9527833 Casper Under models that predict long-term changes in global climate, semi-desert regions are expected to become even drier. The purpose of this study is to predict how natural populations of plants in these regions will respond to drought conditions. Most ecological studies of water use in plants have focused on anatomical or physiological adaptations that allow particular species to occupy dry habitats. There has been little effort to determine how populations and communities will respond to temporal changes in water availability. To do this, we need to understand how drought affects individuals depending on their size, developmental stage, or associations with other plants. The investigators will experimentally reduce the amount of rainfall received by natural populations of the semi-desert herbaceous perennial Cryptantha flava by erecting temporary shelters during spring rainstorms. The study is based on data collected during a natural drought year indicating that larger plants are more severely affected by drought than smaller plants. Comparisions will be made of survival, growth and flowering of individuals of different sizes and in different microhabitats, under shrubs or in the open, both in areas where water has been reduced and in areas receiving natural rainfall. These demographic data will be supplemented by measurements of water use, leaf nitrogen content, and photosynthesis. These sorts of measurements will help to determine why individuals respond to changes in water availability as they do. The experimental reduction in water will be repeated spatially and in two different years. Demographic and physiological measurements will be taken on the same individuals in the year before drought, the drought year, and the year afterwards. This replication of the experiment is important because natural variation in rainfall is likely to occur during the experiment. The study should also lead to identification of lingering effects that extend beyond the drought year itself.
