Gas exchange and water relations of Lemmon's 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 plant species that dominate these areas. We measured gas exchange and xylem water potential of Nebraska sedge (Carex nebrascensis Dewey) and Lemmon's willow (Salix lemonii Bebb) growing in a streamside location in the northern Sierra Nevada over a 2 year period. Standing biomass of both species and leaf area index of Lemmon's willow was also determined. Rooting activity of Nebraska sedge was measured the second year of the study. Measurements were taken during 1988 and 1989 with growing season precipitation 46% and 110% of average, respectively. Photosynthesis was remarkably similar for the 2 species (10.9 and 11.1 11 micromoles m-2 second-1 for Nebraska sedge and Lemmon's willow, respectively) when averaged over all dates for the 2 years. However, the 2 species exhibited different seasonal and yearly patterns of photosynthesis. Nebraska sedge maintained higher rates of photosynthesis during the early portion of the growing season and Lemmon's willow had higher photosynthesis during mid to late summer. Mean seasonal rates of willow photosynthesis were higher than those of the sedge during the drought year, and the opposite was true during the average year. Yearly average photosynthesis varied more for the sedge than for the willow. However, mean seasonal photosynthesis rates for each species were higher in an average year compared to a drought year. Nebraska sedge almost always had more negative values of xylem water potential than Lemmon's willow (overall 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 rates than Lemmon's willow during the spring of 1989. Willow biomass was similar among years, but willow leaf area index and sedge biomass were slightly greater in the wet year (1989) compared to the dry year. Contrasting growth forms and morphology of the 2 species may help explain 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.