with similar
precipitation and minimum temperatures. Narrowleaf trees are able to grow in areas with lower
maximum temperatures than F1 type hybrids.
Broadleaf cottonwoods showed the most plasticity in all climate tolerances. While other
cross types and tamarisk all had a difference of between 35˚ and 36˚ C between the average
lowest and highest temperatures tolerated, broadleaf cottonwoods occupied niches with
temperature extremes spanning 42˚. Mean precipitation at sites occupied by broadleaf
cottonwoods ranged from 13.25 cm to 48.81 cm, exposing some broadleaf cottonwoods to lower
precipitation than the other cross types and tamarisk, while only narrowleaf cottonwoods grew in
areas with higher precipitation. Broadleaf cottonwoods were found along a 1973-m elevational
gradient, surpassing the differences in elevational extremes of the other trees by over 400m.
Climate also influences tree arrangement within the predicted hybrid zone, although there
is less differentiation between the niches (Fig. 5, right side). Broadleaf cottonwoods and
tamarisk occur at similar elevations and maximum temperatures, differing significantly from
narrowleaf cottonwoods, but not from F1 hybrids. Again, tamarisk occupied the driest areas.
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There was no differentiation in minimum temperature tolerance within the hybrid zone. Neither
parent species dominated in the overlap area.
Drought-altered distributions.---Our model suggests that cottonwoods are highly resilient to the
type of long-term drought that periodically occurs in the southwest. However, greater changes in
precipitation and temperature will affect these closely related species in very different ways
Extreme reductions in precipitation will reduce the potential niche of lowland broadleaf
cottonwoods, while temperature increases will have a greater negative impact on upland
narrowleaf cottonwoods. A 50% reduction in average precipitation, which is representative of
the most severe single and multiyear drought events during the last century, removed broadleaf
cottonwoods from 10.3% of their potential niche within riparian corridors, while narrowleaf
cottonwoods lost 3.4% of their riparian coverage. A 75% reduction in precipitation, which
would be more severe than any recent drought events in this region, removed 76.5% of the
broadleaf niche and 20.0% of the narrowleaf niche from riparian areas. Under the 75% reduction
scenario, broadleaf cottonwoods became limited to 18.4% of the riparian landscape while 51.6%
of riparian areas remained suitable for narrowleaf cottonwoods.
Temperature increases impacted the cottonwood cross types in opposite ways. At the
50% precipitation level, a 4˚C temperature increase enabled broadleaf cottonwoods to return to
within 1% of their predrought coverage area by enabling upslope migration. Temperature had
little effect on broadleaf cottonwoods subject to a 75% precipitation reduction, changing the
amount of coverage by < 1%. The same temperature increases caused the higher elevation
narrowleaf cottonwoods, which have a limited amount of upslope land available to them, to lose
an additional 17.5% - 32.0% of their potential niche space. Even with a greater sensitivity to
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rising temperatures, narrowleaf cottonwoods showed greater tolerance to drought. Under the
most severe conditions, narrowleaf habitat still covered 40.3% of riparian corridors.
According to the model, changes in parent species distributions will reduce the size of
their overlap, decreasing the area of the hybrid zone. Under the 50% precipitation scenario, the
hybrid zone lost 9.6% of its area; under the 25% precipitation scenario, the hybrid zone lost
77.5% of its area from riparian corridors. Rising temperatures reduced the size of the hybrid
zone more than drought alone. Under the most severe conditions modelled, the hybrid zone was
reduced to 16.0% of its original size.
Population fragmentation with and without tamarisk.---When we modelled a shift to tamarisk
dominance in areas already infested by this invasive exotic tree, it decreased the total potential
niche area, divided that area into a greater number of patches, and increased the distance between
patches of the cottonwood cross types under all but one climate scenario in the model (Figs. 7,
8). The exception was broadleaf cottonwoods at 25% of average precipitation and + 4˚C, where
tamarisk decreased the total number of patches but had no effect on potential niche area or patch
connectivity. Much of the loss occurred along mainstem river channels and larger tributaries
(Fig. 8).
In all cases where tamarisk increased habitat fragmentation, its effect on number of
patches was similar to or exceeded the effects of drought. For example, a 1.5˚C temperature
increase at 50% of average precipitation divided broadleaf cottonwood forests into 682 patches,
more than 2X the 295 patches produced by the model under current climatic conditions. When
tamarisk locations were subtracted from the modelled locations, the same drought conditions
created 1276 patches, over 4X more than the model under current conditions. The percent of
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patches within 6km of another patch fell from 50.5% under current conditions to 35.5% under
the drought scenario, and then to 14.3% when tamarisk was incorporated in the model.
While drought caused little or no change to the number of narrowleaf patches, and
actually increased the percent of patches within 6km of another patch, tamarisk increased the
number of individual patches and decreased narrowleaf forest connectivity under all scenarios.
Tamarisk divided narrowleaf cottonwoods into 69.6% to 89.1% more patches than climate alone,
and reduced connectivity by 3.5%-21.2%.
Broadleaf dominant refugia.---Very dry conditions will have a far worse impact on the range
size of southwestern broadleaf cottonwoods than on narrowleaf and hybrid cottonwoods (Figs. 6,
7). Areas west of the Rocky Mountains will see greater range reductions than areas to the east.
If severe drought dominates over the long-term, broadleaf cottonwoods will be limited to refugia
at higher elevations and the more mesic plains of eastern Colorado and New Mexico (Fig. 9).
Although some new potential niche spaces will become available to broadleaf cottonwoods if the
temperatures rise, we found 100% overlap of these new niches with narrowleaf cottonwood,
even under the harshest conditions modelled. We do not predict that broadleaf cottonwoods will
become dominant in areas where they must compete for space with narrowleaf cottonwoods and
hybrids. Populations in smaller tributaries will probably experience greater longevity than those
along larger rivers. Lowland areas such as the lower Colorado River region will be the first
affected by decreases in precipitation and increases in temperature. The lower Colorado River is
already dominated by tamarisk, as are most of the largest river corridors in the study area (Fig.
8).
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Effects of drought on closely related, hybridizing species.---Previous research has shown that
individual species will respond differently to climate perturbations, but this study demonstrates
different responses within closely related species and their hybrids. Our model suggests that low
elevation broadleaf cottonwoods will be extremely sensitive to drought conditions that exceed
the norms of past centuries. Broadleaf cottonwoods currently have a large potential niche,
covering 78.3% of riparian corridors in the southwest, and are the most generalist species, able to
inhabit a large range of temperature and precipitation regimes. Even so, extended severe drought
may cause a population crash in their westernmost distribution. In contrast, narrowleaf
cottonwoods will lose far less of their habitat from drought.
The two cottonwood species in this study will also respond differently to increasing
temperatures. The range of narrowleaf cottonwoods in the southwest will shrink if temperatures
rise, while broadleaf cottonwoods can potentially expand upward in elevation. This is consistent
with Berry et al. (2002), who found that arctic-alpine communities in Britain and Ireland will
lose territory while lowland species will gain coverage as climate warming drives their potential
niches toward the pole. However, invasive exotic lowland species such as tamarisk may also
increase their distributions in a warmer climate, which could negate the potential for lowland
cottonwoods to compensate.
Our models predict that broadleaf cottonwood species will be at a competitive
disadvantage with narrowleaf cottonwoods and their hybrids if long-term drought persists.
Narrowleaf cottonwoods, which will suffer less fragmentation and smaller range contractions,
are likely to recover quickly. Broadleaf cottonwoods will experience high levels of
fragmentation and may experience a permanent range shift. Since hybrid cottonwoods
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experience very low levels of drought mortality, dense hybrid forests may prevent broadleaf
cottonwoods from migrating into newly available niche spaces if temperatures rise.
Furthermore, narrowleaf and hybrid cottonwoods, which reproduce clonally in the absence of
flooding, will be more resilient in a fluctuating or dry climate pattern, maintaining aboveground
biomass and rapidly repopulating after drought.
Vegetative reproduction is likely to be an advantageous trait in a fluctuating climate.
Aspen (P. tremuloides) in the southern Rocky Mountains shifted from sexual to asexual
reproduction and moved upward in elevation during a warm, dry period (Elliott and Baker 2004).
In agreement with this hypothesis, molecular studies of isolated Populus populations in Nevada’s
sky islands argue that the parental species have died out leaving only F1 type hybrids growing
hundreds of kilometers from their parental species (Woolbright, unpub. data). Since cottonwood
hybrids produce prolific clonal offspring (Schweitzer et al. 2002) and are drought tolerant (this
paper), the Nevada sites may be relict populations that have used asexual reproduction to survive
continual drying since the Pleistocene (Mensing 2001). Our field observations during recent
drought years confirm that asexually reproductive hybrid and narrowleaf cottonwood stands
continued to reproduce during drought. Broadleaf stands, which depend primarily on sexual
reproduction (Schweitzer et al. 2002), showed the least reproduction during drought.
An invasive exotic species compounds the effects of climate.---We added to our model a
widespread invasive exotic species that gains greater dominance in dry conditions (Cleverly et al.
1997, Glenn and Nagler 2005, Lite and Stromberg 2005) to examine the ways that it might alter
forest connectivity. Recent research suggests that tamarisk acts to alter floodplain processes and
cottonwood abundance at river reach scales (Birken and Cooper 2006). Our work shows that
tamarisk is present in the driest places, and is strongly associated with increased cottonwood
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mortality (Fig. 3). If dry areas increase in size and temperatures become warmer, we predict that
tamarisk with its superior dispersive abilities (Friedman et al. 2005), will rapidly expand its
distribution. Since it is currently dominant along mainstem river channels (NIISS data, and
Gitlin, personal observation), it is acting to isolate cottonwood forest fragments in smaller
tributaries and upland drainages. This could be problematic if trees that are genetically adapted
to the hydrologic regimes, competitive associations, soil chemistry, insolation, and seasonality of
larger rivers are dying off while the surviving trees are adapted to different localized conditions
(Frewen et al. 2000, Lytle and Poff 2004, Rowland et al. 2004). For example, smaller tributaries
might have a more variable water table, less developed understory, less saline soils, cooler
temperatures modified by shady canyon walls, or a differently timed spring flood peak.
Tamarisk dominance decreased the cottonwood potential niche area while increasing the
number of and distance between forest fragments. Tamarisk forests consistently reduce
cottonwood forest cover and connectivity at a magnitude equal to drought, effectively doubling
the effect of drought alone for both narrowleaf and broadleaf cottonwoods. The impact is less
severe at the highest temperature scenario, probably because we kept the tamarisk distribution in
our models static under all projected climate regimes, rather than increasing it to fill new warm
and dry niches.
The tamarisk niche is identical to the broadleaf cottonwood niche except that tamarisk is
less sensitive to aridity and more sensitive to minimum temperature than broadleaf cottonwoods.
Neither of these limitations are likely to pose a problem for tamarisk in a warmer and drought
prone environment. Tamarisk are also capable of prolific root sprouting and this will likely
increase their ability to spread rapidly during and after drought years, gaining dominance in
many of the areas that lose broadleaf cottonwoods (Birken and Cooper 2006). Land and water
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management during and after drought, combined with post-drought climate, will likely determine
whether native trees are able to return to dominance.
Conservation implications.---Extreme weather events, exotic species, land use changes, and
other smaller-scale factors are interacting with long-term global climate changes to alter forest
distributions (IPCC 2001, Breshears et al. 2005, Ferreira et al. 2005, Reinhart et al. 2005,
Mueller et al. 2005, Gitlin et al. 2006, Parmesan 2006). Rather than attempting to predict a
complex future climate scenario and then modelling its effect on species distributions, this study
explores the impact of a specific and recurring climatic stressor, drought, that is likely to become
more severe with climate warming (Cook et al. 2004). By altering the severity of temperature
and precipitation changes, we show how the most vulnerable and resilient areas on the landscape
can be identified so that appropriate conservation actions can be proactively taken. Since
drought is expected to be recurrent but temporary, the long-term survival of these forests will
depend on refugia that can provide post-drought seed sources and corridors with appropriate
conditions to enable post-drought native plant reestablishment (Noss 2001, Lake 2003).
Our models show how a common tree can become rare if subjected to adverse climate
conditions. It is important to protect such dominant species as they will impact the habitat
quality of a large number of dependent species (Noss 2001, Ellison et al. 2005) rather than solely
focusing on rare and endangered species. Trees are often foundation species that determine the
structure and function of their local environment. Genetic diversity of foundation species should
be preserved because high levels of genetic diversity in these species leads to greater biodiversity
of dependent communities (Wimp et al. 2004, Bangert et al. 2005, Whitham et al. 2006,
Crutsinger et al. 2006) and increases adaptability to climate change (Reusch et al. 2005). Forest
fragmentation processes should be mitigated since unnatural levels of fragmentation degrade
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habitat quality, impede forest regeneration, alter phenology, feed back to hydrology, and
decrease adaptability to climate change (Laurance and Williamson 2001, Noss 2001, Hewitt and
Kellman 2004, Herrerías-Diego et. al. 2006).
Prioritizing areas for conservation.---Ideally, restoration projects should be coordinated across
large areas to preserve genetic diversity, create intact habitat corridors, and limit fragmentation
processes (Noss 2001). In the southwest, we suggest three specific approaches to regional
habitat conservation and restoration: 1) in the most drought susceptible regions, secure long-term
water rights for vital habitat areas and restoration projects so they can be sustained during
extended dry periods; 2) in the most drought resilient areas, proactively remove exotic vegetation
and limit destructive land and water projects to maintain healthy diverse refugia capable of
acting as post-drought seed banks; and 3) designate “native riparian corridors” along large river
channels, where active management maintains the historic conditions that promote native plant
recruitment and the many animals dependent upon these dominant species for cover, forage, and
migration. Historic conditions include properly timed flood peaks, meandering channel
morphology, open bare substrates, and a diverse floodplain structure. It is important to realize
that these policies will require not only a commitment of water resources, but also time, money,
and persistent human effort. These actions require a new approach to river management in the
southwest because current restoration projects tend to focus on small reaches that are already
degraded, and are often located in specially designated areas such as parks and wildlife refuges
(Follstad Shah et al. 2006). While the current patchwork of restoration attempts should not be
abandoned, a regional approach will provide a buffer to permit species migrations in a changing
climate.
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The most drought susceptible regions of broadleaf cottonwood habitat in our model are
located in the western portion of our region, primarily in the lower Gila River watershed in
southwestern Arizona; the Virgin, Dirty Devil, and lower San Juan Rivers in Utah; and portions
of the Colorado River. Interestingly, the model distinguishes between the Colorado River in and
out of the Grand Canyon; much of the Colorado River watershed contained within the walls of
the canyon fares well under a 50% reduction in precipitation. Ironically, managed rivers such as
the lower Colorado, where restoration of riparian forests has become a high priority, could have
an advantage in buffering drought impacts by moderating water delivery through dry years.
The most drought resilient areas lie along eastern Colorado and northeastern New
Mexico. These are largely agricultural areas where healthy riparian forests should be maintained
to preserve water quality while also preserving seed sources for future cottonwood recruitment
events. Isolated areas, including highlands along the Mogollon Rim region of Arizona,
mountainous areas in southeastern Arizona and western New Mexico, the uppermost reaches of
the Gila River watershed, and portions of the Pecos River and its watershed, also show a great
deal of drought tolerance. These areas should receive careful attention, including eliminating
exotic species before they become dominant and carefully planning new land developments to
conserve water. The Mogollon Rim region, a transition zone at the southern edge of the
Colorado Plateau, harbors the most drought resilient riparian forests in the western states of Utah
and Arizona, and therefore merits special conservation consideration. Likewise, the Pecos River
should be considered as a candidate “native riparian corridor” where native vegetation and
cohesive species migration routes could be restored and preserved.
Not only is it important to locate conservation and restoration projects across climatic
gradients, it is also important to preserve and create pockets of high genetic diversity to buffer
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for future climatic extremes (Noss et al. 2001, Reusch et al. 2005, Harris et al. 2006). Deriving
source trees for restoration projects from warmer drier areas, or using drought-tolerant
individuals, will increase the likelihood of creating a new population that is tolerant of what’s to
come (Seliskar et al. 2002). Increasing the genetic diversity of a dominant plant population can
increase that population’s resilience to climate variability, with positive effects reaching to other
trophic levels (Reusch et al. 2005). Without interventional efforts to maintain and encourage
evolutionary processes, adaptation may not keep pace with rapid climate change (Parmesan
2006).
Since hybrid zones contain both parental species and their hybrid offspring, they are areas
of increased genetic diversity and associated community biodiversity as they tend to accumulate
the species supported by both parental species (Lewontin and Birch 1966, Whitham et al. 1999,
Wimp et al. 2004, Bangert et al. 2005). If dryness persists and temperatures rise, the cottonwood
hybrid zone will shrink as the amount of overlap between parental species contracts. Our
findings that naturally occurring hybrids are better able to survive drought and are better able to
regenerate via asexual reproduction (Fig. 2), further argues for their increased role in preserving
a threatened habitat type. As human demands for water combine with climate perturbations to
reduce water availability for these dominant riparian species, drought resilient hybrid zones may
become vital to the floodplain communities which depend on them. However, if artificially
dammed lakes interrupt gene flow along rivers or if extensive regional mortality reduces forest
connectivity, new hybrid population creation could be limited (Imbert and Lefèvre 2003, Merritt
and Wohl 2006).
Although the importance of hybridization in plant evolution has long been debated,
evidence continues to mount that hybrids represent a major evolutionary pathway in plants
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(Rieseberg 1997, Hendry et al. 2000, Lexer et al. 2003, Rieseberg et al. 2003) and that their
conservation is important (Whitham et al. 1991). Repeated evidence for rapid speciation via
hybridization in response to new environments has been documented (Rieseberg 1997, Hendry et
al. 2000, Lexer et al. 2003), and molecular studies argue that hybridization has been important in
the speciation of Populus (Smith and Sytsma 1990). Therefore, conservation of extant hybrid
Populus stands should be considered a high priority when designing river management projects.
Empirically prioritizing restoration actions across large regions will empower land
managers to work together across land ownership boundaries to provide the greatest benefits
over time. By modelling a widely distributed tree that is a hotspot of biodiveristy, we identified
procedures that can mitigate great biodiversity risks. If climate change and interactions with
exotics are ignored, conservation project goals may be unattainable and create unpredictable
ecosystem trajectories. As our understanding of past climate and predictions for the future
become refined, we must adjust our methods and objectives accordingly. If we can work with
anticipated climate variability, we may be able to maximize project success.
Acknowledgements— We thank Arches NP, Capitol Reef NP, Great Sand Dunes NP and
Preserve, Hubbell Trading Post NHS, New Mexico State Forestry Division, Ouray NWR,
Petrified Forest NP, Zion NP, TNC Tabeguache Preserve, and TNC Hassayampa River Preserve
for providing access to trees, and the financial support of the Merriam-Powell Center for
Environmental Research and NSF grants DEB-0078280, DEB-0236204, and DEB-0425908.
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FIGURES
FIG. 1. Sampling locations that were input into the Desktop GARP modelling software.
FIG. 2. A survey of cottonwood mortality and reproduction on the Colorado Plateau during the
drought years of 2003 and 2004 revealed higher survival of hybrids than parental species, while
broadleaf cottonwood stands showed the least reproductive success. (a, b) Average F1 mortality
was 4% both years, and differed significantly from mortality of parental species (2003 χ2 =
14.889; P = 0.0006; 2004 χ2 = 26.449; P < 0.0001). (c) Mortality within each of the three rivers
observed in 2004. (d) Presence of young trees within parental and hybrid stands in 2003.
Parental species differed in reproductive success (χ2 = 4.174; P = 0.0410).
FIG. 3. Increased tamarisk cover predicts increased broadleaf cottonwood mortality on the
Colorado Plateau (R2 = .62, n = 13, P = 0.001).
FIG. 4. (a) A GARP potential niche distribution for narrowleaf (black) and broadleaf (light grey)
cottonwoods, with overlap between the species (medium grey) showing extent of hybrid zone,
shows that broadleaf cottonwoods currently have the largest niche of the cross types. (b)
Potential niche map masked to show riparian areas only (cross type designations same as in (a).
FIG. 5. A comparison of climate parameters at cottonwood and tamarisk locations shows that
tamarisk occupy the driest areas. Broadleaf cottonwoods have the most habitat variability, but
share much of their habitat with tamarisk. Upper and lower box boundaries indicate 25th and75th
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percentiles; midline indicates median value. Whiskers represent 10th and 90th percentiles.
Letters show significant differences by Tukey-Kramer tests (P = 0.05).
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FIG. 6. Potential niche models for narrowleaf cottonwood (black), broadleaf cottonwood (light
grey), and their hybrid zone (medium grey) under six drought scenarios show how precipitation
and temperature changes will affect narrowleaf and broadleaf cottonwoods in opposite ways.
The two precipitation levels, 50% and 25% of average, were chosen to represent the driest years
of the last century and then an extreme drought exceeding anything in recent history.
Temperature increases are based on those predicted by the International Panel on Climate
Change at doubled CO2 concentrations. Precipitation and temperature levels deviate from the
March to October averages.
FIG. 7. Analyses of riparian corridors with FRAGSTATS show that decreased precipitation will
be more detrimental to the potential niche area of broadleaf cottonwoods (left column), while
increased temperatures will limit narrowleaf cottonwood habitat (right column). Tamarisk will
increase fragmentation for both species. Connectivity is a measure of the percent of patches
within 6km of another patch. Open symbols represent models with tamarisk-infested regions
removed from potential niche distributions (see Fig. 7).
FIG. 8. (a) Cottonwood potential niches were masked to show riparian areas only and then
overlain with a representation of the current tamarisk distribution (heavy black lines) from the
National Institute for Invasive Species Science. (b) Cottonwood potential niches with both non-
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riparian and tamarisk-infested areas masked out to show the places most likely to be cottonwood-
dominated.
FIG. 9. Vulnerability surface showing the probability that a given area will remain as potential
niche space for broadleaf cottonwoods. Highest drought susceptibility is shown in red and most
resilient areas are shown in blue. New potential niche spaces that could become available under
increased temperature regimes are shown in green. White areas are not potential niches under
any modelled conditions.
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