Global change progressively increases foliar nitrogen-phosphorus ratios in subtropical forests

Foliar nitrogen (N) to phosphorus (P) ratios (N/P) indicate N versus P limitation in terrestrial ecosystems. Quantifying the long-term dynamics of foliar N/P and their potential drivers is crucial for predicting nutrient status in forest ecosystems under global change. Using 1811 herbarium specimens collected during 1920-2010 in subtropical forests of China, we detected significant increases in foliar N/P (21.2%) and decreases in foliar P concentrations (23.1%). Foliar N/P increased more in evergreen species (22.9%) than in deciduous species (16.9%). Changes in atmospheric CO2 concentrations (Pco2), N deposition and mean annual temperature (MAT) dominantly contributed to the increased foliar N/P of evergreen species, while Pco2, MAT and vapor pressure deficit, to that of deciduous species. Under future Shared Socioeconomic Pathway scenarios, increasing MAT would continuously increase foliar N/P by more 6.5-26.0%. The results suggest that global change progressively and non-uniformly aggravates the N-P imbalance of plant species in subtropical forests.