This project seeks to describe and compare long-term changes in forest composition, terrestrial nitrogen (N) availability, and stream nitrate-N export in two neighboring USGS-gaged, forested watersheds in the Adirondack Mountains of New York (USA) during a period of declining acid deposition. Shifts in tree species composition were examined in 15 permanent plots at 5-year intervals from 2000 – 2015 using the importance value (IV) index. Longterm changes in terrestrial N availability (1980 – 2014) were assessed through δ15N analysis of tree rings (20 trees per watershed). Wet atmospheric N deposition data were acquired from NADP/PRISM annual gradient maps (1986 – 2013) and combined with statistically modeled stream nitrate-N yields (1999 – 2012) to construct annual N input-output budgets.
We found pronounced differences in forest composition between watersheds; American beech had the highest IV in the southern watershed, and both American beech and red spruce had high IVs in the northern watershed. Both of these dominant tree species are generally understood to be in decline in the Adirondacks. Atmospheric N deposition has decreased by 40% since 1986, and the δ15N data suggest that terrestrial N availability has also decreased over time in both watersheds, yet the two tributaries are showing divergent trends in stream nitrate-N export. These results point to the need to integrate regional (N deposition) and site-specific (forest composition) drivers to explain long-term changes in terrestrial N availability and stream nitrate-N export in forested watersheds.