Exploring continental-scale stand health – N: P ratio relationships for European forests

forest health, N: P ratios, nitrogen (N) deposition, plant stoichiometry, visual crown assessment. 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-deposi-tion levels beyond their retention capacity. We used data from a long-term forest monitoring programme (ICP Forests) to test the rela-tionship 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 tre