Intra-annual variability in isotopic and total nitrogen in tree rings of old growth Sitka spruce from coastal British Columbia

摘要

Tree-ring studies using increment cores have increasingly measured elemental (N) and isotopic (delta N-15) nitrogen values to evaluate environmental changes in the nitrogen cycle. The paucity of nitrogen in wood has constrained tree-ring analyses to annual resolution. Based on 77 rings and 310 sub-rings, we provide evidence for substantial intra-annual variability in N and delta N-15 values in the heartwood of 11 geographically widely separated, old growth Sitka spruce [Picea sitchensis (Bong.) Carriere] trees from coastal British Columbia. The range of N and delta N-15 values within rings (intra-annual level) was on average equal to or up to seven-times the range among rings (inter-annual level). Most rings showed a seasonal reduction in N values from early to late growth and a corresponding increase to the early growth of the following season (P & 0.05 to 0.001, Mann-Whitney-Wilcoxon test). By contrast, intra-annual changes in delta N-15 values were highly variable among years and among trees, ranging from pronounced oscillations (4.0 parts per thousand) to reasonable consistency. Our results allude to the potential importance of such intra-ring data for interpreting seasonal trends in nitrogen use and increasing understanding of ecological processes in the marine-terrestrial interface.