watercourse-upland and elevational gradients in spring vegetation of a mojave-great basin desert...

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Watercourse-Upland and Elevational Gradients in Spring Vegetation of a Mojave-Great Basin Desert LandscapeAuthor(s): Scott R. Abella, Jill E. Craig, Sara L. McPherson Jessica E. SpencerSource: Natural Areas Journal, 34(1):79-91. 2014.Published By: Natural Areas AssociationDOI: http://dx.doi.org/10.3375/043.034.0109URL: http://www.bioone.org/doi/full/10.3375/043.034.0109

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Volume 34 (1), 2014 Natural Areas Journal 79

Natural Areas Journal 34:79–91

•Watercourse-Upland

and Elevational Gradients in Spring

Vegetation of a Mojave-Great Basin Desert Landscape

Scott R. Abella1,5,6

1Department of Environmental andOccupational Health

University of Nevada Las VegasLas Vegas, Nevada 89154-3064

Jill E. Craig1,2

Sara L. McPherson1,3

Jessica E. Spencer1,4

2City of Boulder City401 California Ave.

Boulder City, Nevada 89005

3College of ForestryOregon State University

Corvallis, Oregon 97331-5704

4U.S. Army Corps of Engineers701 San Marco Blvd.

Jacksonville, Florida 32207

•

R E S E A R C H A R T I C L E

5 Corresponding author: [email protected]

6 Current address: National Park Service, Washington Office, Natural Resource Stewardship and Science Directorate, Bio-logical Resource Management Division, 1201 Oakridge Dr., Fort Collins, Colorado 80525

ABSTRACT: Springs in arid lands provide critical habitat for a variety of species and functions to humans, yet the ecology and management needs of springs to maintain these values are poorly understood. To examine plant communities along spring watercourse-upland gradients, we sampled 12 springs at low (desert) and high (forest) elevations on the Desert National Wildlife Refuge in the Mojave and Great Basin Deserts in southern Nevada. In contrast to the commonly reported positive relationship between native and exotic species richness in sampling studies, we did not find strong correlations (r2 < 0.05) between native and exotic richness at any distance from watercourses. Additionally, exotic species cover was lower nearest (0 and 2 m) watercourses than at uplands 20 m from watercourses, which also differs from the hypothesis that watercourses are more heavily invaded than uplands. Exotic species were more pervasive at low-elevation compared to high-elevation springs, but the proportion of total plant cover comprised by exotics was still small (0.03 – 0.06) at low-elevation springs. Species distri-butions and ordinations suggested that compositional watercourse-upland gradients were often readily detectable, but the composition of springs was individualistic. Some springs contained wetland species such as Juncus saximontanus, while other springs contained species of dry-site affinity. This study also illustrated challenges associated with estimating reference conditions for arid-land springs, as there are no known data prior to the development (i.e., modifying surface flow) of the springs and no known unmodified springs on this landscape.

Index terms: conservation, exotic species, riparian, seeps, wetlands

INTRODUCTION

The rarity of water made springs critically important to Native American habitation and Euro-American settlement of Ameri-can Southwest landscapes. In his recount of travels in southern Nevada’s Mojave and Great Basin Deserts with the Wheeler expeditions, Lyle (1878:18, 27) encapsu-lated the significance of springs within arid lands: “To those who have experienced the pangs of thirst, while journeying over the desolate wastes that characterize this section, it will not be surprising that remi-niscences of water should linger longest in the memory of the traveler… The springs of this inhospitable region are so few, that at one time or another, each one becomes, as it were, the polar star of the desert trav-eler, towards which he turns his face with inflexible determination.” Humans used water from springs for many purposes, such as agriculture, mining, watering of livestock, and direct human consumption (Meretsky 2008). Springs remain critically important to humans for agriculture, ranch-ing, drinking water, and recreation (Sada 2001). Springs also are used by wildlife and other organisms and supply critical habitat for endemic fish, invertebrates, and plants (Deacon et al. 2007). Springs already have a long history of anthropogenic impacts, and conserving springs in the face of esca-lating pressure from current and proposed ground-water pumping, climate change, and exotic species invasions remains

challenging (Shepard 1993; Mayer and Congdon 2008; Dudley 2009). Advancing our understanding of the ecology of springs underpins the development of effective conservation strategies (Patten et al. 2008a; Unmack and Minckley 2008).

A key ecological feature of springs is variation along distance-from-watercourse (moving perpendicular away from the wa-tercourse towards uplands) and elevational gradients within landscapes (Bradley 1970; Patten et al. 2008a). Patten et al. (2008b) categorized the spring-upland gradient of Mojave and Great Basin Desert springs into four segments. Closest to the spring and associated downhill-flowing water-course, wetlands receive surface flow or have water-saturated soils and contain obligate wetland species such as Juncus mexicanus Willd. ex Schult. & Schult. f. (Mexican rush) and Schoenoplectus americanus (Pers.) Volkart ex Schinz & R. Keller (chairmaker’s bulrush). Moving further from the watercourse, mesoriparian (wetland-upland transition) communities have drier but still moist soils, a shallow water table, and a mix of herbaceous and woody species. The phreatophytic upland zone, not directly influenced by surface water, has a moderately deep water table and species composition commonly in-cluding Sporobolus airoides (Torr.) Torr. (alkali sacaton) and Atriplex spp. (saltbush). Lastly, uplands are furthest from the spring and contain non-riparian communities typi-

80 Natural Areas Journal Volume 34 (1), 2014

cal of the regional desert vegetation. In ad-dition to this watercourse-upland gradient immediately surrounding springs, spring communities can markedly differ across broader elevational gradients from desert to forested communities within landscapes, but spring community-elevational relation-ships are thought to be complex and require further study (Meretsky 2008). While high elevation communities in general are often thought to be less invaded by exotic spe-cies because of extreme climates and re-moteness from anthropogenic activities in lowlands, this may be changing (Pauchard et al. 2009).

Along the watercourse-upland gradient, the moist, resource-rich areas near springs are predicted to be most prone to exotic plant invasions based on a general supposi-tion that species- and resource-rich areas within landscapes are the most invasible (Stohlgren et al. 1999; Hood and Naiman 2000; Tabacchi and Planty-Tabacchi 2005). When ‘hotspots’ of native species diversity are most invasible by exotic plants, this is a concern for native biodiversity conserva-tion if native species are reduced or locally extirpated by the invasion (Fridley et al. 2007). Moreover, alteration of these unique areas may impact their functions (e.g., hy-drology) that can have broader implications across landscapes (Weissinger et al. 2012). Evaluating the relative status of invasion of springs compared with uplands is hindered by a paucity of data regarding native-exotic species relationships and few surveys of exotic plants around desert springs (Fleish-man et al. 2006). The general postulate that native and exotic species richness are positively correlated remains uncertain for desert springs and is important to evaluate because exotic species invasions could impede spring conservation.

Most springs in the Southwest have been anthropogenically modified by some type of historical or more recent development (Sada et al. 2001; Weissinger et al. 2012). These developments typically entail exca-vating springs to create small reservoirs or placing some type of artificial basin within the spring, or capturing some or all the spring outflow and piping the water to a storage reservoir (Sada et al. 2001). Surface flow is typically reduced or elimi-

nated by these developments. Ranchers and wildlife managers often intended that the developments provide better sources of free water for livestock or wildlife such as Ovis canadensis (bighorn sheep; Broyles 1995; Rosenstock et al. 1999; Dolan 2006). Rosenstock et al. (1999) suggested that a key research need is assessing the effects of the developments on plant communities (e.g., whether the spring-upland gradient in plant composition remains detectable). The authors highlighted challenges to these assessments, including few to no undevel-oped springs available for comparison and difficulties of experimentally manipulating springs. Assessments of existing spring conditions are important because most springs are presently heavily modified, which is the condition confronting resource managers when planning spring conserva-tion strategies.

This study was undertaken to improve ecological understanding of plant com-munities of springs on a desert-forest landscape, including low-elevation arid and high-elevation semi-arid springs, and to evaluate status of exotic species invasion as a measure of the ecological condition of springs. The specific objectives were

to compare plant community composition along distance-from-watercourse gradients and to examine native/exotic richness and cover relationships along watercourse-upland gradients. We discuss the results in relation to the development history of the springs and suggest implications for evaluating natural reference conditions and conservation of springs.

METHODS

Study Area

We conducted this study within the 307,743-ha eastern portion of the Desert National Wildlife Refuge, managed by the U.S. Fish and Wildlife Service, in southern Nevada, U.S.A., in a transitional area between the northern Mojave and southern Great Basin Deserts (Figure 1). Topography is basin and range, consisting of valleys between north-south trending mountain ranges. Elevations range from 745 m near the Las Vegas Valley to 3020 m on Hayford Peak in the Sheep Mountain Range (Ackerman et al. 2003). Major up-land plant communities, from low to high elevation, include Larrea tridentata (Sessé

Figure 1. Location and map of the eastern portion of the Desert National Wildlife Refuge, southern Nevada, and 12 springs sampled by this study numbered according to Table 1.


& Moc. ex DC.) Coville (creosote bush) and Coleogyne ramosissima Torr. (black-brush) shrublands, Pinus monophylla Torr. & Frém. – Juniperus osteosperma (Torr.) Little – Juniperus scopulorum Sarg. (pin-yon pine-Utah juniper-Rocky Mountain juniper) woodlands, and Pinus ponderosa Lawson & C. Lawson (ponderosa pine), Pinus longaeva D.K. Bailey (Intermoun-tain bristlecone pine), and mixed conifer forests (Bradley 1964; Ackerman et al. 2003). Precipitation averages 11 cm/yr at the lowest elevations, with greater amounts (often exceeding 30 cm/yr including snow) at the higher elevations (Ackerman et al. 2003). Livestock grazing is not authorized on the refuge, but exotic Equus asinus (wild burro) and native herbivores such as Lepus californicus (jackrabbit) and Ovis canadensis nelsoni (desert bighorn sheep) inhabit the area.

Ackerman et al. (2003) listed approxi-mately 30 mapped springs within the study area, all of which had been developed with the intent to improve the availability of free water. The springs were typically developed by installing some type of catch basin and pipes to move water (Figure 2). We randomly selected equal numbers of low- (in desert communities) and high-elevation (forest communities) springs for sampling. With two springs unable to be located, we sampled 7 low-elevation and 5 high-elevation springs (Figure 1, Table 1). Partial records of spring discharge have been kept for some springs since 1946 by the Fish and Wildlife Service (Corn Creek, Nevada). The more recent, available records indicate that surface discharge varies among the springs from only saturating the soil at the spring head, to 128 L/h (Table 1).

Data Collection

Along the watercourse downhill from each spring, we established three, 40-m long transects perpendicular to and crossing the watercourse. Transects were equidistant from each other, starting at a randomly selected distance within the first third of the stream length from the spring source (Figure 3). The downhill length of water-courses varied among springs from < 5 m

(e.g., Quail Spring) to 34 m (Wiregrass Spring). The width of the watercourses also varied among springs, so we sampled 1-m × 1-m quadrats at 0, 2, 10, and 20 m from the center of and perpendicular to the watercourse. We used a quadrat size of 1-m2 to accommodate the small areas of spring-associated vegetation present at some springs (Figures 2, 3). We sampled a total of 6, 1-m2 quadrats at each distance at each spring (24 quadrats/spring). In June 2007 for the high- and October 2007 for the low-elevation springs, we visually catego-rized the areal percent cover of each plant species, including any dead annual plants rooted in each quadrat, using Peet et al.’s (1998) modified cover classes: 1 = 0.1%, 2 = 0.1 – 1%, 3 = 1 – 2%, 4 = 2 – 5%, 5 = 5 – 10%, 6 = 10 – 25%, 7 = 25 – 50%, 8 = 50 – 75%, 9 = 75 – 95%, and 10 = 95 – 100%. Nomenclature and classification of growth form and native/exotic status followed NRCS (2012). Using ArcMap ver-sion 9.2 (Esri Co., Redlands, California), we obtained the elevation, slope gradient, and aspect of springs from 10-m digital el-evation models. From the Fish and Wildlife Service (Corn Creek, Nevada), we obtained records of the developmental history of the springs and historical photographs dating to the mid – 1900s.

Data Analysis

Based on relative cover, calculated as the proportion of cover each species contrib-uted to total cover, we compared species composition among distances from water-courses for low- and high-elevation springs using non-metric multidimensional scaling ordination in PC-ORD’s ‘autopilot’ slow and thorough mode (McCune and Mef-ford 1999). We further ordinated 0- and 2-m (near spring) species composition with environmental variables and species input as vectors for joint plots to display variables correlated with community composition. We calculated the cumula-tive number of species/6-m2 and mean total and individual species cover for each distance-from-watercourse at each spring. To assess relationships between native and exotic species richness and cover separately for each distance from watercourses, we used bivariate correlation.

RESULTS

Totals of 8 exotic and 81 native taxa were recorded across all sites. The frequencies of individual species varied with distance from watercourses and among springs, both within and between elevation categories (Appendix 1). Predominant species closest to watercourses at the 0- and 2-m distances included Bromus tectorum L. (cheatgrass, an exotic species), Muhlenbergia as-perifolia (Nees & Meyen ex Trin.) Parodi (scratchgrass), Ericameria paniculata (A. Gray) Rydb. (Mojave rabbitbrush), Salix spp. (willow), and Phragmites australis (Cav.) Trin. ex Steud. (common reed) at some low-elevation springs. Upland communities 20 m from watercourses at low elevations contained species such as Bromus rubens L. (red brome, an exotic species), Coleogyne ramosissima, Ephedra nevadensis S. Wats. (Nevada jointfir), and Atriplex canescens (Pursh) Nutt. (fourwing saltbush, a species broadly distributed along the watercourse-upland gradient). Juncus mexicanus, Juncus saximontanus A. Nelson (Rocky Mountain rush), Aq-uilegia formosa Fisch. ex DC. (western columbine), and Ribes cereum Douglas (wax currant) dominated near watercourses at high-elevation springs. In high-elevation uplands, Leptosiphon nuttallii (A. Gray) J.M. Porter & L.A. Johnson (Nuttall’s linanthus, widely distributed along the watercourse gradient), Symphoricarpos longiflorus A. Gray (desert snowberry), and Elymus elymoides (Raf.) Swezey (squirreltail) were among the dominants. Only two exotic species (Taraxacum of-ficinale F.H. Wigg. [common dandelion] and Poa pratensis L. [Kentucky bluegrass]) occurred within quadrats at high-elevation springs, with four of five high-elevation springs not containing any exotic species within quadrats.

Distribution patterns of individual species translated to differences in overall species composition among springs (Figure 4). Results portrayed a pattern detectable at many springs of moist-affinity species nearest watercourses changing to dry-af-finity species closer to uplands, but some springs did not display this type of moist-dry compositional gradient. While the 0-m distance generally grouped together


among springs, compositional patterns with distance from watercourse varied among springs and were often, but not always, approximately linear trajectories. For ex-

ample, Wiregrass Spring displayed little difference in species composition within the 0 – 2 m and 10 – 20 m distances, but these broad near and far distance categories

were clearly distinguished (spring 11 in Figure 4b). Yellow Jacket Spring, in con-trast, exhibited a trajectory where species composition at 20 m moved slightly back

Figure 2. Examples of sampled springs in the Desert National Wildlife Refuge, southern Nevada: (a) Rye Patch spring in 1948 following installation of a metal tub, (b) same view in 2007, (c) Quail spring in 1948 following construction of a covered, concrete tub, (d) same view in 2007, (e) watercourse dominated by Muhlenbergia asperifolia at Upper White Blotch Spring in 2007, and (f) looking upstream at Wiregrass Spring, in a high-elevation Pinus ponderosa forest in 2008. Photos (a) and (c) by O.V. Deming and (f) by A. Sprunger; others by J.E. Craig.


to 0 m composition in the ordination space (spring 12 in Figure 4b).

Native species richness was most fre-quently greatest at the furthest distances (10

and 20 m) from watercourses compared to closer distances, while native plant cover showed no consistent trend (Figure 5). Exotic species were detected at only one high-elevation spring (Wiregrass; Appen-dix 1). At low-elevation springs, exotic species cover was lowest nearest (0 m: 1 ± 1%; 2 m: 3 ± 6%, mean ± SD) compared to furthest from watercourses (10 m: 10 ± 2%; 20 m: 8 ± 1%). Native and exotic richness were not strongly correlated at any distance from watercourses (Figure 6). In contrast, native and exotic cover exhibited r2 values ≥ 0.32 for all distances except 2 m.

DISCUSSION

Distance and Elevational Gradients

While gradients in species composition were not necessarily linear and varied among springs, ordinations and individual species distributions revealed that an over-all wetland-upland gradient was present,

as previously reported for Mojave-Great Basin Desert Springs (Patten et al. 2008b). Some of the species we detected concur with Patten et al.’s (2008b) list of typal wetland species. For instance, Patten et al. (2008b) listed Juncus mexicanus as charac-terizing wetland areas nearest springs, and this species occupied 80% of high-elevation springs in our study. Also similar to Patten et al. (2008b), we detected Phragmites australis near wetlands and in transitional areas to uplands.

In addition to the elevation-related climate of greater precipitation and cooler tempera-tures at the high-elevation springs (Acker-man et al. 2003), several other factors could be related to differences in vegetation be-tween the low- and high-elevation springs. For example, we obtained supplemental data from the Fish and Wildlife Service (Corn Creek, Nevada) on the distance from each spring to the nearest road, as roads can be vectors for exotic species inva-sions (Craig et al. 2010). Low-elevation

Table 1. Characteristics of springs sampled on the Desert National Wildlife Refuge, southern Nevada.

Figure 3. Sampling design for characterizing veg-etation at springs in the Desert National Wildlife Refuge, southern Nevada.


springs on average were 0.6 ± 0.3 km (± SD, range 0.3 – 1.2 km) from the near-est road, whereas high-elevation springs were 5.9 ± 0.6 (range 5.2 – 6.7 km). The cover of exotic species was increasingly negatively related to distance from road with increasing distances from springs, with r2 values of 0.01 (0 m), 0.09 (2 m), 0.21 (10 m), and 0.38 (20 m). Differences between the desert matrix community at the low elevations versus the forest matrix community at the high elevations could also affect the watercourse-upland gradients. At low-elevation springs, native plant cover was similar nearest watercourses (25%) to uplands 20 m away (24%); whereas, cover was 2-fold greater nearest watercourses (29%) compared to uplands (14%) at the

high-elevation springs. Shading of the understory by trees in uplands at the high elevations might have reduced understory plant cover (Bakker and Moore 2007).

Native/Exotic Species Relationships

Native and exotic species richness were not strongly correlated at any of the distances from watercourses. This finding differs from the commonly reported positive rela-tionship between native and exotic species richness in sampling studies similar to ours (Fridley et al. 2007; Souza et al. 2011). However, this result concurs with at least one other study in wetland ecosystems where Matthews et al. (2009) found that native and exotic richness were uncor-

related across 28 wetland sites in Illinois. This result also is consistent with Brooks et al.’s (2006) study of artificial watering sites in the western Mojave Desert, where native/exotic richness was not correlated (r2 = 0) in below-shrub microsites and actually negatively correlated (r2 = 0.48) in interspaces between shrubs. Additionally, Brooks et al. (2006) found that richness of exotic annuals was not significantly related to distance from water and native richness declined near water. We also report a simi-lar trend where at low-elevation springs, only two (Polypogon monspeliensis (L.) Desf. [annual rabbitsfoot grass] and Tama-rix ramosissima Ledeb. [saltcedar]) of the six exotic species displayed any trend to exhibit greater frequency near compared to far from watercourses. These two species are known to occupy moist sites, whereas the other four low-elevation exotic species, such as Bromus tectorum, inhabit dry and moist sites (Bossard et al. 2000). Occur-rence of species such as Bromus across a broad moisture gradient may account for their lack of relationship with distance from water. At high elevations, only one of five springs had exotic species recorded. This low level of invasion was consistent with previous surveys in the study area (Acker-man et al. 2003; Abella et al. 2009). Thus, there also was not strong evidence that watercourses were more heavily invaded than the surrounding uplands.

In comparison to richness, native and ex-otic cover were more strongly correlated, at least at three of four distances from watercourses (Figure 6). The distance (2 m) where there was no correlation was influenced by an unusually high exotic cover value (18%, largely Tamarix ramosis-sima) at Whitespot Spring. Removing this point resulted in a positive relationship like the other distances (r2 = 0.56). Similar to richness, exotic species comprised only a small portion of total plant cover. This por-tion varied little among distances, ranging from 3% of the total plant cover at 0 m to 6% of the total plant cover at 2 m. These results for cover differ from Brooks et al. (2006), who found that the portion of total plant cover that was exotic increased from 35% far (800 m) from watering points to 87% at the watering points.

Figure 4. Non-metric multidimensional scaling ordination of plant species composition with distance (0 m and 20 m are labeled) from watercourse (a, b) and joint plots showing correlations of environmental variables and species with community composition at 0 (c) and 2 m (d) from watercourses at springs of the Desert National Wildlife Refuge, southern Nevada. In joint plots, the direction of a vector denotes plots to which the variable was positively correlated and the length of a vector is proportional to the strength of the correlation (minimum r2 for display = 0.25). Species are abbreviated as the first three letters of the genus and species and can be matched up to the species list in Appendix 1 (e.g., MUHASP = Muhlenbergia asperifolia). Springs are numbered 1 to 12 corresponding to Table 1. Among the ordi-nations, stress for a 2-dimensional solution ranged from 8-17 and the 2 axes cumulatively represented 45-90% of the variance of the original distance matrices.


Our finding that neither native nor exotic species were consistently abundant nearest springs compared to uplands, and in fact that richness and cover were often lowest near springs, implies that the environment near some springs was not most favorable for plant establishment (Figures 5, 6). There could be a variety of reasons for this, such as grazing or trampling by watering animals near springs – or minimal surface water present at many springs, such that resources were not necessarily greatest near springs. Free surface water is a key feature for supporting both aquatic and terrestrial biodiversity in arid land springs (Keleher and Rader 2008).

Developmental History

Data on the conditions of the springs prior to their modification by developments are not available, and similar to many landscapes, current unmodified springs

also are not available for comparison (Rosenstock et al. 1999). The develop-ments likely have influenced hydrology, vegetation, and habitat of the springs in unknown, but probably profound, ways. Typically, developments such as occurred at the study springs reduce, eliminate, or alter (e.g., by piping water away from the springs) surface flow (Sada et al. 2001). Additionally, based on descriptions of construction activities and photos of the post-construction springs, vegetation might have been removed directly by the construction activities (e.g., Figure 2a,c). We detected most of the wetland species Ackerman et al. (2003) documented in their flora of the study area, and these authors noted that there were few riparian species on the refuge relative to its size and hy-pothesized that this paucity was related to extensive development of the springs. On the other hand, the possibility that reduced surface flow has decreased the invasibility of the springs by exotic species cannot be presently dismissed.

Conservation Implications

A positive finding of this study for indig-enous ecosystem integrity was that springs were not heavily infested by exotic plants compared to what might be expected from literature on invaded riparian ecosystems (e.g., Hood and Naiman 2000; Tabacchi and Planty-Tabacchi 2005) and on the concept that springs are resource-rich sites that of-ten are most prone to invasion (Matthews et al. 2009). Conversely, from an ecosystem standpoint, a negative observation is that the springs have been heavily modified by anthropogenic activities (Ackerman et al. 2003). Because of the extensive anthropogenic development of the springs and a lack of reference information prior to development, it is difficult to estimate precisely which (if any) other species, and amount of vegetation, might be present if the springs had not been developed. Our finding that the springs generally contained little wetland vegetation is consistent with conclusions of a botanical survey of the study area (Ackerman et al. 2003). Devel-opment of natural springs with the intent to augment free water available to wildlife is controversial because of negative ecosys-tem-level tradeoffs and unclear benefits to wildlife over the natural hydrology of the springs with which wildlife have evolved (Broyles 1995; Rosenstock et al. 1999; Dolan 2006). It is important to recognize that developing natural springs is a different situation from creating artificial watering points for wildlife (Rosenstock et al. 1999). These artificial watering points are often created to counteract habitat degradation, such as fragmentation that limits wildlife movement to natural water sources.

Several conservation and management strategies for the springs might be possible depending on management objectives and keeping in mind that reference conditions are poorly known because of a lack of un-disturbed springs for comparison (Keleher and Rader 2008; Weissinger et al. 2012). First, passive management is possible, al-though this does not necessarily mean that the springs will not still change in some way. Second, springs could be largely pas-sively managed but monitored and treated for exotic species to attempt to maintain the current status of relatively low levels

Figure 5. Native plant species richness and cover at low- and high-elevation springs with increasing distance from watercourses (from 0 to 20 m) in the Desert National Wildlife Refuge, southern Nevada. Springs are numbered 1 to 12 corresponding to Table 1. ‘A’ is the average of low- or high-elevation springs. Error bars are standard deviations.


of invasion. Third, especially at springs with the least amount of native wetland vegetation (Figure 5), actively augmenting or establishing wetland plants may be a management goal. This could be attempted to improve function, including shading the spring, stabilizing soils, or improv-ing habitat for species benefitting from riparian vegetation (Loheide et al. 2009). Achieving ≥ 50% survival when outplant-ing greenhouse-grown riparian species is possible in the Mojave Desert (Abella and Newton 2009), and establishing wetland plants in higher elevation forests also is feasible (Steed and DeWald 2003). Plants might need to be protected from herbivores and provided with supplemental water to become established, especially if the cur-rent altered hydrology of the springs limits establishment. Seeding has not been tested

as extensively as planting for augmenting vegetation around springs, but if propagule availability is more limiting than other fac-tors, seeding might be effective (Turnbull et al. 2000).

Fourth, hydrological treatments could be attempted to enhance surface flow (Sada et al. 2001). These types of treatments might range from thinning dense tree stands at the high elevations (Brown et al. 1974) to recontouring topography of the springs (Unmack and Minckley 2008). These treat-ments should be considered experimental (Unmack and Minckley 2008) and account for potential influences on water levels of climate change and ground-water pumping (Deacon et al. 2007; Mayer and Congdon 2008). Other types of activities, such as transporting water to springs with little

or no flow and establishing a slow-release water system, could be part of intensive management strategies (Bainbridge 2007). Elements of all these strategies could also be considered to address spring-specific management needs in a comprehensive landscape framework. Management strate-gies would depend on functional objectives, ideally optimizing ecosystem values of the springs. These values would likely include providing free water to wildlife, support-ing native wetland plant communities, and promoting habitat for amphibians and other organisms.

ACKNOWLEDGMENTS

This study was funded as part of the Weed Sentry Program by the Clark County Mul-tiple Species Habitat Conservation Plan through a cooperative agreement between the National Park Service (Lake Mead National Recreation Area [LMNRA], in particular K. Turner and A. Newton) and the University of Nevada Las Vegas, and was supported by A. Newton and C. Norman of LMNRA. We thank the Desert National Wildlife Refuge, U.S. Fish and Wildlife Service, in particular refuge manager A. Sprunger, for facilitating the study and providing access to records and photos of the springs. We also thank D. Bangle for identifying plant specimens, S. Altman for creating figures, and S. Altman, D. Sarr, and an anonymous reviewer for helpful comments on the manuscript.

Scott Abella was an Associate Research Professor at the University of Nevada Las Vegas and is presently an Ecologist with the Washington Office of the National Park Service. He has an applied ecological research focus in the areas of ecological restoration, plant ecology, and exotic spe-cies management.

Jill Craig completed her M.S. in Landscape Architecture at the University of Wiscon-sin-Madison and worked as a Research Assistant at the University of Nevada Las Vegas during this study. Currently, she is the Environmental Compliance Coordina-tor for Boulder City, Nevada.

Figure 6. Relationship of native and exotic species richness and cover at distances from the watercourse at low- and high-elevation springs in the Desert National Wildlife Refuge, southern Nevada.


Sara McPherson completed her B.A. in Environmental Studies at the University of Nevada Las Vegas and is currently pursuing a Master of Natural Resource Management at Oregon State University. Her research focuses on understanding the distribution and management of exotic plant species.

Jessica Spencer worked as a Research Assistant at the University of Nevada Las Vegas during this study, leading field crews for the mapping, monitoring, and treatment of exotic plant species on federal lands in southern Nevada. Currently, she is a Biolo-gist with the Invasive Species Management Branch in the Jacksonville District of the U.S. Army Corps of Engineers.

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Abella, S.R., J.E. Spencer, J. Hoines, and C. Nazarchyk. 2009. Assessing an exotic plant surveying program in the Mojave Desert, Clark County, Nevada, USA. Environmental Monitoring and Assessment 151:221-230.

Ackerman, T.L., J. Bair, and A. Tiehm. 2003. A flora of the Desert National Wildlife Refuge, Nevada. Mentzelia 7:1-90.

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watercourse-upland and elevational gradients in spring vegetation of a mojave-great basin desert...

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