Nitrogen isotope variation of tree rings as a potential indicator of environmental change

The δ15N-values of tree rings have been measured for two eastern hemlocks (Tsuga canadensis) from a site in Hanover, New Hampshire, U.S.A. δ15N-values of tree rings show a total range of −0.1 to +6.8%. Both trees show a systematic decrease of δ15N-values from the early 1960ʹs to the last year of growth. Possible explanations for the observed decrease of δ15N-values over the last 30 years include a decrease in the δ15N-values of available nitrogen, or isotope fractionation accompanying translocation. sults demonstrate the potential utility of nitrogen isotope analysis of tree rings as a method to investigate the long-term biogeochemical behavior of nitrogen. Tree-ring δ15N-values signatures can potentially procide information concerning the effects of atmospheric deposition of nitrogen upon forested ecosystems at actual levels of nitrogen deposition, as opposed to high levels of pulsed nitrogen addition over a relatively short period, as is typically the case in fertilizer studies. Additional advantages of tree-ring studies over fertilizer studies include the ability to study very long time periods, wide geographical application and the lack of need for a long-term, intensive experimental effort.