Carbon Storage And Water Use Strategy Of The Betula Ermanii Treeline In Changbai Mountain

The formation mechanism of alpine line,namely the formation mechanism of the distribution pattern of alpine trees,has been one of the hot topics of climate change.Although a large number of related research results have been achieved,there are still many uncertainties in the physiological mechanism of timberline trees to adapt to the stress environment.The alpine Betula ermanii forest line is a typical forest line composed of deciduous broad-leaved trees in northeast China.Previous studies have focused on its adaptation mechanism to low temperature stress,but few studies have been conducted on its adaption on water stress.Therefore,in this paper,taking B.ermanii as the research object,the carbon storage and water use strategies of B.ermanii,the concentrations of non-structural carbohydrates（NSC）in different tissues and organs,and their response to drought stress were analyzed.The main results are as follows:（1）There were significant differences in water use efficiency（WUE）between seedlings and trees under different altitude.With the increase of altitude,leafδ¹³C of B.ermanii changed from tree significantly higher than seedling to seedling higher than that of tree,and there was no significant difference at forest line（altitude 2000m）.Leaf WUE of B.ermanii seedlings and trees increased along the increase of elevation gradient,indicating that B.ermanii might have water limitation on the forest line,and B.ermanii seedlings are more vulnerable to water stress than large trees.（2）Control experiments demonstrated that drought and its duration had significant effects on photosynthetic characteristics and NSC storage and distribution of B.ermanii seedlings.Drought significantly reduced the net photosynthetic rate（Pn）and stomatal conductance（Gs）of B.ermanii seedlings and increased their water use efficiency（WUE）.Drought dramatically improved the contents of soluble sugar and total NSC in leaves,barks,stems and roots of B.ermanii seedlings,but significantly reduced their starch content.The Pn,Gs and WUE decreased rapidly as the drought continued,whereas the soluble sugar,starch and NSC increased and then declined.At the end of the experiment,90%of the leaves turned yellow,and the ratios of soluble sugar to starch in the stems,barks and roots in the drought treatment were significantly higher than those in the control treatment.These results demonstrated that B.ermanii might be drought-avoidance species that could reduce water loss by rapidly reducing stomatal conductance and improving WUE,when they encountered drought stress.It also implied that B.ermanii might have evolved priority storage strategy to cope with water deficit through improving the content of soluble sugar in organs and increasing the conversion rate between starch and sugar.With the extension of drought stress,seedlings tended to die,since water stress might exceed the threshold of the plant self-regulation capacity.However,the content of NSC in organs did not decrease,suggesting that the death of B.ermanii under drought stress might not be caused by carbon starvation.（3）With the increase of diameter class,the contents of NSC,soluble sugar and starch in leaves and branches significantly increased except starch content in leaves.Soluble sugar and NSC contents in roots existed obvious differences under different diameter classes,nevertheless,no significant differences of starch in roots were observed under different diameter classes.NSC,soluble sugar and starch contents of trunk significantly decreased with the increase of diameter class,and the tree heartwood hold higher NSC,soluble sugar and the ratio of soluble sugar to starch,indicating that there might be a large NSC pool in the internal woody tissue of the tree trunk.It has been reported that NSC in heartwood could still be used to maintain survival under severe stress.Therefore,in the future climate change conditions,B.ermanii trees might be more resistant to stress and easier to survive than seeding.