Effects of nitrogen addition on the photosynthetic responses of two dominant subtropical forest tree species to drought.

Drought and nitrogen deposition are global change factors that influence forest dynamics by altering tree physiology and growth. Nevertheless, their interactive effects on tree physiology (especially leaf photosynthetic traits) remain poorly understood. Here, we examined the effects of drought (D), nitrogen addition (N) and their combination (DN) on the leaf photosynthetic traits of two dominant evergreen broadleaved tree species (Schima superba and Michelia macclurei) in a subtropical forest in southern China following a 2-year treatment period. For S. superba, compared to the control (CK), D significantly decreased the leaf photosynthetic rate (Asat), stomatal conductance (gs), stomatal size (SS) and leaf nitrogen content (LNC), but increased soluble sugar content (LSS). Although N exerted no significant effects on any leaf functional traits relative to CK, DN significantly alleviated drought-induced declines in Asat, gs, SS and LNC of S. superba when compared with D. This indicates a significant interactive effect between drought and nitrogen supply: nitrogen addition only mitigated drought stress damage under water limitation, rather than improving leaf photosynthetic performance under well-watered conditions. In contrast, M. macclurei showed no significant differences in leaf photosynthesis-related traits across the D, N, DN and CK treatments. The divergent responses of the two species to D might be related to inherent differences in their hydraulic traits (turgor loss point-TLP and minimum conductance-gmin) and root characteristics. The deep-rooted S. superba displayed significantly higher TLP and gmin than the shallow-rooted M. macclurei. Overall, our results suggest that S. superba is more sensitive to D than M. macclurei. The negative effects of D on leaf photosynthesis of S. superba were significantly alleviated by N in this subtropical forest.