Phosphorus-driven changes in leaf phosphorus fractions shape species-specific hydraulics in a subtropical forest

Phosphorus (P) is essential for plant physiological function, yet how P allocation among leaf fractions shapes trait responses in subtropical forests remains unclear. Phosphorus (P) is an essential nutrient for plant photosynthetic and physiological functions. In subtropical ecosystems where P is limited, the allocation of P among different functional fractions in leaf is a potentially adaptive strategy for plants to cope with low soil P availability. However, the effect of imbalanced anthropogenic P input on leaf P fractions and their contributions to leaf functional traits remains unclear. Here, we investigated the effects of P additions (0, 25, 50, and 100 kg P ha-1 yr-1) on leaf P allocation, photosynthetic, hydraulic and water-relation traits in two dominant tree species (Schima superba and Magnolia Sumatrana) and two shrub species (Melicope pteleifolia and Psychotria rubra) within a nutrient addition experiment platform in a subtropical forest. With P additions, leaf total P concentration markedly increased in trees and shrubs; metabolic P proportion increased in trees; inorganic P concentration increased in shrubs. However, P addition did not increase the photosynthetic rate or the maximum leaf hydraulic conductance in any species. Tree species showed more negative turgor loss points and higher bulk modulus of elasticity with P addition. Furthermore, S. superba exhibited significantly lower relative water content and higher relative water deficit at the turgor loss point, while no significant response was observed in M. sumatrana, P. rubra, and M. pteleifolia under P addition. These results indicate cellular adjustments that enhance turgor maintenance and mechanical support, potentially contributing to the stabilization of leaf water relations. Although P addition did not affect leaf water-relation traits in shrub species, it increased leaf mass per area. These results indicate that P addition alters leaf P allocation and water-relations traits in a species-dependent manner. Predicting forest responses to P addition will therefore require species-specific trait-based approaches rather than assuming uniform ecosystem-wide effects.