Nutrient limitation in global forests: current status and future trends

Nutrients are essential regulators of the structure and function of natural ecosystems and play a key role in constraining future terrestrial productivity and carbon sequestration in response to rising CO2 concentrations and climate change. Nitrogen (N) and phosphorus (P) widely limit plant growth in global terrestrial ecosystems. Therefore understanding spatial patterns and future trends of N and P limitation can shed light on the future dynamics of forests and other terrestrial biomes. Based on current literature, here we (1) review the concept and mechanisms of nutrient limitation and how vascular plants adapt to nutrient limitation, (2) summarize the direct and indirect approaches to diagnose nutrient limitation, (3) synthesize the current understanding of global patterns of N and P limitation, and (4) discuss the future trends in N and P limitation in boreal, temperate, and tropical forests. Phosphorus limitation mainly occurs in tropical, subtropical, and temperate broadleaved forests, while N limitation mainly occurs in boreal and temperate conifer forests. Therefore P is limiting in a larger area than N in global forests and the area of P-limited forests will likely expand due to continuingly rising atmospheric CO2 and climate change. More observational, experimental, and modeling efforts are needed to better understand forest dynamics in response to changing nutrient limitation in the context of global change.