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 effect