Alstad K P, Welker J M, Williams S A, Trlica M J
Department of Forestry, Northern Arizona University, Flagstaff, AZ 86011, USA, , , , , , US.
Department of Renewable Resources, University of Wyoming, Laramie, WY 82071, USA e-mail:
Oecologia. 1999 Aug;120(3):375-385. doi: 10.1007/s004420050870.
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 (δO). 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 δO 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.
为了确定放牧或水文条件是否会影响柳树的生理表现，以及柳树和禾本科植物（苔草属）是否利用并可能竞争相似的水资源，我们在三个生长季节中对植物水分和碳关系的原位季节性模式进行了量化。我们的研究旨在探讨可能导致科罗拉多州落基山国家公园山地河岸栖息地木本植物再生不良的生理因素。由于这些系统对确保向下游用户输送淡水起着作用，因此维护其完整性至关重要。我们使用水的稳定同位素（δO）对植物水势、碳固定瞬时速率、叶片碳同位素分馏（Δ）、叶片氮含量和水源进行了量化。柳树的碳和水分关系受到麋鹿冬季放牧的显著影响，在某些情况下还受到与活跃溪流距离相关的景观位置的影响。麋鹿对柳树的冬季放牧显著提高了夏季植物水势和气体交换的综合指标（Δ），尽管放牧并未持续影响光合作用的瞬时速率、叶片氮含量或柳树使用的水源。未观察到地表水条件的实验操作对柳树生理有影响，这可能是由于我们研究期间的湿润性质。使用双组分线性混合模型，根据δO值我们计算得出柳树似乎约80%的水分依赖于溪流。相比之下，禾本科植物苔草近50%的水分来自降雨，这表明这两种生命形式的水源利用存在差异。基于这些发现，麋鹿的冬季放牧可能通过改变地上部分与地下部分的比例改善了柳树的水分平衡，进而导致更高的水势和更长季节的气体交换程度，而实验性筑坝总体上对柳树植物的生理表现没有影响。此外，由于柳树和苔草的水源显著不同，对水的竞争可能不会影响柳树的生长、发育和再生。因此，在我们的研究条件下，食草作用对柳树的生理表现有积极影响，但这些生理变化是否会转化为柳树更好的再生和存活率仍不清楚。然而，在诸如积雪减少和溪流流量降低等更干燥的环境条件下，柳树的放牧抗性和生态系统再生可能会受到极大阻碍，因为柳树对溪水的依赖使其容易受到地表水和水文条件变化的影响。
