Document: KAR-3-32-32

Differences in leaf gas exchange and water relations among riparian woody species along a diverted mountain stream in the eastern Sierra Nevada.

ALSTAD, K.P.* and S.C.HART

Northern Arizona University, Flagstaff, AZ 86011 USA ¹

Abstract:
Leaf gas exchange and water potential were measured on five woody species above and below a hydropower plant (2020 m) along Bishop Creek (eastern Sierra Nevada mountains, CA) where the greatest potential contrast in stream flow levels were possible. The study included Pinus jeffreyii, Populus trichorcarpa (both mature and saplings), Betula occidentalus, Sheperdia argentea, and Salix spp. The Pinus, which are only present within riparian areas at this elevation, appear to be an obligate riparian species. The Betula and Populus species are subdominants, and sapling-sized poplar occur in both the low and the high flow areas, above and below the power plant respectively. The Sheperdia species are only a small component of the community in both areas, although these species have a notable presence in historically water stressed riparian ecosystems of neighboring watersheds. Predawn and midday water potential levels were measured once each month during the growing season (June through September) on all five species using a pressure chamber. Net photosynthesis (Pn) and stomatal conductance to water vapor (Gw) were collected monthly under full sunlight conditions using a LICOR portable photosynthesis system. Predawn leaf water potential values were higher in Populus (large and small), Salix, and Betula species in the high flow reach during June and July. Midday Pn also reflected the differences in stream flow level in June with higher rates in Betula, small Populus, and Salix species in the high flow site. Water diversion did not alter leaf gas exchange and water relations for Pinus and Sheperidia individuals at any time during the growing season. Sheperdia maintained the highest seasonal rates of Pn and Gw while maintaining the most negative leaf waterpotential values of all the species examined. These results suggest potential physiological mechanisms by which Sheperdia may out-compete neighboring riparian species under water-stressed conditions.

Keywords: plant water relations, riparian ecosystem, gas exhange

This abstract is being presented at: 3:30 PM in session:
WATER RELATIONS