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

The δ 15N-values of tree rings have been measured for two eastern hemlocks ( Tsuga canadensis) from a site in Hanover, New Hampshire, U.S.A. δ 15N-values of tree rings show a total range of −0.1 to +6.8%. Both trees show a systematic decrease of δ 15N-values from the early 1960's to the last year of growth. Possible explanations for the observed decrease of δ 15N-values over the last 30 years include a decrease in the δ 15N-values of available nitrogen, or isotope fractionation accompanying translocation. The results 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 δ 15N-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.