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 & GE; 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.

Automated summary

The availability of nitrogen and phosphorus has a significant impact on forest productivity and function, but predicting tree responses is challenging due to trait plasticity among different size classes and tree species. A field experiment in an evergreen broadleaved forest showed that phosphorus addition increased the relative growth rate of small trees. However, nitrogen and nitrogen-phosphorus combination treatments did not affect tree growth rate. Small trees exhibited greater trait plasticity in response to phosphorus addition, while large trees did not show any changes in functional traits. The results suggest that small subtropical trees have a higher capacity to respond to nutrient alterations and may potentially affect species abundance in the future.