Individual tree species responses to concurrent nitrogen and sulfur deposition across the contiguous United States
Kevin Horn1, R. Quinn Thomas2, Linda H. Pardo3, Christopher M. Clark4, Mark E. Fenn5, Gregory B. Lawrence6, Steven S. Perakis7, Erica A.H. Smithwick8, Douglas Baldwin9, Sabine Braun10 and et al.11
Atmospheric deposition of nitrogen (N) and sulfur (S) concurrently impact temperate forests through similar and diverse mechanisms. Both N and S compounds contribute to soil acidification; however, N deposition also affects individual tree physiology and stand competition through eutrophication. Tree responses to S and N deposition can be difficult to separate, especially since N and S deposition often originate from common sources. Additionally, tree responses to deposition are controlled by individual species physiologies and the influence of local soil and climatic factors. Understanding the impacts of S and N deposition, both separate and concerted, on demographics of tree species will help identify how forests and forest resources are changing, and facilitate setting response thresholds for N and S deposition.
Here we examine tree growth and survival rates against modeled N and S deposition and climate at the species level for the contiguous United States. Of the 94 species examined, growth and/or survival of 74 and 66 tree species were associated with S and/or N deposition, respectively. Only 5 species showed no relationship with S or N deposition in either growth or survival. Of the 94 tree species, 30 tree species increased in growth with N deposition, 6 decreased, and 19 increased then decreased while 39 showed no relationship with N deposition. For survival, 5 increased, 6 decreased, and 26 increased then decreased while 57 did not change across N deposition gradients. Only non-positive responses to S deposition were allowed in the models, for which 41 and 51 of the 94 species showed negative growth and survival responses, respectively. When N deposition was considered without S deposition in the models, growth and survival responses associated with N deposition decreased from an average rate of 17 Δkg C/Δkg N to 9.9 Δkg C/Δkg N and -0.065 Δ%P(s)/Δkg N ha-1yr-1 to -0.441 Δ%P(s)/Δkg N ha-1yr-1. Surprisingly, the presence of S deposition in the models affected observed N deposition responses regardless of the correlation between N and S inputs. Nonetheless, forest sensitivities to N and S deposition seemed to be associated with species-specific mechanisms as co-occurring species often demonstrated contrasting responses. These relationships of tree species dynamics across highly variable N and S deposition gradients provide estimates for N and S deposition impacts on forests in the U.S. and identify quantifiable thresholds for species responses to N and S deposition.
1Forest Resources and Environmental Conservation, Virginia Tech, Cheatham Hall, Blacksburg, VA 24061, firstname.lastname@example.org 2Forest Resources and Environmental Conservation, Virginia Tech, email@example.com 3USDA Forest Service, Northern Research Station, Burlington, VT 05405, firstname.lastname@example.org 4US EPA, National Center for Environmental Assessment, Washington, DC 20460, email@example.com 5USDA Forest Service, Pacific Southwest Research Station, Riverside, CA 92507, firstname.lastname@example.org 6U.S. Geological Survey, Troy, NY 12180, email@example.com 7Forest and Rangeland Ecosystem Science Center, US Geological Survey, Corvallis, OR 97331, firstname.lastname@example.org 8Department of Geography, The Pennsylvania State University, University Park, PA 16802, email@example.com 9Department of Geography, The Pennsylvania State University, University Park, PA 16802, firstname.lastname@example.org 10Umeå Plant Science Centre, Umea, Swedish University of Agricultural Sciences, Sweden, email@example.com 11Umeå Plant Science Centre, Umea, Swedish University of Agricultural Sciences, Sweden, firstname.lastname@example.org