Ecological and hydrological characteristics of ecosystems interact in several important ways to influence species and ecosystem responses to precipitation regime.  Soil characteristics and the surface structure associated vegetation strongly influence the vertical and horizontal distribution of available water following precipitation.  Plant root systems provide a conduit for redistribution of water between soil compartments that differ in water potential.  Finally, physiological differences among species determine drought tolerance and stomatal regulation of water loss, and thus feed back to the surface structure of the ecosystem by determining growth and survival under different precipitation regimes.  We are manipulating precipitation inputs in black grama grassland, creosote shrubland and piñon-juniper woodland at the Sevilleta LTER to understand aspects of these interactions. Rainout shelters have been used to impose extended drought by diverting ambient precipitation while overhead sprinkler systems have been employed to increase water inputs on adjacent plots.  In each case, intensive measurements of soil water content and potential have been combined with measures of plant function and productivity to determine the effect of the altered precipitation regime.

Results/Conclusions

In the grassland and shrubland, patterns of available soil water before treatment showed that available soil water following precipitation was concentrated under grass and shrub canopies with less water in bare inter-plant spaces.  During 5 years of drought treatment, grass cover decreased more than shrub cover.  Ongoing measurements of post-drought recovery will determine whether decreased grass cover persists and its influence on available water in the system.  In piñon-juniper woodland, where treatments have recently started, we hypothesized that physiological differences between species would lead to differential response to our treatments.  Treatment responses were immediately evident in soil water content and potential and in plant water potential and sap flow.  Stomatal regulation of transpiration was isohydric in piñon and anisohydric in juniper and plot scale water use by piñon was much greater than juniper.  These data suggest that the gas exchange and water transport characteristics of these species are likely to lead to different responses as drought progresses.  Moreover our data indicate that the high piñon mortality, such as observed during recent severe drought in the southwest US, will lead to significant changes in site water balance and hydrology.