Global variability of leaf functional traits under nitrogen and phosphorus co-limitation

Nutrient availability drives leaf trait variation, yet how plants reconcile trait plasticity and trade-offs under individual or combined nitrogen (N) and phosphorus (P) limitations remains elusive. Our global synthesis, comprising 1380 observations across 105 sites, revealed that N and P co-limitation prevailed in 52% of the field studies. Under single-nutrient limitation, trait responses depend primarily on the limiting nutrient, whereas under co-limitation, both nutrients shape responses and often interact synergistically. Relieving nutrient constraints shifts traits toward acquisitive strategies while maintaining scaling exponents (slopes) in leaf economic trait relationships across most nutrient regimes. However, it induces shifts in elevations (intercepts) that reflect changes in resource-use efficiencies. Crucially, scaling exponents for trait pairs involving mass-based P concentration (P _mass ) reconfigure trait coordination: N addition enhanced photosynthesis in low-P _mass species, whereas P addition disproportionately reduced photosynthetic area per unit P investment in high-P _mass species. These patterns are likely driven by P _mass -related differences in nutrient allocation and functional differentiation, underscoring the need to integrate co-limitation and P-centered trait coordination into predictions of ecosystem responses to anthropogenic nutrient inputs.