Leaf phenotypic plasticity coupled with integration facilitates the adaptation of plants to enhanced N deposition

The response of leaf functional traits can provide vital insight into the adaptive strategies of plants under global change. However, empirical knowledge on the acclimation of functional coordination between phenotypic plasticity and integration to increased nitrogen (N) deposition is still scarce. The variation of leaf functional traits of two dominant seedling species, Machilus gamblei and Neolitsea polycarpa, across four N deposition rates (0, 3, 6, and 12 kg N ha^-1yr^-1), along with the relationship between leaf phenotypic plasticity and integration were investigated in a subtropical montane forest. We found that enhanced N deposition promoted the development of seedling traits toward the direction of resource acquisition, including improved leaf N content, specific leaf area and photosynthetic performance. Appropriate N deposition (≤6 kg N ha^-1 yr^-1) might induce the optimization of leaf functional traits to promote the capability and efficiency of nutrient use and photosynthesis in seedlings. However, excessive N deposition (12 kg N ha^-1 yr^-1) would result in detrimental effects on leaf morphological and physiological traits, thus inhibiting the efficiency in resource acquisition. A positive relationship occurred between leaf phenotypic plasticity and integration in both seedling species, implied that higher plasticity of leaf functional traits likely led to better integration with other traits under N deposition. Overall, our study emphasized that leaf functional traits could rapidly respond to changes in N resource, while the coordination between leaf phenotypic plasticity and integration can facilitate the adaptation of tree seedlings in coping with enhanced N deposition. Further studies are still needed on the role of leaf phenotypic plasticity and integration in plant fitness for predicting ecosystem functioning and forest dynamics, especially in the context of future high N deposition.