COS 207-1 - Foliar Chemistry and Resorption Following 10 Years of Experimental Nitrogen and Phosphorus Addition in New Hampshire Forests

Abstract: Long assumed to be nitrogen (N) limited, hardwood forests in the northeastern United States have also responded to phosphorus (P) and calcium (Ca) additions, perhaps due in part to a legacy of anthropogenic N deposition. These nutrient cycles are coupled, meaning that changes in the availability of one nutrient could influence availability of the others in complex ways. Foliar and litter chemistry measured over time in fertilization experiments can provide insight on responses to altered nutrient availability, on nutrient limitation, and on ways in which nutrient cycles are interrelated. We compared foliar chemistry, litter chemistry, and resorption efficiency in six northern hardwood tree species and at the community level in a long-term N and P fertilization study across 10 forest stands distributed across three sites in New Hampshire. We measured N, P, Ca, magnesium (Mg), and potassium (K) in foliage from sunlit portions of dominant and codominant trees and in fresh litter collected during peak litterfall. We calculated resorption efficiency for N and P. Foliar P concentrations increased following P addition for all species, but decreased following N addition for all species except red maple (Acer rubrum L.). Foliar Ca decreased in American beech (Fagus grandifolia Ehrh.), sugar maple (Acer saccharum Marsh.), and yellow birch (Betula alleghaniensis Britt.) under N addition, resulting in lower community-weighted mean of Ca under N addition. Litter P was higher under P addition and lower under N addition for three species, and community-weighted P was lower under N addition. However, while litter N was lower under P addition for two of the same three species, community-weighted N was not lower under P addition. Phosphorus resorption efficiency was lower under P addition for all species, but higher under N addition in many stands for several species. Litter N was highest in N-addition plots for all species except red maple, which had higher N concentrations in NP plots than N plots. These results indicate that both N and P may be limiting in this forest type and highlight differences in species’ responses to changes in nutrient availability; these differences are important in light of nutrient limitations exacerbated by global change. Ongoing analyses of 10-year tree growth responses will help to better understand these nutrient cycling responses.