Summary1. Recent elevated temperatures and prolonged droughts in many already water-limited regions throughout the world, including the southwestern United States, are likely to intensify according to future climate-model projections. This warming and drying can negatively affect perennial vegetation and lead to the degradation of ecosystem properties. 2. To better understand these detrimental effects, we formulate a conceptual model of dryland ecosystem vulnerability to climate change that integrates hypotheses on how plant species will respond to increases in temperature and drought, including how plant responses to climate are modified by landscape, soil and plant attributes that are integral to water availability and use. We test the model through a synthesis of fifty years of repeat measurements of perennial plant species cover in large permanent plots across the Mojave Desert, one of the most water-limited ecosystems in North America. 3. Plant species ranged in their sensitivity to precipitation in different seasons, capacity to increase in cover with high precipitation and resistance to decrease in cover with low precipitation. 4. Our model successfully explains how plant responses to climate are modified by biophysical attributes in the Mojave Desert. For example, deep-rooted plants were not as vulnerable to drought on soils that allowed for deep-water percolation, whereas shallow-rooted plants were better buffered from drought on soils that promoted water retention near the surface. 5. Synthesis. Our results emphasize the importance of understanding climate-vegetation relationships in the context of biophysical attributes that influence water availability and provide an important forecast of climate-change effects, including plant mortality and land degradation in dryland regions throughout the world.

Abstract-Contaminants used at low elevation,

Lake Mohave, on the lower Colorado River in Nevada and Arizona, was created by the construction of Davis Dam for power generation, flood control, and water supply. Management has led to the periodic lowering of the water level of the reservoir (drawdown), such that it reveals a gradient of zones around the margins of the reservoir that range from frequently inundated to frequently dry. The initial filling of Lake Mohave flooded the preexisting native riparian woodlands of Populus-Salix (cottonwood-willow), creating a new shoreline and plant community. We analyzed the spatial distribution of the plant species that dominate the plant community (i.e., native Salix gooddingii C.R. Ball [Goodding's willow] and nonindigenous Tamarix ramosissima Ledeb. [saltcedar]) and the soil components to discern patterns. Data analyses and modeling indicate that there are 3 emergent patterns in the distribution and composition of vegetation and soils. First, even though both S. gooddingii and T. ramosissima were present in the inundated zones, there were more mature S. gooddingii individuals in the frequently inundated reaches, while T. ramosissima presence and cover increased with distance from the water's edge. Salix gooddingii seedlings were not observed, but T. ramosissima seedlings were present in all zones. The only regeneration of S. gooddingii was vegetative. Naturally occurring Populus fremontii S. Watson (Fremont cottonwood) was completely absent in the drawdown and upland plant communities. Second, soil salinity and pH values range from 49.4 to 0 dS ⋅ m-1 and 6.4 to 9.4, respectively, and varied significantly with landform type and geographic location along the reservoir. Patterns in soil chemistry may be related to shore geomorphology that either shelters or exposes soils to wave action, which mechanically agitates, aerates, and flushes soils. Presence of Salix gooddingii in the frequently inundated zones and the co-occurrence of T. ramosissima and relatively high soil salinity concentration reflect patterns among plant flood tolerance and soil responses to periodic inundation. While reasons for the absence of P. fremontii are unknown, the absence of S. gooddingii seedlings may be related to the fact that seed release coincides with the period when the reservoir is at its highest, thereby limiting recruitment. Third, the only regeneration of S. gooddingii appeared to have occurred following herbivory (Castor canadensis Kuhl [North American beaver]) and wind damage. We conclude with suggestions for the conservation of novel riparian ecosystems as surrogates for lost native ecosystems. These suggestions include manipulating reservoir water levels to simulate natural fluvial processes so that nonnative plant establishment is inhibited, excessive soil salts are flushed from the system, and native transplants can be established.