Exploring continental‐scale stand health – N : P ratio relationships for E uropean forests

Summary Understanding the relationship between nitrogen ( N ) availability and stand health in forest ecosystems is crucial, because a large proportion of European forests is subjected to N ‐deposition levels beyond their retention capacity. We used data from a long‐term forest monitoring programme ( ICP Forests) to test the relationship between an index of N availability, foliar nitrogen : phosphorus ( N  :  P ) ratios, tree defoliation and discoloration. We hypothesized a segmented response of stand health to N  :  P ratios and an improved model‐fit after correcting for climatic covariates. In accordance with the hypothesis, we found a segmented response with a breakpoint for conifer defoliation at N  :  P ratios as low as 7.3. Inclusion of climatic variables improved the fit of the models, but there was significant collinearity with N  :  P . Increases in N availability appear, at least for conifers, to have a negative effect on tree health even under N ‐limiting conditions. Regulation of N ‐deposition levels is consequently as timely as ever. We propose that increases in tree defoliation, other than resulting in serious plant fitness issues, may represent early diagnostic symptoms of N ‐addition related imbalances.