Water Balance-carbon Metabolism Relations Of Different Tree Species Under Drought And Frost Adverse Conditions

Recently,forest decline related to global climate change has drawn people's great attention.Mechanism hypotheses about drought-induced tree mortality have been proposed: hydraulic failure hypothesis,carbon starvation hypothesis and biotic attack hypothesis.This study focused on water-carbon relation to explore water-carbon changes and the interaction of them during drought,relation of seasonal embolism change and nonstructural carbohydrate(NSC),and effects of simulative carbon limitation on water balance.Carbon(C)balance and the maintenance of hydraulic architecture are indispensable for plants' survival.Drought and frost are the two main environmental stresses inducing cavitation embolism and impeding long-distance water transport.Hydraulic architecture and carbon dynamic are greatly interrelated during drought,and C plays an important role in coping with frost and in seasonal embolism refilling.Global climate change results in reduced precipitation or changes of rainfall patterns in arid and semi-arid region and more frequency of extreme climatic events,so increasingly more attention has been paid on drought-induced tree mortality mechanism,as well as effects of frost on xylem water-transport tissues and seasonal embolism refilling mechanism.Therefore,studies about water-carbon relations under drought and frost conditions will facilitate us to further understand trees' strategies of coping with adversities,and to provide research thinking and experimental evidence for comprehensively understand different responses-and interaction of hydraulic architecture and C dynamic under the two types of adversity.This study was conducted on potted saplings subjected to drought and carbon limitation conditions and on field trees undergoing winter frost with an entry point of water relations and C dynamics under adversities.Two tree species with different drought response strategies,black locust(Robinia pseudoacacia)and arborvitae(Platycladus orientalis)were exposed to fast drought and slow drought to analyze the role of hydraulic failure and carbon starvation in driving saplings to die.Shading and girdling were conducted on potted black locust and arborvitae saplings to create carbon limitation,in order to explore effects of C limitation on hydraulic architecture.Three wood types of tree species,ring-,diffuse-and non-porous(conifers)trees were used to explore xylem embolism change pattern and NSC dynamics over winter,in order to reveal response strategies and seasonal embolism refilling mechanisms of different tree species.Based on above studies,interrelations of hydraulic architecture and C dynamic under drought,carbon limitation and frost conditions were comprehensively analyzed.Main results are as follows:1.In this experiment,hydraulic characteristics and NSC dynamics during different types of drought were measured to analyze and determine the role of hydraulic failure and carbon starvation in driving plants to die.The percentage loss of conductivity(PLC)in both species was at its maximum at mortality under both fast-and slow drought.For black locust,PLC > 95%,and for arborvitae it was ~45%.The predawn water potential reached approximately-8 MPa at mortality.Before complete defoliation,black locust had a rapid increase of PLC with ~90% at complete defoliation,but continued to survive for a long time.Arborvitae slowly increased its PLC until mortality without defoliation.NSC concentrations in stems and roots of both rapidly-and slowly dehydrated black locust declined to a very low level near death.In contrast,the NSC concentrations in the needles,stems,and roots of arborvitae at mortality under fast drought did not significantly differed from that of control,whereas the NSC concentrations in the stems and roots of arborvitae at mortality under slow drought were significantly lower than those of the control.The results revealed that both species experienced hydraulic failure under fast-and slow drought,but C played different roles in the two species.Availability of stored NSC helped black locust to survive,ultimately leading to carbon depletion,so carbon starvation was also a mortality mechanism for black locust,whereas arborvitae died of hydraulic failure despite C level.2.The role of NSC in drought resistance and maintenance of hydraulic architecture was explored by artificially changing NSC level of black locust and arborvitae saplings.For both species,biomass in all organs,especially in fine roots,was significantly reduced by shading and girdling.NSC concentration in roots of the two species was markedly decreased by shading and girdling,and NSC concentration in stem was increased.Root hydraulic conductivity of black locust seedlings subjected to shading and girdling accounted for 3.7% and 2.9% of the control,respectively,and that shading-and girdling-treated arborvitae accounted for 21.9% and 7.6% of the control,respectively.For the two species,root and branch PLC significantly increased by shading and girdling treatments with root PLC severer than branch.Meanwhile predawn and midday water potential significantly decreased.Leaf stomatal conductance was also significantly reduced,and with shading and girdling treatments black locust accounted for 33.7% and 26.1% of the control respectively,and arborvitae accounted for 46.9% and 23.4% of the control,respectively.The results revealed that both shading and girdling greatly reduced NSC in roots,and the carbon limitation constrained the growth of new roots,hence reduced the ability of water uptake and transport in roots,which deteriorated hydraulic architecture of roots and stem,and further impeded long-distance water transport.As a result,carbon uptake was in turn constrained.Thus,plant's survival under adverse conditions was influenced.In addition,girdling-induced NSC accumulation above girdles could not alleviate stem PLC for both species.The effects of shading on black locust was greater than arborvitae due to arborvitae's more resistance to shading3.Xylem response to frost fatigue and mechanisms of seasonal embolism refilling was explored by measuring seasonal PLC change patterns of different tree species growing in the field.Almost all of tree species were susceptible to frost-induced embolism,and for ring-porous species,PLC in later winter was near 100%,for diffuse-porous species,it was ~80%,for conifers,it was below 50%,however,arborvitae showed no seasonal PLC change.There was significant increase of P50(xylem water potential at the point of 50% PLC)from August to February in ring-and diffuse-porous species,and in May P50 was markedly reduced along with formation of new vessels,while P50 of conifers had no seasonal change.There was no significant reduction of PLC until formation of new vessels in ring-porous trees,while diffuse-porous trees and conifers are not the case showing PLC reduction in March or April before development of new xylem tissues.Except diffuse-porous trees had positive xylem pressure of 15-25 kPa in April,the others did not.Osmotic potential of xylem sap in diffuse-porous trees decreased to a minimum in March or April,accompanied with increase of soluble sugar concentration and amylase activity.Results showed that resistance to winter-induced embolism ranked in order of conifers > diffuse-porous trees > ring-porous trees,and different wood types of species recovered hydraulic conductivity through different mechanisms: ring-porous trees mainly depending on new vessels,diffuse-porous trees depending on root pressure-and osmotic gradient-drived active embolism refilling and development of new vessels,conifers relying on osmotic gradient-drived active embolism refilling and development of new tracheids.Recovery of frost fatigue in ring-and diffuse-porous species mainly relied on development of new vessels.