Long-term changes in ecosystem nitrogen availability and nitrate export in two neighboring watersheds in the Adirondack Mountains

Background/Question/Methods

This project seeks to identify the controls on nitrogen (N) retention in forests by highlighting the long-term changes in forest composition, atmospheric N deposition, stream nitrate export, and soil N availability in two neighboring USGS-gaged watersheds (the North and South Tributaries) of Buck Creek in the Adirondack Mountains. Shifts in tree species composition were assessed using the importance value (IV) index in 15 permanent observation plots in each watershed in 2000, 2005, and 2010. The N input-output budget was constructed using atmospheric N deposition data from NADP and statistically modeling annual stream N yields. Long-term changes in N availability were assessed through δ¹⁵N analysis of 40 tree cores (20 trees per watershed). 

Results/Conclusions

We found pronounced differences in forest composition between watersheds; American beech had the highest IV in the South, and both American beech and red spruce had high IVs in the North. Both of these dominant tree species, however, are generally in decline due to beech bark disease and unexplained spruce decline. Atmospheric N deposition has decreased by 40% since 1986, yet the two streams are showing divergent trends in stream nitrate export. Even though both catchments are experiencing forest decline, δ15N analysis of annual tree rings suggests that ecosystem N availability has also decreased over time in both catchments since 1980. These results emphasize the complexity of N biogeochemistry, and the need to integrate regional (N deposition and climate) and site-specific (forest composition and health) drivers of N retention in forests.