Characterizing Nutrient Budgets for Treeline Watersheds of the Colorado Rockies.

In high-elevation environments, basin-scale variability in streamwater chemistry and nutrient budgets relate to features of the physical environment, soil development, nutrient uptake, and biogeochemical cycling that differentiate alpine and forest ecosystems. At the Fraser Experimental Forest in central Colorado over a three-year period, we measured atmospheric deposition and streamwater export from eighteen tributaries of the Fraser River at the headwaters of the Colorado River. The proportion of basin area located above treeline ranged from 0 to nearly 70%.  Across that range, streamwater nitrate concentrations spanned two orders of magnitude and were strongly and positively correlated to the proportion of a basin situated above treeline (r² = 0.94).  Seasonal patterns for most chemical constituents are synchronized with the period of peak snowmelt in early spring.  We also developed terrain models and hydrological distance surfaces to characterize upland and riverine geomorphic variability in order to develop basin-wide characterizations of the factors that regulate biogeochemical and hydrologic processes.  This method accounts for non-linear relationships between spatially-explicit physical features of a watershed and point estimates of parameters such as soil depth, extent of exposed rock or steep slopes, and Normalized Difference Vegetation Index (NDVI) and generates a representation of the landscape relationships that contribute to biogeochemical fluxes from a watershed.  High-elevation watersheds are sensitive indicators of changing climatic conditions and atmospheric inputs to alpine and treeline ecosystems.  Greater understanding of interactions between terrain features and watershed processes will lead to better monitoring of ecosystem change in complex mountainous terrain.