Are small trees more responsive to nutrient addition than large trees in an evergreen broadleaved forest?

Nitrogen (N) and phosphorus (P) availability exert a profound influence on forest productivity and function. However, forecasting tree responses is still challenging because trait plasticity among different size classes and tree species remains largely unclear. We conducted a field experiment in an evergreen broadleaved forest fertilized with four treatments: +N (100 kg N ha−1 yr−1), +P (50 kg P ha−1 yr−1), N+P (100 kg N ha−1 yr−1 + 50 kg P ha−1 yr−1), and a control. We selected four species across size classes (small trees, DBH < 15 cm; large trees, DBH ≥ 15 cm) to quantify the relative growth rate (RGR) and functional traits related to the leaf economic spectrum (LES). +P increased the RGR in small trees rather than in large trees, and its effect on RGR varied with tree species in small trees. In contrast, +N and N+P did not affect the RGR in small or large trees. Small trees exhibited greater trait plasticity in light-saturated photosynthetic rate (Pmax) and photosynthetic N and P use efficiencies (PNUE and PPUE) than large trees under P addition. In contrast, nutrient addition did not change LES traits in large trees. Most notably, treatment-induced changes in RGR were significantly correlated with changes in Pmax and PNUE in small trees and with leaf mass per area (LMA) in large trees in this subtropical forest, suggesting that trait plasticity drives tree growth responses. Our results highlight that small subtropical trees have a greater capacity to respond to nutrient alterations and have the potential to change species abundance under future N and P deposition.