δ15Nitrogen in tree rings of temperate forest trees: Indicators of past disturbance

Background/Question/Methods

Forest disturbances can result in a temporary increase in soil nitrification and nitrate (N0₃^-) losses from forest soils to nearby streams. Nitrate leaching can alter soil nutrient balances, lead to soil acidification and contribute to declines in biodiversity of nearby aquatic ecosystems. Previous studies have shown that clear-cutting induced nitrification, followed by NO₃^- losses from soils, can lead to elevated δ¹⁵N values of remaining N in soils and foliage. As trees reforest an area following a clear-cut, nitrification rates tend to decline, bringing soil and foliar δ¹⁵N values to background levels. In this study, we sought to determine whether δ¹⁵N values of bole wood of trees could be used as an indicator of past pulses of elevated nitrification followed by NO₃^- leaching. Since trees obtain N from the soil, we expected that immediately following a clear-cut, δ¹⁵N in tree rings would spike and then return to pre-disturbance levels. We sampled bole wood from individuals of American Beech (Fagus grandifolia) and sugar maple (Acer saccharum), two dominant tree species of northeastern hardwood forests. Wood was collected from mature trees growing in forest sites without recent disturbance and from trees growing following a forest cut at the Hubbard Brook Experimental Forest in New Hampshire.

Results/Conclusions

Our results show that for both species, δ¹⁵N values were greater in wood from the cut watershed compared to the undisturbed sites for the first three years following the forest cut. These results suggest that δ¹⁵N of bole wood from sugar maple and American beech trees can be used as indicators of past pulses of nitrification followed by NO₃^- losses, illustrating that δ¹⁵N within tree rings can potentially be used as an indicator of past disturbances in temperate forest ecosystems.