Michael E. Loik, Department of Environmental Studies, University of California, Santa Cruz, CA 95064

Anthropogenic increases in carbon dioxide have and will continue to alter the global climate (Schneider 1989, Kacholia & Reck 1997).  Although predictions vary, it is likely that increased levels of CO2 and other greenhouse gases will result in a 1.5° to 3°C increase in global air temperatures by the middle of the next century (Watson et al. 1990).  Such warming is likely to cause altered weather patterns, increases in air and soil temperatures, changes in soil water content, with dramatic implications for ecosystems (Peters & Lovejoy 1992).  These changes are expected to influence the establishment, survival, and reproduction of plants, resulting in the enhanced recruitment, extinction, or geographic migration of certain species (Clark et al. 1998).

For the arid and semi-arid ecosystems of the southwestern United States, predicted increases in temperature may be of less consequence compared to expected precipitation changes (Easterling et al. 2000).  The major mesoscale inputs of precipitation include Continental Pacific frontal systems from the Gulf of Alaska in winter, Maritime Tropical summer monsoon precipitation from the Gulf of California, and El Nino Southern Oscillation precipitation from the mid-Pacific at varying frequencies and intensities (Ahrens 1991).  The importance of each of these inputs varies spatially and temporally across the southwestern U.S.  Model predictions suggest that seasonal patterns of precipitation change will not be the same across the entire region.  Although there is great variation among models, recent models have predicted that precipitation from the summer monsoon will increase by about 15% and that such systems will track more to the west than at present (Arritt et al. 2000).  This would result in greater summer precipitation in the western Great Basin Desert and the eastern slope of the Sierra Nevada.  On the other hand, winter changes are quite uncertain: there could be more or less snow in the Sierra Nevada in the future.  (However, current state-of-the-art computer models suggest that there will be less snow.)

Water availability in arid ecosystems is often the crucial limiting processes for plant recruitment, photosynthesis and growth, nutrient dynamics, and net ecosystem productivity (Smith et al. 1997; Reynolds et al. 1999). Natural plant establishment may be decreased by enhanced levels of stress in a future climate (Smith & Nowak 1990, Larcher 1995).  Moreover, water availability will be important for success in restoring damaged ecosystems in arid and montane habitats (Chambers 1997).  Although it seems intuitive that plants will conduct more photosynthesis when given extra water, not all species respond to additional water.  In particular, the tendency to take up water is dependent upon root architecture: woody trees with deep roots do not respond to small-magnitude summer rains, whereas some shrubs do, and many if not all herbaceous species and succulents with shallow roots exhibit considerable responses (Flanagan et al. 1992, Lin et al. 1996, Williams & Ehleringer 1996).  It is important to understand the differential responses of plants to future increases in precipitation to provide information on the potential for changes in ecosystem processes such as biomass accumulation, fluxes of energy and water, potential invasive species encroachment, and changes in fire regimes.  Such information will also inform models of migrations of ecosystem borders along natural elevation gradients (Allen & Breshears 1998, Weltzin & McPherson 2000), and will also contribute to the development of predictions and management plans regarding recovery of damaged habitats.

In this research, we measured soil and plant processes altered by 50 of snow depth manipulation using snow fences. We conducted experiments to measure the impacts of increased and decreased winter precipitation treatments on soil water content, soil surface temperature, canopy temperature, plant water potential, photosynthesis, biomass accumulation, and NDVI for Artemisia tridentata (Great Basin Sagebrush, family Asteraceae) and Purshia tridentata (Antelope Bitterbrush, family Rosaceae).  

Literature Cited

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