Large old trees influence patterns of δ13C and δ15N in forests

Large old trees are the dominant primary producers of native pine forest, but their influence on spatial patterns of soil properties and potential feedback to tree regeneration in their neighbourhood is poorly understood. We measured stable isotopes of carbon (δ13C) and nitrogen (δ15N) in soil and litter taken from three zones of influence (inner, middle and outer zone) around the trunk of freestanding old Scots pine (Pinus sylvestris L.) trees, to determine the trees' influence on below-ground properties. We also measured δ15N and δ13C in wood cores extracted from the old trees and from regenerating trees growing within their three zones of influence. We found a significant and positive gradient in soil δ15N from the inner zone, nearest to the tree centre, to the outer zone beyond the tree crown. This was probably caused by the higher input of 15N-depleted litter below the tree crown. In contrast, the soil δ13C did not change along the gradient of tree influence. Distance-related trends, although weak, were visible in the wood δ15N and δ13C of regenerating trees. Moreover, the wood δ15N of small trees showed a weak negative relationship with soil N content in the relevant zone of influence. Our results indicate that large old trees control below-ground conditions in their immediate surroundings, and that stable isotopes might act as markers for the spatial and temporal extent of these below-ground effects. Copyright © 2008 John Wiley & Sons, Ltd.