Species divergence in seedling leaf traits and tree growth response to nitrogen and phosphorus additions in an evergreen broadleaved forest of subtropical China

Tree competitiveness generally depends on trait plasticity in response to environmental change. The effects of nitrogen (N) and phosphorus (P) on leaf trait variability by species is poorly understood, especially in China's subtropical forests. This study examined the seedling leaf traits and net primary productivity of all trees >5 cm DBH of two dominant species, Schima superba and Castanopsis carlesii, in an evergreen broadleaved forest fertilized with nitrogen (+ N), phosphorus (+ P), and nitrogen plus phosphorus (N + P). The effect of N on seedling leaf traits was stronger than P, while fertilization in general was species dependent. Leaf mass per unit area decreased with N for S. superba seedlings but not for C. carlesii. Leaf N, P, and N/P ratios changed with N addition for both species. All four N fractions of carboxylation, bioenergetics, cell wall, and other N metabolites in C. carlesii leaves responded significantly to fertilization, while only the cell wall in S. superba leaves responded. Other leaf functional traits, including light-saturated photosynthetic rates, water, N, and P use efficiencies, chlorophyll and nonstructural carbohydrate contents increased with N addition in S. superba and by P addition in C. carlesii. Canopy closure at the stand-level increased due to N. Litter biomass and relative growth rate of S. superba was not affected by any treatments, while both for C. carlesii significantly decreased with N + P addition. Collectively, nutrient limitation may vary at a small scale among species in a subtropical forest based on their responses of seedling traits and net primary productivity to fertilization. Seedling traits are not correlated with the net primary productivity of larger trees except for N fractions, because low light conditions induced by fertilization reduces the proportion of N allocated to photosynthesis in seedlings. In addition, acclimation differences of tree species may increase the uncertainty of community succession.

Automated summary

This study investigated the effects of nitrogen and phosphorus fertilization on seedling leaf traits and net primary productivity of two dominant tree species in China's subtropical forests. The results showed that nitrogen had a stronger effect on seedling leaf traits than phosphorus, and the response to fertilization varied among species. Other leaf functional traits, canopy closure, litter biomass, and relative growth rate also showed species-specific responses to fertilization.