This proposal is directed at understanding the ability of common arid region plants to utilize summer precipitation events and the consequences of different water sources for growth and for the efficiency of dry matter production. Preliminary data indicate that several of the dominant perennial species in the Intermountain West appear not to utilize summer precipitation, even though summer rain is a major component of the annual water input to these ecosystems. It is proposed to examine several hypotheses related to a plant's utilization of summer precipitation and how this relates to the efficiency of biomass production along a gradient of increasing summer precipitation. Research will be centered on using stable isotope analyses to determine both water sources and efficiency of dry matter production. Field studies will be conducted at five sites, which range from 15% to 52% summer precipitation, but which are otherwise similar in total precipitation and temperature. The study will focus on dominant tree-subtree species in the ecosystem transitional between desert and temperate forest ecosystems: Juniper (Juniperus osteosperma), pinyon pine (Pinus edulis) and scrub oak (Quercus gambelii). There is strong potential for applied aspects of the proposed research. Stable isotope analyses provide integrated long-term estimates of plant parameters involving water use and efficiency of dry matter production. The information and techniques derived from this study will be useful in determining the kinds of characters to be used for evaluation of forest timber and pulp species. Foresters and agronomists are already beginning to use these stable isotope approaches for integrated analyses of different genetic lines.
