Landscape-driven Variation in Leaf C:N:P Stoichiometry of Rocky Mountain Conifers

Organic carbon (C) and reactive nitrogen (N) are vital to ecosystem productivity but are not evenly distributed across landscapes. Anthropogenic N pollution and rising atmospheric CO2 levels are rapidly altering nutrient cycles and distribution by increasing N supply in already eutrophic systems and forcing oligotrophication in those that are nutrient poor. Detecting patterns in nutrient cycling and oligotrophication across landscape gradients can prove difficult due to high variability at smaller scales. This project aims to identify landscape-driven patterns of nutrient source and limitation in a high elevation forest, where aspect and slope are major drivers of the ecosystem water and energy budgets. The most recent 2-year cohorts of foliage from Pinus contorta, Abies lasiocarpa, and Picea englemanniiwere sampled across a north facing slope, at foot- and toe-slope positions, and a South facing slope from back-slope to toe-slope. Samples were analyzed for concentrations and stable isotope ratios of C and N, while phosphorus extractions for further stoichiometric analysis are currently underway. Slope position strongly dictates nitrogen concentration, d15N, and intrinsic water-use efficiency derived from d13C values; trees with higher slope positions were potentially more N and water limited than those on the toe slope. Stoichiometry and d15N indicate that P. contorta may have higher uptake and a different source of N than the other species. This better understanding of foliar C:N:P stoichiometry across forested landscapes may help guide ecosystem management in varying nutrient limited environments with ongoing global change.