Species-specific asynchronies in the response of tree growth and mortality to N deposition at the continental-scale
Nitrogen (N) deposition has been shown to alter tree growth and mortality in species across North America and is even proposed to aid in enhancing sequestration of atmospheric CO2 by increasing forest biomass. Increased bio-availability of N can stimulate growth rates in trees but can also produce negative effects through mechanisms such as altering soil chemistry, increasing susceptibility to pathogen infection, and affecting inter- and intra-species dynamics. As a result, N deposition can drive shifts in forest composition and health as responses differ by tree species and environmental conditions. To identify species-specific responses of N deposition on tree growth and mortality across the contiguous United States we used FIA (Forest Inventory and Analysis) data with modeled N deposition rates and various geospatially available climate and soils data. Tree response data spanned the years 1984 to 2013 and contained 114 species that had at least 1,000 individual trees measured multiple times and a maximum of 476,690 individual trees for a single species. We hypothesize that species will vary in their sensitivity to N deposition and those that are sensitive will indicate negative, positive, and threshold responses to N deposition within their exposed range.
Preliminary results of our continental-scale analysis indicate that increased N deposition is associated with clear decreases, increases, and threshold responses in growth rates of individual tree species. Of the 114 species examined, 29 exhibited decreased growth over their range of N deposition, 26 increased growth over their range of N deposition, and 16 species displayed a threshold response that initially increased growth under low N deposition but negatively affected growth at higher deposition rates. Mortality rates are also expected to vary across species but not necessarily in the same patterns with growth. Further analysis of these results will aid in projecting forest carbon budgets and species diversity, and focus subsequent investigations to determine mechanisms of response including similarities across functional groups.