Divergent allocations of nonstructural carbohydrates shape growth response to rainfall reduction in two subtropical plantations

Nonstructural carbohydrates (NSC) are indicators of tree carbon balance and play an important role in regulating plant growth and survival. However, our understanding of the mechanism underlying drought-induced response of NSC reserves remains limited. Here, we conducted a long-term throughfall exclusion (TFE) experiment to investigate the seasonal responses of NSC reserves to manipulative drought in two contrasting tree species (a broadleaved tree Castanopsis hystrix Miq. and a coniferous tree Pinus massoniana Lamb.) of the subtropical China. We found that in the dry season, the two tree species differed in their responses of NSC reserves to TFE at either the whole-tree level or by organs, with significantly depleted total NSC reserves in roots in both species. Under the TFE treatment, there were significant increases in the NSC pools of leaves and branches in C. hystrix, which were accompanied by significant decreases in fine root biomass and radial growth without significant changes in canopy photosynthesis; while P. massoniana exhibited significant increase in fine root biomass without significant changes in radial growth. Our results suggested that under prolonged water limitation, NSC usage for growth in C. hystrix is somewhat impaired, such that the TFE treatment resulted in NSC accumulation in aboveground organs (leaf and branch); whereas P. massoniana is capable of efficiently utilizing NSC reserves to maintain its growth under drought conditions. Our findings revealed divergent NSC allocations under experimental drought between the two contrasting tree species, which are important for better understanding the differential impacts of climate change on varying forest trees and plantation types in subtropical China.