Evaluating temporal patterns in wood δ15N in Swedish forests as an indicator of changing N limitation

Boreal forests play an important role in the global carbon (C) cycle, and their productivity is strongly limited by nitrogen availability.  Thus, understanding whether nitrogen availability in boreal forests is changing has important implications for understanding past, present, and future trends of forest growth. We utilized a unique archive of tree cores collected by the Swedish National Forest Inventory, to evaluate temporal patterns (1950-2017) of wood δ15N, which is commonly used as an indicator of N limitation. First, we focused on an area of ca. 55,000 sq. km in central Sweden to evaluate how sensitive the wood δ15N approach is to tree age and two alternative sampling methodologies: a) analysis of single trees sampled in the present, versus b) tree chronologies constructed from multiple trees of the same age sampled during different decades.  By analysing 1038 woods samples, and covering two key boreal tree species (Picea abies and Pinus sylvestris), we found strong trends of declining δ15N through time, suggestive of progressive N limitation.  We further found that temporal patterns were highly sensitive to method choice, where the multiple tree approach supported by the tree core archive showed much stronger temporal patterns than reliance on more conventional contemporary sampling approaches, where N mobility appeared to obscure temporal patterns.  We further found that temporal trends were relatively insensitive to tree age class. Using the more powerful Multiple Tree Approach, we further evaluated δ15N values from an additional 1000 P. abies and P. sylvestris wood samples covering the entire forested area of Sweden and spanning the same time period, to investigate how temporal patterns in wood δ15N varied in areas with historically high N deposition (Southern Sweden) versus low N deposition (Northern Sweden).  These data help address current debates regarding whether temporal patterns in δ15N are indicative of oligitrophication (i.e. progressive N limitation), or are instead the result of changing δ15N signatures from nitrogen deposition inputs.