GAS-EXCHANGE AND WATER RELATIONS OF LEMMONS WILLOW AND NEBRASKA SEDGE

There is considerable interest in riparian zones in the western United States, yet little information is available on the autecology of plan t species that dominate these areas. We measured gas exchange and xyle m water potential of Nebraska sedge (Carer nebrascensis Dewey) and Lem mon's willow (Salix lemonii Bebb) growing in a streamside location in the northern Sierra Nevada over a 2 year period. Standing biomass of b oth species and leaf area index of Lemmon's willow was also determined , Rooting activity of Nebraska sedge was measured the second year of t he study. Measurements were taken during 1988 and 1989 with growing se ason precipitation 46% and 110% of average, respectively. Photosynthes is was remarkably similar for the 2 species (10.9 and 11.1 mu moles m( -2) second(-1) for Nebraska sedge and Lemmon's willow, respectively) w hen averaged over all dates for the 2 years. However, the 2 species ex hibited different seasonal and yearly patterns of photosynthesis. Nebr aska sedge maintained higher rates of photosynthesis during the early portion of the growing season and Lemmon's willow had higher photosynt hesis during mid to late summer. Mean seasonal rates of willow photosy nthesis were higher than those of the sedge during the drought year, a nd the opposite was true during the average year. Yearly average photo synthesis varied more for the sedge than for the willow. However, mean seasonal photosynthesis rates for each species were higher in an aver age year compared to a drought year. Nebraska sedge almost always had more negative values of xylem water potential than Lemmon's willow (ov erall average was -2.6 MPa and -1.25 MPa for Nebraska sedge and Lemmon 's willow, respectively). Trends in transpiration and conductance were similar among species, except that Nebraska sedge maintained higher r ates than Lemmon's willow during the spring of 1989. Willow biomass wa s similar among years, but willow leaf area index and sedge biomass we re slightly greater in the wet year (1989) compared to the dry year. C ontrasting growth forms and morphology of the 2 species may help expla in differences in gas exchange and xylem water potential. The ability of willows to tap groundwater and the concentration of sedge roots in the upper soil profile probably accounts for the differential response to drought.