Carbon and water relations of Salix monticola in response to winter browsing and changes in surface water hydrology: an isotopic study using delta C-13 and delta O-18

To ascertain whether browsing or hydrologic conditions influence the physio logical performance of Salix and whether Salix and graminoids (Carex) use a nd possibly compete for similar water resources, we quantified the in situ seasonal patterns of plant water and carbon relations over three growing se asons. Our studies were designed to address the physiological factors which may be responsible for poor woody plant regeneration in montane riparian h abitats of Rocky Mountain National Park, Cole. As these systems act to insu re the delivery of fresh water to downstream users, the maintenance of thei r integrity is critical. We quantified plant water potentials, instantaneou s rates of carbon fixation, leaf carbon isotope discrimination (Delta), lea f nitrogen content and water sources using stable isotopes of water (delta( 18)O). The carbon and water relations of Salix were significantly affected by winter browsing by elk and in some cases by landscape position with rega rd to proximity to active streams. Winter browsing of Salix by elk signific antly increased summer plant water potentials and integrative measures of g as exchange (Delta), though browsing did not consistently affect instantane ous 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 stu dy period. Using a two-member linear mixing model, from delta(18)O values w e calculated that Salix appears to rely on streams for approximately 80% of its water. In contrast, the graminoid Carer derives almost 50% of its wate r from rainfall, indicating divergent water source use by these two life fo rms. Based on these findings, winter browsing by elk improved Salix water b alance possibly by altering the shoot to root ratio which in turn leads to higher water potentials and higher degrees of season-long gas exchange, whi le experimental damming had in general no effect on the physiological perfo rmance of Salix plants. In addition, as the water sources of Salix and Care x 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 perform ance 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 fl ows, the browsing resistance of Salix and ecosystem regeneration may be gre atly hindered because the reliance of Salix on stream water makes it vulner able to changes in surface water and hydrological conditions.