Cascading Effects of Climate Change on Forest Ecosystems: Biogeochemical Links Between Trees and Moose in the Northeast USA
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The relationship between herbivores, plants and nutrient dynamics, has been investigated in many systems; however, how these relationships are influenced by changing climate has had much less attention. In the northeastern USA, both moose populations and winter climate have been changing. Moose, once extirpated from the region, have made a comeback; while locally, snow depth and duration of snow cover have declined. There is considerable uncertainty in how these changes will interact to influence forested systems. We used small experimental plots and transects along with snow removal (to elicit soil freezing and expose potential forage plants), mechanical browsing, and fecal additions (labeled with 15N) to examine ecosystem responses. We found that snow removal changed moose browsing behavior, with balsam fir more heavily browsed than sugar maple or Viburnum under low snow conditions. Soil freezing alone did not significantly alter N dynamics or selected plant responses, but there were significant interactions with moose activity. The combined effects of moose fecal additions, mechanical browsing, and soil freezing resulted in higher levels of NO3 − leaching under fir and maple, whereas Viburnum had essentially no response to these multiple factors. Our results suggest that declines in snow depth can initiate a cascade of ecosystem responses, beginning with exposure of plants to increased browsing that then triggers a series of responses that can lead to higher N losses, precipitated by decreased N demand in plants compromised by soil freezing damage. Balsam fir may be particularly susceptible to this cascade of multiple stresses.
Key wordsclimate change snow pack depth soil freezing N cycling herbivores moose 15N stable isotopes
We would like to thank Lisa Martel and Jackie Wilson for extensive assistance in both the field and laboratory. We would also like to thank John Pastor and an anonymous reviewer for helpful comments that have significantly improved this paper. This project was funded by National Science Foundation (NSF) through Grant DEB 00-75387 (Ecosystem Studies) and Grant DEB 98-10221 (Long Term Ecological Research). This research was conducted at the HBEF, which is owned and operated by the Northeastern Research Station, USDA Forest Service. This paper is a contribution to the Hubbard Brook Ecosystem Study.
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