Forests in northeastern North America are influenced by varying climatic and biotic factors; however, there is concern that rapid changes in these factors may lead to significant changes in ecosystem processes such as decomposition and nutrient cycling. Climate change (especially warming) is predicted to increase rates of decomposition in northern latitudes. Warming in winter may result in complex effects including decreased levels of snow cover and an increased incidence of soil freezing that will affect decomposition. Along with these changes in climate there are concomitant changes in the biota in the region due to a variety of factors. A notable example has been the increase in moose densities. Our research focused on how forest structure and nutrient cycling responds to these changes in climate and moose activity. Our approach evaluated how the multiple stresses of climate change and herbivory affected ecosystem processes. We measured decomposition rate, release and fate of nitrogen (N) from 15N labeled sugar maple (Acer saccharum) leaf litter and moose feces over 20 months in reference and snow removal (to induce soil freezing) plots in two separate experiments at the Hubbard Brook Experimental Forest in New Hampshire, USA. Experimental plots contained saplings of balsam fir (Abies balsamea), sugar maple or Viburnum alnifolium, and selected plots received simulated browsing. To understand ecosystem response to these multiple stresses, the contribution of 15N released from moose feces was measured in soil lysimeters, soils, microbes and plant tissues.
Results/Conclusions
Soil freezing decreased the decomposition of maple litter, but stimulated N transfer to soil and microbial biomass. Moose feces decomposed more rapidly than maple litter, and feces N moved into the mineral soil more than N derived from litter. There was a clear indication that combined ‘multiple stresses’ (i.e., soil freezing and browsing) elicited the most marked responses. Plots dominated by balsam fir had high lysimeter NO3- concentrations compared to plots with sugar maple or Viburnum plots, indicating that individual plant species affect the responses to these multiple stresses. These results suggest that soil freezing, especially when combined with the impact of an increasing large herbivore population, increased the transfer rate of N into the mineral soil. Such changes show the importance of soil freezing and moose activity in altering the spatial and temporal patterns of ecosystem N cycling.
