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Landscape variables as predictors of characteristics of native-grass communities in xeroriparian areas of the Sonoran Desert.

The stream channels of southwestern Arizona are often braided ephemeral washes that have flowing water only after major rainfall. These washes, which can be classified as Type-1 ephemeral streams, end in upland alluvial fans (similar to those described in Schwinning et al., 2011) rather than being part of a more traditional network connected to increasingly larger streams or rivers. Another difference is that upland species form the floral communities in the washes instead of riparian obligate vegetation (see Shaw and Cooper, 2008). Concentrations of vegetation associated with washes, henceforth xeroriparian areas, occur as small grasslands on trapped sediment within the washes and remain distinct from the surrounding sparsely-vegetated Sonoran Desert. In this type of desert system, plants and resources can move along the paths of flow similar to more common mesic riparian systems, but the water needed to form these connections does not occur frequently and when it does rain enough for water to flow in channels, it often takes the form of flashfloods. These flashfloods resculpt the channels every few years, removing existing xeroriparian vegetation and making new pockets of sediment that develop into new xeroriparian areas.

The sparse surrounding upland vegetation contributes to the xeroriparian areas in moderating the resources that flow into them as well as being the putative source of seeds for revegetation after flashfloods. While slope and the surrounding community of plants control vegetative composition of most arid vegetative types (Aguiar and Sala, 1999), the composition of the matrix (those in closest proximity) communities of plants is not necessarily the best predictor of xeroriparian presence or composition because the conditions in the washes are different from the adjacent uplands (Beauchamp and Shafroth, 2011). This, when combined with their relative rarity, makes finding and managing xeroriparian areas challenging.

Despite their small proportion of the total area of land, xeroriparian areas in the Sonoran Desert are unduly important. Xeroriparian areas increase stability of soil by providing stabilizing vegetative cover and favor aggregation of soil (e.g., Carter et al., 1994; Whitford et al., 1998; Sponseller and Fisher, 2006). This allows them to play a pivotal role in the distribution of run-off water and the rates of infiltration across the landscape (e.g., Whitford et al., 1998). They also serve as reservoirs that regulate microbial communities and nutrient concentration and moisture in soil within the washes (e.g., Belnap and Phillips, 2001).

While these effects of the ecosystem are important, managers of land primarily value the xeroriparian areas because they form the main grassland critical to many local populations of wildlife (Szaro and Belfit, 1986; Johnson and Haight, 1987; Cartron and Finch, 2000; deVos and Miller, 2005). This includes several protected and endangered species of wildlife, including Sonoran pronghorn antelope (Antilocapra americana sonoriensis), cactus ferruginous pygmy owl (Glaucidium brasilianum cactorum), and flat-tailed horned lizard (Phrynosoma mcallii), as well as abundant unprotected species of wildlife, most notably desert mule deer (Odocoileus hemionus eremicus; Ordway and Krausman, 1986; Ragotzkie and Bailey, 1991; Marshal et al., 2006).

Recent management of these diverse species of wildlife has included the construction of basins for catching water and the hauling of water to refill new and existing natural catchments to increase populations of wildlife. The impacts of adding water for wildlife on the xeroriparian forage are not well understood. While there is limited research on wildlife, research on livestock in grasslands of southeastern Arizona (Bock and Bock, 1993; Valone and Sauter, 2005), central Mexico (Mata-Gonzalez et al., 2007), and northern California (Gelbard and Harrison, 2003) shows that even moderate grazing can have a significantly negative impact on composition and distribution of communities of plants. Further, negative impacts have been shown to be strongly correlated with decreasing distance to water (e.g., Todd, 2006). The impact of grazing by wildlife in other regions has been shown to have a similar effect especially when watering points have been added to allow more use of seasonally dry areas by wildlife (Smith, 1996).

In addition to reducing or removing grasses, grazing can augment invasions of alien plants (Richardson et al., 2007) that can reduce or replace palatable natives (e.g., Richardson et al., 2007) and reduce or eliminate the value of critical habitat in xeroriparian areas. Xeroriparian areas, while generally dry and isolated, have nonetheless suffered invasions in other regions (vanDevender, 1997). Another concern is that invaders can act as fine fuels that allow fire to spread into vegetation otherwise too sparse to carry a fire (e.g., Brooks et al., 2004; Rogstad et al., 2009).

A better understanding of the factors that control the presence and distribution of xeroriparian areas in the desert Southwest will allow for better identification and management of these important areas. To determine the landscape variables and compositional elements of communities of the native grasses, data on xeroriparian grass were collected to answer the following questions: do differences exist in composition, cover, and density of species of grass within a xeroriparian community with increasing distance from a catchment of water for wildlife; do differences exist in composition, cover, and density of species of native grasses between xeroriparian communities that contain different amounts of species richness and cover of exotic (nonnative) grass; do differences exist in composition, cover, and density of species of grass between xeroriparian communities with different adjoining matrix communities.

MATERIALS AND METHODS--The Barry M. Goldwater Range-East (BMGR-E) encompasses 98,549 ha (380.5 square miles; 243,520 acres) in southwestern Arizona (Fig. 1). It is located in the Major Land Resource Area 40 of the Sonoran Desert (Natural Resources Conservation Service, 2006) and has soils that are primarily classified as Aridisols, with small areas of Entisols and Andisols. The BMGR-E receives 150-200 mm of precipitation per year, with precipitation being divided almost equally between cool-season (October-March) and warm-season (April-September) precipitation (data collected at Gila Bend and Ajo; Western Regional Climate Center, http://www.wrcc.dri. edu).

We established 40 plots of xeroriparian grass distributed throughout the BMGR-E. All plots were located within one of two adjoining matrix communities: Creosote-Bursage Desert Scrub (CBDS); Palo Verde-Mixed Cacti-Mixed Scrub on Bajadas (PMCSB). Each plot was placed into one of the following categories based on their distance from a catchment of water for wildlife: <1 km; 1-4 km; 4-6 km; >6 km. We chose these matrix communities and distances to obtain data consistent with Smith and Morrison (2006). We established all plots in areas where grazing by domestic livestock had been excluded. Artificial water holes or modified natural tinajas used by wildlife are potential sources of disturbance that may affect the surrounding communities of grass. The catchments of water for wildlife in this study area are artificial water holes maintained (filled) by the Arizona Game and Fish Department. These structures consist of large pieces of sheet metal placed on hillslopes that collect and channel precipitation into underground storage tanks and then gradually feed the water to aboveground drinkers. The plots where vegetation was measured were not located immediately adjacent to these artificial water holes nor were they immediately downslope of them. The sample plots were located in the xeroriparian bottoms of stream channels that surrounded the catchments at various distances from the artificial catchments in areas of relatively homogenous moisture of the soil. All input of moisture on all locations of plots appeared to be solely from rainfall and natural runoff; none was attributable to the tanks or groundwater.

A 25-m radius circular plot (1,963.5 [m.sup.2]) was centered at each sampled wash bed and extended into the surrounding xeroriparian community. We collected data during the winters of 2007 and 2008. Within the plot, we established five quadrats, 1.5-m x 1.5-m, to measure density (Fig. 2; Smith and Morrison, 2006). We conducted a complete tally of individual clumps of grass for each species of native and exotic grass present within each quadrat. We also recorded the percentage of the area covered (foliar cover) within each quadrat by each species of grass present. We recorded 12 native perennials, 5 native annuals, and 1 exotic annual within the plots. Native perennial grasses recorded included (in order of decreasing abundance): Aristida purpurpea (purple threeawn); Leptochloa panacea (mucronate sprangletop); Muhlenbergia porteri (bush muhly); Bouteloua rothrockii (Rothrock's grama); Dasyochloa pulchella (low woolygrass); Heteropogon contortus (tanglehead); Pleuraphis rigida (big galleta); Aristida ternipes (spidergrass); Bothriochloa barbinodis (cane bluestem); Bouteloua repens (slender grama); Setaria vulpiseta (plains bristlegrass); and Tridens muticus (slim tridens). Native annual grasses included: Muhlenbergia microsperma (littleseed muhly); Aristida adscensionis (sixweeks threeawn); Bouteloua barbata (sixweeks grama); Eragrostis cilianensis (stinkgrass); and Bouteloua aristidoides (needle grama). The only exotic grass that occurred within the study area was the annual Mediterannean grass, Schismus barbatus (nomenclature from United States Department of Agriculture PLANTS database, Natural Resources Conservation Service, http://plants.usda.gov). It is not uncommon to find communities of plants as depauperate as observed in these areas with low rainfall (e.g., Stohlgren et al., 2005).

We used a multivariate analysis of variance (MANOVA) to assess relationships between the landscape variables and eight observed variables for grasses: foliar cover for native perennials; foliar cover for native annuals; foliar cover for exotic annual; density for native perennials; density for native annuals; density for exotic annual; species richness for native perennials; species richness for native annuals. There was only one species of exotic grass so the richness variable would be masked by data on the other exotic grass in the analysis and was not included. The MANOVA could correct for the expected partial correlation between the observed variables. We analyzed data using R version 2.9.1 (R Foundation for Statistical Computing, http:// cran.r-project.org/mirrors.html). We used a least-squares linear model approach to hypothesize associations between exotic annual cover and density and the remaining variables.

RESULTS--The MANOVA showed that the adjoiningmatrix community had a significant effect (Pillai Trace = 0.41, [F.sub.8,28] = 2.41, P = 0.040) on the observed variables. Distance to catchments of water for wildlife (Pillai Trace = 0.41, [F.sub.24,90] = 0.59, P = 0.931) did not show a statistically significant relationship with the observed variables. A univariate analysis of variance (ANOVA) showed that only native annual density had a statistically significant relationship with the adjoining-matrix community (F = 4.28, df = 1, P = 0.046; Table 1). Post-hoc testing showed substantial deviation from the assumption of normality in the univariate residuals for all observed variables except native perennial richness. Based on the Shapiro-Wilk normality test, the nonnormal observed variables (native perennial cover, native annual cover, exotic annual cover, native perennial density, native annual density, exotic annual density, and native annual species richness) were resistant to the usual transformations. The data, respectively, had 52.5, 20.0, 30.0, 60.0, 47.5, 35.0, 52.5, and 17.5% zeros reflecting the sparse distribution of plants in this desert grassland. In additon to the data being sparse, there were several observations with data an order of magnitude higher than the rest which contributed to the overall skewness of the data. Tree-based regression was selected as a method that was robust to this kind of skewed data (see Maindonald and Braun, 2007), and the analysis was conducted in reverse order (to determine which of the observed variables predicted the matrix community) as a check on the validity of the MANOVA and ANOVA. The tree-based regression classification similarly chose native annual density as the best predictor of the matrix community; this variable predicted 41% of the CBDS matrix while simultaneously incorrectly predicting 10% of the PMCSB as CBDS based on a native annual density > 0.98 plant/[m.sup.2].

To make the analyses complete, a similar analysis was conducted on the controlled variable, distance from water, despite the lack of significance from MANOVA. Native perennial richness showed some evidence of association (P = 0.050) in the univariate ANOVAs and the residuals met the assumption of normality. The tree-based regression classification again showed native perennial richness as having predictive power on distance from water. The analysis classified sites in the category of > 6 km as having native perennial richness of [less than or equal to] 2 on 92% (11 of 12) of the sites compared to 33% (2 of 6) on the sites 4-6 km and 50% (6 of 12) on the sites < 1 km. This would make a better relationship except that the remaining category, 1-4 km, has 80% (8 of 10) < 2 which makes it intermediate. A fitted line between the average native perennial richness and the centroids of the categories shows a decline in average native perennial richness with increasing distance from water, but the overall correlation ([R.sup.2] = 0.04) gives this relationship little support.

The exploratory analysis showed that higher exotic annual cover was associated (P = 0.032) with higher native annual density (an increase of 0.23 [+ or -] 0.10 plants/ [m.sup.2] per additional percentage of exotics). Exotic annual cover also was associated (P = 0.014) with decreasing native richness (a loss of combined annual and perennial richness of 0.18 [+ or -] 0.07 per additional percentage of exotics). No other variable had significant predictive power (P > 0.05). Exotic annual density was not associated with any of the variables (P > 0.05). The residuals had the same issues previously mentioned.

DISCUSSION--Increasing distance from watering locations for wildlife could be expected to have a positive effect on the structure and composition of the xeroriparian areas because less use by wildlife is expected. However, our data show no significant association between the observed characteristics of communities of plants and distance from water for wildlife. We did find a tenuously supported relationship between decreasing native perennial grass richness with increasing distance from a source of water for wildlife (> 6 km). Native perennial richness can increase through use by wildlife because wildlife have been shown to act as a dispersal agent for seeds of grass (e.g., Cain et al., 2000; Vellend et al., 2003; Myers et al., 2004). However, if wildlife are facilitating the spread of grasses around the catchments of water, one would expect reductions in the distribution and abundance of the populations of grass nearest the water due to use by wildlife (Ordway and Krausman, 1986; Ragotzkie and Bailey, 1991; Marshal et al., 2006) which we did not detect.

This lack of detectable impact of wildlife similarly may explain why the data showed no significant relationship between distance from catchments of water and abundance of exotic species of grass. Disturbance near the catchments could be expected to facilitate the invasion of exotic species because disturbed areas are known hotspots for the introduction and spread of exotic and potentially invasive species (e.g., Stohlgren et al., 1998; Lovich and Bainbridge, 1999; Gelbard and Harrison, 2003). It also is possible that the exotic species of grass currently present on the range (Schismus barbatus) is not found near the catchments of water (located in the PMCSB) as it favors a different habitat (the CBDS).

The overall lack of a compelling relationship between distance to water for wildlife and the observed variables could be due in part to insufficient sample size in terms of the number of sites visited and in the size of each plot. More sites would allow a better chance of finding the appropriate distribution to meet statistical assumptions although this may make the sampling untenable given the large distances involved. A better option may be expanding the size and scope of plots, which, given the diffuse distribution of xeroriparian areas along with the sparse vegetation upon them, may similarly improve the distribution of data by limiting the number of zero observations in each variable. The combination of these approaches will quickly progress toward a complete census of the xeroriparian area which may present other analytical challenges. Future work also should consider treating the distance to water for wildlife as a continuous variable.

The cover of Schismus, an annual and the only exotic species of grass, was positively associated with native annual density. This seems likely to be due to the common requirements for these annual grasses. Where the native annuals Bouteloua barbata, E. cilianensis, L. panacea, and M. microsperma grow, Schismus also can establish itself. The fact that cover of Schismus was predictive while density was not likely reflects that cover is a better metric of total uptake of resources than is density which counts small and large plants equally (Fehmi, 2010).

The loss of native richness with increasing exotic cover may be explained by the fact that Schismus occurs in higher amounts on sites less suitable (i.e., drier) for the native plants. This would imply that Schismus is occupying vacant niches rather than competitively taking resources from the native species. This explanation is supported by Gurevitch (1986) who showed that native and exotic grasses utilize different areas of the xeroriparian habitats across the Goldwater Range. Native annuals are associated with the warmer, xeric valley bottoms of CBDS. The exotic annual grasses prefer the cooler, mesic uplands of PMCSB and are infrequently found in the warmer, xeric valley bottoms of CBDS. The native perennials also prefer the uplands of PMCSB but are unlikely to be affected by the exotic species of grass encountered in this study (Schismus barbatus) as they are larger in size and likely less susceptible to negative competitive feedbacks (e.g., shading, useage of water) with the smaller exotic species (e.g., Corbin and D'Antonio, 2004).

Differences between the xeroriparian communities CBDS and PMCSB also may be due to differences in received runoff. Grasses growing in a wash surrounded by a matrix community with little vegetative cover (e.g., the CBDS) may receive more runoff water from the surrounding areas than do grasses growing in a wash surrounded by a community with greater vegetative cover (e.g., the PMCSB), because there is less vegetation to impede movement of water and facilitate infiltration following rains. This is similar to the infiltration of water occurring at greater amounts in and around clusters of vegetation relative to bare soil, leading to net displacement of surface-water runoff to vegetated patches as reported by Bergkamp et al. (1996) and Rietkerk et al. (2002).

However, the associated loss of native richness with increasing exotic cover that was detected may represent a real change in the community of plants as the exotic species takes resources formerly going to native species. This may imply a decline in these native-dominated systems due to competition from the invading exotic species similar to the significant competitive interactions between exotic annual grasses and native annual grasses in other water-limited ecosystems (e.g., Brooks, 2000). The sites with higher native richness also may have better resisted invasion. Anderson and Inouye (2001) found that as a result of competitive interactions for resources, sites with increased native richness had decreased exotic invasibility and that sites with greater native cover had low exotic cover, density, and species richness. Gurevitch (1986) also found significant differences in the density and cover of native and exotic grasses as a result of competitive interactions for resources.

The structure and composition of communities of native and exotic grasses in xeroriparian areas on the Barry M. Goldwater Range-East are influenced by the adjoining-matrix community of plants. Distance to catchments of water for wildlife was not found to have a significant effect on the observed variables, although a tree-based regression suggested that native perennnial richness showed some evidence of association with distance from water. Of the eight variables observed, only native annual density had a statistically significant relationship with the adjoining matrix community. Exotic annual cover was associated with higher native annual density as well as decreasing native richness, suggesting either differences in niches or competitive interactions between the native and exotic grasses in this system. The vacant-niche explanation seems better supported than one involving competition but a combination of factors is likely involved. A manipulative experiment will be needed to determine these interspecific interactions. These data reflect the sparse distribution of vegetation in this desert grassland as well as the variable nature of the structure of vegetative communities as influenced by availability of resources (primarily runoff from rainwater).

We thank W. Carroll, A. Habgood, and M. L. Osmer for assistance with collection of data and R. K. Whittle and C. W. Black for logistical support and providing access to high-quality remotely sensed imagery to assist with sampling in the field. We also thank D. P. Guertin and S. E. Smith for helpful discussions and comments. This research was funded by the United States Air Force on behalf of Luke Air Force Base, Glendale, Arizona.

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Submitted 31 August 2012. Acceptance recommended by Associate Editor Mara L. Alexander 11 March 2013.

Eva M. Levi * and Jeffrey S. Fehmi

School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721

* Correspondent: evamarie@email.arizona.edu

TABLE 1--Results of univariate analysis of variance of data
for xeroriparian grass (percentage of foliar cover, density,
and species richness) at Barry M. Goldwater Range-East,
Arizona.

Dependent   Sum of       Mean       F       df   P
variable    squares      squares

Native annual cover

Matrix          0.0003     0.0003   <0.01    1   0.997
Distance       41.81      13.94      0.78    3   0.512
Residuals     623.38      17.81             35

Native perennial cover

Matrix        308.43     308.43      1.89    1   0.178
Distance       70.28      23.43      0.14    3   0.933
Residuals   5,724.10     163.50             35

Exotic annual cover

Matrix          1.27       1.27      2.59    1   0.116
Distance        0.13       0.04      0.09    3   0.965
Residuals      17.09       0.49             35

Native annual density

Matrix          7.30       7.30      4.28    1   0.046 *
Distance        1.50       0.50      0.29    3   0.830
Residuals      59.74       1.71             35

Native perennial density

Matrix          7.48       7.48      1.90    1   0.176
Distance        2.47       0.82      0.21    3   0.889
Residuals     137.42       3.93             35

Exotic annual density

Matrix         16.49      16.49      0.79    1   0.379
Distance       10.49       3.49      0.17    3   0.917
Residuals     727.70      20.79             35

Native annual richness

Matrix          1.00       1.00      1.48    1   0.231
Distance        0.17       0.06      0.09    3   0.967
Residuals      23.60       0.67             35

Native perennial richness

Matrix          2.10       2.10      1.10    1   0.301
Distance       16.39       5.46      2.87    3   0.050
Residuals      66.61       1.90             35

* Significant at P [greater than or equal to] 0.05.
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