Nitrogen addition promotes conservative resource-use strategies via aggravating phosphorus limitation of evergreen trees in subtropical forest

Leaf traits generally reflect plant strategy of resource acquisition and conservation. It remains unclear whether leaf-trait syndrome of closely related species in a specific forest ecosystem is consistent with the predictions of global leaf economics spectrum (LES). Moreover, the extent to which nitrogen (N) deposition affects leaf traits and the resource-use strategies they characterize needs further validation. Hence, based on the Nutrient Enrichment Experiments in China's Forests (NEECF) platform established in 2011, we monitored the changes of diameter at breast height of adults and measured eight leaf functional traits of adults and seedlings of the dominant tree species (i.e., Castanopsis eyrie and Castanopsis sclerophylla) in two subtropical forests after nine years of N fertilization. Our results showed that: (1) leaf-trait syndrome of the two dominant species followed the predictions of global LES theory at both adult and seedling stages, and that of the primary C. eyrie forest was more conservative than the secondary C. sclerophylla forest; (2) for both species, especially the adults, N addition decreased leaf phosphorus (P) concentration, with a subsequent increase in Carbon:P (C:P) and N:P ratios, and thus promoted a trend towards the conservative end on the LES; (3) N addition had no significant effect on the relative growth rate (RGR) of adults for both species. Our results confirmed the existence of LES patterns among closely related evergreen species at the local scale. Although nine years of high N addition did not change the RGR of adults in subtropical evergreen forests, changes of their leaf-trait syndrome indicated an aggravated P limitation as plants tended to favor a more conservative resource-use strategy.