Hemlock

Multiple interacting stressors are generating unprecedented challenges to ecosystem resilience, necessitating efforts to understand how ecosystems will respond to concurrent biotic and abiotic changes. To address this need, we examined the effects of Eastern hemlock (Tsuga canadensis) loss due to an exotic insect on nitrogen retention at three elevations (low, mid, high) subject to increasing atmospheric nitrogen deposition in mixed hardwood stands in western North Carolina. We found that nitrogen pools and fluxes varied substantially with elevation: total forest floor and mineral soil nitrogen increased and forest floor and soil carbon to nitrogen ratio decreased with elevation, suggesting that these high elevation pools are accumulating available nitrogen. Contrary to expectations, subsurface leaching of inorganic nitrogen was minimal overall and was not higher in stands with hemlock mortality. Moreover, although nitrogen loss increased with nitrogen availability in reference stands, there was no relationship between nitrogen availability and loss in stands experiencing hemlock decline. Higher foliar nitrogen and observed increases in the growth of hardwood species in high elevation stands suggest that hemlock decline has stimulated nitrogen uptake by healthy vegetation within this mixed forest, and may thereby contribute to decoupling the relationship between nitrogen deposition and ecosystem nitrogen loss.