Response of the nitrogen isotopic composition of tree-rings following tree-clearing and land-use change.

Clear-cutting of forests affects the nitrogen cycle and the nitrogen isotopic composition of bioavailable ammonium and nitrate in the soil. Here, we have used nitrogen isotopic variations of tree-rings in red oak (Quercus rubra) and white oak (Quercus alba) as indicators of changes in the nitrogen cycle on a local scale. The delta15N values of late-wood from trees at two remnant forest stands in Ontario, Canada, that underwent large-scale tree-clearing and permanent land-use change at different times were measured. Trees from the perimeter of each stand record a marked 1.5-2.5 per thousand increase in the delta15N values of their tree-rings relative to the values in trees from the center of the stand, with the shift synchronous with the tree-clearing and land-use change. This shift was most likely due to increased rates of nitrification and nitrate leaching in the soil as a result of tree-clearing combined with permanent changes in hydrology and probable fertilizer use accompanying the change in land-use. Nitrogen concentration in tree-rings was not affected bytree-clearing and the associated change in land-use. These results indicate that changes in nitrogen cycling in forest ecosystems, whether due to climate change, land-use change, or other environmental changes (increased O3, other atmospheric pollutants, insects, etc.), can be faithfully monitored with nitrogen isotopic compositions of tree-rings and that dendrogeochemical analysis can be incorporated into studies of the effects of long-term anthropogenic effects on forest ecosystems.