| The upper range limit of species distribution along elevation is ultimately a species-specific low-temperature boundary.Its formation and maintenance are the result of ecophysiological adaptability of species to long-term low temperatures.Up to now,however,the physiological mechanism of the formation of upper elevational range limit based on source-sink balance has not been explicitly elucidated.In this study,we selected three different functional tree species(shade-intolerant broad-leaved deciduous tree,shade-tolerant evergreen conifer,shade-intolerant deciduous conifer)in the Qinling Mountains of north-central China.In situ manipulation and ex situ warming experiment have been carried out to investigate whether the "source-sink" relationship dynamics of non-structural carbohydrate(NSC)in trees is a phenotypic physiological response or an adaptive evolution to low temperature stress,and further clarify whether it is an active or passive carbon allocation strategy for the trees at the upper range limits.The aim of this study is to reveal the cold-adaption mechanism of carbon allocation of the upper tree-limit formation to low temperatures and its responses to climate warming,as well as the potential species-specific characteristics.The main conclusions of this study are as follows:(1)"Source-sink" dynamic balance of non-structural carbohydrateThe magnitude of NSC seasonal variation was greater in leaves,twigs and roots of the deciduous tree species(Larix chinensis and Betula albo-sinensis),while NSC in root has the largest seasonal variation in the evergreen tree species(Abies fargesii).The NSC content did not decrease with the elevation increaing.Although the NSC content of leaves of the three species decreased with elevation in the middle of the growing season,the effect of elevation on the model was not significant,and the interaction between species and season was significant.Thus,the short-term carbon shortage was attributed to the seasonal source-sink dynamics.At the end of the growing season,the NSC content of each organ did not decrease with the elevation.These findings did not support the carbon limitation hypothesis.The NSC content in each organ was the lowest at the beginning of the growing season,indicating the reserves of NSC in each organ were involved in cold protection in winter,germination in early spring and regeneration of branches and leaves.(2)Active or passive carbon allocation mechanismDeclining temperature with increasing elevation did not reduce Asat in any of the species.We found NSC increased with elevation in major ripe tissues(e.g.,roots and stems)but not in leaves.The defoliation showed that C storage took priority over growth.Such preferential carbon allocation,directly caused by growth decline,always existed in the deciduous tree species.In the evergreen tree species,however,growth decline resulted from preferential carbon allocation to storage was only detected in the second year of defoliation and then disappeared as the intensity of defoliation increased.Our results showed that trees prioritized sustaining stores of C more highly than allocation of growth,regardless of the trees’ C or sink limitations.At the cold range limits,the prioritized carbon allocation to storage in deciduous tree species was in response to low temperature stress,while in evergreen tree species,the prioritization of carbon allocation was only a transient physiological response to defoliation disturbances.(3)Phenotypic plastic or ecologically adaptive carbon allocation mechanismGenetic differentiation and phenotypic plasticity between low-and high-elevation populations of L.chinensis and A.fargesii were found in growth.The growth of lowelevation species decreased with elevation,while the growth of high-elevation species did not decrease with elevation.These results indicated that low-elevation species showed stronger phenotypic plasticity than high-elevation species,while high-elevation ones showed greater genetic differentiation in growth to low temperature stress.Although the signs of phenotypic plasticity for NSC pools and R values of different organ in L.chinensis were found,the R values in species-level are not affected by elevation and source of species: local adaptation played a crucial role in R values of L.chinensis.On the contrary,the relative importance of phenotypic plasticity of the NSC pool and R value in A.fargesii was far greater than genetic differentiation: The individual level NSC pool and R value of increased with the elevation.In addition,strong co-gradient plasticity was involved in the R value of A.fargesii.These results proved that L.chinensis responded to local low temperature by genetic differentiation,and ultimately maintained the R value within a certain range(0.12-0.28),while the evergreen tree species A.fargesii can still assimilate supply in winter,and it is not necessary to store enough NSC pool in advance.Only under cold pressure will the NSC pool be actively increased to respond to environmental changes.(4)Response of growth and carbon storage to warmingExperimental warming mainly promoted lateral branch elongation of three species with different functional types.However,for deciduous tree species,the lateral branch growth increased significantly in the second growing season under experimental warming,which indicated that the growth of deciduous tree species showed a lagging response to climate warming.The response of NSC pool in tissues of L.chinensis to experimental warming was not obvious,and the distinct response of NSC pool in B.albo-sinensis to warming occured mainly in the dormant season but not in growing season.Thus,NSC pools of deciduous tree species are not sensitive to warming in the growing season,and the trees maintain a certain proportion of NSC in response to low temperature stress.Both the NSC and R values decreased with experimental warming in the evergreen species A.fargesii,indicating that the warming broke the carbon allocation mechanism of preferential NSC storage,and such carbon "bet-hedge" was a plasticity response to climate warming. |
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