Global distribution and drivers of forest biome foliar nitrogen to phosphorus ratios (N:P)

Aim: The aim was to create global maps of foliar nitrogen-to-phosphorus (N:P) ratios across ecosystems, based on modelled climate, soil, and N and P deposition data, to identify global drivers of woody vegetation N:P ratios and to explore the role of genetic legacy (phylogenetics) in foliar N:P ratios of woody plants. Location: Woody cover globally. Time period: Present; data collected from 1990 to 2016. Major taxa studied: Woody plants. Methods: We compiled a database of 20,851 foliar N:P records and assigned them into boreal, temperate coniferous, temperate broadleaved and tropical groups. We applied neural networks to predict N:P global distribution maps, generalized linear models to assess environmental drivers and generalized linear mixed models to disentangle the effect of genetic legacy. Results and main conclusions: Foliar N:P ratios are negatively associated with latitude, with higher N:P ratios occurring in tropical forests and lower N:P ratios in boreal forests. Globally, N:P ratios indicate greater levels of P limitation than N limitation. The influence of environmental factors varied among the four forest biomes, probably owing to contrasting combined environmental conditions; this finding would have been obscured had we conducted a single "forest biome" analysis. Genetic legacy explained significant variation in woody plant foliar N:P ratios, and we suggest its inclusion in future studies to improve N:P ratio predictions.