The heterogeneous response in leaf traits among seasons and plant types of an evergreen broadleaf forest in western China to nitrogen addition was regulated by fertilization intensity

Previous studies have focused on the differing response patterns of leaf functional traits (LFTs) to nitrogen (N) addition under spatiotemporal or species classification variations. However, in N-rich forest ecosystems, it remains unclear whether continuous N input regulates the sensitivity of various plant types in different seasons to simulated N deposition.

We examined how N addition at 0, 20 (LN), and 40 kg N hm^–2 a^–1 (HN) affected the variations in LFTs and trait-trait covariations among seasons (April and August) and plant types in a N-rich evergreen broadleaf forest in western China.

Along the vertical vegetation gradient within the forest, LFTs that exhibit significant seasonal variations are most prevalent in trees, followed by shrubs, while they are rare in herbs. Most plants had higher C and P concentration in August than in April. The HN treatment reduced the seasonal variation in C concentration of trees and herbs, while it accentuated that of shrubs. Additionally, HN significantly decreased the differences in C and P between trees and both shrubs and herbs, while enhancing the differences in leaf N between shrubs and herbs in August. Only the scaling exponent of the N-P allometric function (i.e., the major regression slope of (logLNC = α logLPC + logβ)) decreased with increasing N addition.

Various ecological adaptation strategies and environmental sensitivities among plant types resulted in heterogeneous responses of plants to N addition. Meanwhile, continuous N input enhancing (weakening) the differences in certain leaf traits among species and across seasons.