Variation in the Structure and Nitrogen Dynamics of Mountain Riparian Zones

DOCTORATE DISSERTATION:Increased land development and air pollution are intensifying nitrogen inputs to aquatic ecosystems of the American mountainous west. As a result, the health of mountain lakes and streams are at risk. Recognition of riparian zones as important modulators of stream water chemistry and critical habitat has increased interest in riparian management and restoration. Riparian classifications that reflect differences in ecosystem function can be used to prioritize management and restoration efforts and to develop landscape models of ecological processes. I examined structural and floristic variation in riparian zones of Lake Tahoe Basin and developed an ecosystem type classification for the area. I used this classification to test the hypothesis that ecosystem types can be used to organize the riparian landscape into areas with similar N dynamic and water quality effects. The greatest degree of structural and floristic variation in riparian zones of the Tahoe Basin fell along a gradient of valley form and stream sinuosity. I identified 12 ecosystem types along that gradient and examined N dynamics in five of the 12 types. During 1998 and 1999, I applied N fertilizer to plots representative of these five ecosystem types in a split-plot design. I measured soil N transformations, groundwater chemistry and site factors expected to control N dynamics in each plot. I found significant differences among ecosystem types in N process rates under background conditions. Differences in denitrification were most highly correlated to soil moisture content and groundwater level, while differences in net nitrification were most highly correlated to soil redox. Three ecosystem types had indistinguishably high denitrification responses to added N, whereas the denitrification response of another type was significantly lower than the rest. Groundwater N flux also varied significantly among types under N fertilized conditions: alder-rich ecosystem types had the highest, and grass-meadow ecosystem types had the lowest, groundwater N flux. Results from this research reveal the large structural and functional differences that exist among mountain riparian ecosystem types. These results also suggest that classification of riparian zones into ecosystem types may be useful in predicting landscape scale patterns of riparian zone N dynamics and water quality effects

博士学位论文：土地开发规模扩张与空气污染加剧，正持续加重美国西部山地水生生态系统的氮输入负荷，进而威胁山地湖泊与溪流的生态健康。河岸带（riparian zones）作为溪流水体化学的重要调节因子与关键栖息生境，其生态价值日益受到重视，这也推动了河岸带管理与修复领域的研究热度提升。能够反映生态系统功能差异的河岸带分类体系，可用于确定管理与修复工作的优先级，并助力构建生态过程的景观模拟模型。本研究针对太浩湖盆地（Lake Tahoe Basin）的河岸带开展了结构与区系变异调查，并构建了该区域的生态系统类型分类体系；依托该分类体系，本研究验证了一项假说：基于生态系统类型可将河岸带景观划分为氮动态与水质效应特征相似的区域单元。太浩湖盆地河岸带的结构与区系变异，沿河谷形态与溪流曲度梯度呈现出最为显著的差异，本研究沿该梯度共划分出12种生态系统类型，并对其中5类的氮动态开展了研究。1998年至1999年间，本研究采用裂区设计，对代表这5类生态系统类型的样地施加氮肥，并测定了各样地的土壤氮转化过程、地下水化学特征，以及预期调控氮动态的立地因子。研究结果表明，在背景氮浓度条件下，不同生态系统类型的氮循环过程速率存在显著差异：反硝化作用（denitrification）的差异与土壤含水量及地下水位的相关性最高，而净硝化作用（net nitrification）的差异则与土壤氧化还原电位（soil redox）的相关性最强。其中3类生态系统对添加氮肥的反硝化响应程度无显著差异且均处于较高水平，而剩余1类的反硝化响应则显著低于其他类型。施氮条件下，不同类型的地下水氮通量同样存在显著差异：富含桤木的生态系统类型地下水氮通量最高，而草本草甸型生态系统类型则最低。本研究结果揭示了山地河岸带不同生态系统类型间显著的结构与功能差异，同时表明，基于生态系统类型的河岸带分类方法，或可用于预测河岸带氮动态与水质效应的景观尺度格局。