Abstract: Nitrogen (N) deposition alters the soil environment for forest trees, particularly in tropical regions, leading to variations in leaf traits. However, the adaptive responses of plantation tree species to chronic N deposition, via leaf trait modifications, remain poorly understood. We conducted a decade-long experiment involving N additions in two typical plantations dominated by Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) in South China, to investigate species-specific leaf trait plasticity under N deposition. Our results showed that long-term N addition did not affect N and phosphorus (P) concentrations, sugar and starch levels, intrinsic water use efficiency (iWUE), and leaf mass per area (LMA), but lowered leaf total Carbon (C) content in both EU and AA. Moreover, it resulted in divergent traits between them, showing an increase in tannin and phenolics but a decrease in leaf water content (LWC) in AA but no such variations in EU. These differential responses were attributed to their unique leaf traits that EU contains high chemical defensive compounds and AA, as an N-fixing tree species, exhibits higher resource levels. The reduced leaf total C was redirected towards defense, without compromising iWUE through unchanged sugar and starch levels, particularly in AA. Our findings demonstrate that long-term N addition intensifies the coupling between C and water, resulting in a shift in C allocation in trees. Consequently, long-term N addition triggers different defensive strategies: a conservative defense in EU and an active defense in AA. This finding offers new insights into the adaptive mechanisms of forest plants under global change scenarios.

Key words: nitrogen deposition, intrinsic water use effciency (iWUE), defense, carbon balance, plantation trees, South China