Responses Of Tree Non-structural Carbohydrates To Drought And The Regulation Mechanisms

Non-structural carbohydrates(NSCs)are important buffering substances for plants to tackle climate changes and play a crucial role in maintaining tree survivals under environmental stress.With globally increased intensity and frequency of extreme drought events,a better understanding of NSC responses to drought and the underlying mechanisms is greatly essential for accurately predicting global vegetation dynamics and ecosystem carbon cycling under future climate change.In ecosystems,many factors(e.g.,tree species,drought intensity and duration)affect NSC responses to drought.Among them,differences in water use and carbon storage capacity at different tree developmental stages may have great impacts on NSC dynamics in response to drought.However,little is known about whether and how such differences in tree developmental stages(e.g.,mature trees and saplings/seedlings)affect NSC responses to drought,and our knowledge on the key factors in influencing tree NSC responses and the underlying mechanisms remains elusive.In this study,we collected NSC data under drought and performed a meta-analysis to explore the general patterns of tree NSC response to drought and examine the effects of the key factors,especially tree developmental stages.Furthermore,we conducted a large-scale field throughfall exclusion experiment to compare the differences in NSC response to drought between mature trees and understory seedlings growing in the same ecosystem.To clearly reveal the function of NSC in tree resistance and resilience to drought,we conducted a drought-rewatering gradient potted experiment on two species to detect tree NSC responses during persistent drought and recovery and explore the carbon-water coupling relationships.The main results are as follows:(1)Using a meta-analysis on tree NSC under drought from 32 studies on mature trees and 94 studies on seedlings at a global scale,we analyzed the response magnitudes of tree NSC to drought and investigated the key factors influencing the response magnitudes.The results showed that,drought significantly increased soluble sugar concentrations by 7.3% and decreased starch concentrations by 18.96%,although it had no significant effects on the total NSC concentrations.Under drought conditions,both the increase of soluble sugars and the decrease of starch in seedlings were larger than those in mature trees.For seedlings,starch consumption increased with the increase of drought intentisy.The tree height,drought intensity and tree xylem resistance to embolism were the key factors to regulate the NSC responses to drought in both mature trees and seedlings,while,in seedlings,saturated vapor pressure deficit(VPD)also played key roles.These results demonstrated the remarkable differences of NSC responses to drought between mature trees and seedlings with great NSC loss for seedlings/saplings,and the key factors that regulate NSC responses were also varying with tree developmental stages.(2)To examine NSC responses to drought in trees at different developmental stages in the same conditions,we investigated drought impacts on NSC dynamics in mature trees and understory seedlings of four subtropical broadleaf evergreen species by a large-scale throughfall exclusion(TFE)experiment.The results showed that drought did not change NSC storage or the distribution among tissues in mature trees,but reduced the total NSC and soluble sugar in the three species of seedlings by 14.70 ± 3.66% and 16.93 ± 3.85%,respectively.Under drought,photosynthesis,growth rates and leaf water potential kept stable in mature trees,while seedling photosynthesis significantly decreased.Total NSC and soluble sugars of seedlings were positively correlated with photosynthesis.NSC responses to drought were species-specific.Unlike seedlings of other three species(Schima superba,Castanopsis sclerophylla,Castanopsis carlesii)showing decreases of NSC under drought,the seedlings of Lithocarpus glaber with the highest drought resistance tended to maintain or even slightly increased NSC levels under drought.These results suggested that drought had different effects on canopy trees and understory seedlings in a natural forest ecosystem,further verifying the importance of tree developmental stages in assessing drought effects on tree NSC storage and forest C cycling.(3)To further explore the mechanisms of NSC responses to drought and its relationship with tree drought resistance,we monitored the continuous carbon-water dynamics of two common subtropical tree species with different drought resistance to persistent drought to examine the relationship of NSC dynamics with other physiological processes during drought and recovery.The results showed that neither the higher drought-resistant species Castanopsis sclerophylla(C.sclerophylla)nor the lower drought-resistant species Phoebe chekiangensis(P.chekiangensis)consumed NSC under persistent drought.Compared with P.chekiangensis,C.sclerophylla suffered less physiological damage,survived longer,and accumulated more NSC.The NSC accumulation in two species was negatively correlated with the change in leaf water potential,stomatal conductance and photosynthesis.In addition,the NSC,especially the soluble sugar consumption was positively related to the drought-induced physiological damage.For both P.chinensis and C.sclerophylla,trees that suffered longer drought treatments consumed more solubles sugars for recovery.These results indicated that trees tended to maintain or even accumulated NSC under drought,and that the species with higher drought resistance had stronger NSC accumulation ability for post-drought recovery.Therefore,NSC,especially the soluble sugar consumption,may regulate the hydraulic recovery of trees after drought release.By combining the meta-analysis,throughfall exclusion experiment and greenhouse drought-rewatering experiments,our study systematically elucidated the responses of tree NSC storage to drought and its regulation mechanisms.Our results highlighted the importance of tree developmental stages in NSC responses to drought,with higher susceptibilities of NSC to drought in seedlings than mature trees.Under drought,NSC in seedlings/saplings was more easily to change by growth environment,drought intensity and tree drought resistance.NSC storage and consumption also regulated tree recovery after drought release,which determined tree survivals under drought stress together with tree hydraulics.These results largely deepen our understandings of responses of tree NSC storage and the whole-tree carbon balance to drought,and provide a theoretical and scientific basis for accurately predicting vegetation dynamics and the global carbon cycles in the future.