Abstract

Keywords

Tree-ring, foliage, nitrogen, carbon, isotope, archive

Abstract

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 (d15N) and carbon (d13C) 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 d15N and d13C values were less negative compared to foliar d15N and d13C 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 d15N were not reflected in tree-ring d15N, with much higher variations in tree-rings compared to foliage. Soil N availability and N deposition were partially able to explain changes in foliar d13C, while there were no significant correlations between environmental variables and either tree-ring or foliar d15N. Our results suggest an uncoupling between the N isotopic composition of tree-rings and foliage. Consequently, tree-ring d15N values are inappropriate proxies of foliar d15N values under low-to-moderate N deposition loads. Furthermore, at such low levels of deposition, tree-ring d15N values are not recommended as archives of tree responses to soil C/N or bulk N deposition.

LWF Classification

Network: LWF, Sites: Alptal;Beatenberg;Bettlachstock;Lausanne;Othmarsingen;Vordemwald, Category: ISI,