Carbon and water relations of Salix monticola in response to winter browsing and changes in surface water hydrology: an isotopic study using δ13C and δ18O.

To ascertain whether browsing or hydrologic conditions influence the physiological performance of Salix and whether Salix and graminoids (Carex) use and possibly compete for similar water resources, we quantified the in situ seasonal patterns of plant water and carbon relations over three growing seasons. Our studies were designed to address the physiological factors which may be responsible for poor woody plant regeneration in montane riparian habitats of Rocky Mountain National Park, Colo. As these systems act to insure the delivery of fresh water to downstream users, the maintenance of their integrity is critical. We quantified plant water potentials, instantaneous rates of carbon fixation, leaf carbon isotope discrimination (Δ), leaf nitrogen content and water sources using stable isotopes of water (δ18O). The carbon and water relations of Salix were significantly affected by winter browsing by elk and in some cases by landscape position with regard to proximity to active streams. Winter browsing of Salix by elk significantly increased summer plant water potentials and integrative measures of gas exchange (Δ), though browsing did not consistently affect instantaneous rates of photosynthesis, leaf nitrogen or the sources of water used by Salix. No effect of experimental manipulations of surface water conditions on Salix physiology was observed, likely due to the mesic nature of our study period. Using a two-member linear mixing model, from δ18O values we calculated that Salix appears to rely on streams for approximately 80% of its water. In contrast, the graminoid Carex derives almost 50% of its water from rainfall, indicating divergent water source use by these two life forms. Based on these findings, winter browsing by elk improved Salix water balance possibly by altering the shoot to root ratio which in turn leads to higher water potentials and higher degrees of season-long gas exchange, while experimental damming had in general no effect on the physiological performance of Salix plants. In addition, as the water sources of Salix and Carex were significantly different, competition for water may not influence the growth, development, and regeneration of Salix. Thus, under the conditions of our study, herbivory had a positive effect on the physiological performance of Salix, but it is still unclear whether these changes in physiology transcend into improved Salix regeneration and survivorship. However, under drier environmental conditions such as lower snowpacks and lower stream flows, the browsing resistance of Salix and ecosystem regeneration may be greatly hindered because the reliance of Salix on stream water makes it vulnerable to changes in surface water and hydrological conditions.