Can tree-ring δ15N be used as a proxy for foliar δ15N in European beech and Norway spruce?

For long-term environmental investigations, tree-ring delta (15) N values are inappropriate proxies for foliar delta (15) N for both Fagus sylvatica and Picea abies under moderate N loads. Currently it is unclear whether stable nitrogen isotope signals of tree-rings are related to those in foliage, and whether they can be used to infer tree responses to environmental changes. We studied foliar and tree-ring nitrogen (delta N-15) and carbon (delta C-13) isotope ratios in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) from six long-term forest monitoring sites in Switzerland together with data on N deposition and soil N availability, as well as a drought response index over the last two decades. For both species, tree-ring delta N-15 and delta C-13 values were less negative compared to foliar delta N-15 and delta C-13 values, most likely due to recycling and reallocation of N within the tree and fractionation processes associated with the transport of sucrose and the formation of tree-rings, respectively. Temporal trends recorded in foliar delta N-15 were not reflected in tree-ring delta N-15, with much higher variations in tree-rings compared to foliage. Soil N availability and N deposition were partially able to explain changes in foliar delta C-13, while there were no significant correlations between environmental variables and either tree-ring or foliar delta N-15. Our results suggest an uncoupling between the N isotopic composition of tree-rings and foliage. Consequently, tree-ring delta N-15 values are inappropriate proxies of foliar delta N-15 values under low-to-moderate N deposition loads. Furthermore, at such low levels of deposition, tree-ring delta N-15 values are not recommended as archives of tree responses to soil C/N or bulk N deposition.