| Global climate change has induced severe and prolonged droughts and changes in precipitation patterns,which have caused widespread tree growth decline and even mortality in many forest biomes with profound effects on the ecosystem functions and regional carbon(C)budgets.Currently,two major leading hypotheses of plant mortality mechanisms have been proposed,i.e.,the hydraulic failure of tree hydraulic system occurs when water loss induces progressive cavitation and conductivity loss of the xylem,and the C starvation is induced by the imbalance between carbohydrate demand and supply due to stomatal closure.Nevertheless,different tree species may develop different strategies to cope with drought stress,which may substantially change their survival,growth,potential damage and the underlying physiological and ecological mechanisms under global climate change.To date,however,there are few studies on the response strategies and mechanisms involved to drought stress for the tree species in Northeast China,which have seriously hindered the modeling and prediction of structure,function and distribution of the forest ecosystems under climate change scenarios.Manchurian ash(Fraxinus mandshurica)and Dahurian larch(Larix gmelinii)are two major tree species co-existing in both natural and planted forests in northeastern China;they also represent two distinct plant functional types with contrasting anatomical,morphological,and physiological traits;so they are expected to have different response strategies and mechanisms to drought stress.In this dissertation,we used the seedlings of Manchurian ash and Dahurian larch as case studies,and investigated their hydraulic and economic characteristics with a water-exclusion experiment.The aims are to explore the tree water regulation strategies and the underlying mechanisms,and to reveal potential coordinated interactions between leaf,stem and root hydraulics and C assimilation under drought stress,so as to provide theoretical bases for forest conservation and restoration in this region.The main results are as follows:(1)There were significant differences in hydraulic characteristics between ash and larch.Firstly,ash had higher wood density and lower xylem embolism vulnerability(i.e.,Ψ50(the water potential associated with 50%loss of hydraulic conductivity)was more negative);and its leaf Ψ50 was less negative than Ψgs88(the water potential associated with stomatal closure).These results indicated that ash adopted a drought resistance strategy under drought stress by decreasing the leaf hydraulic conductivity(Kleaf)that caused partial or complete stomatal closure,and thus maintained the stem hydraulic functioning.Conversely,larch had higher Ψgs88,and the Ψgs88 was less negative than the Ψ50 of leaf and stem.These results indicated that larch adopted an avoidance strategy to cope with drought stress by sensitive stomatal responses to drought and maintained the leaf hydraulic functioning by decelerating the decline of water potential.Secondly,ash had a significantly higher leaf Ψ50 than stem Ψ50,exhibiting a leaf-stem hydraulic vulnerability segmentation to preserve the hydraulic integrity of stem.On the contrary,larch had no vulnerability segmentation mechanism,but used the stomata as "safety valves" to maintain the hydraulic function of whole plant and delayed the dehydration progress under drought.(2)Ash and larch displayed different hydraulic strategies and related traits in response to drought.The responses of stomatal conductance of the two tree species to changes in soil moisture were investigated by using three different classification indexes.First,The σ(i.e.the slope of the regression relationship between pre-dawn water potential and midday water potential)for ash was not significantly different from 1(P>0.05),while that for larch was significantly(P<0.05)less than 1.Second,the hydroscape area(HA)for ash was 1.68,which was significantly higher than that of larch(1.17).Third,the leaf stomatal safety margin(SSM50)for ash was less than 0,while that for larch was greater than 0.These results indicated that ash tended to have a more anisohydric regulation behaviour,while larch a more isohydric one.(3)Ash and larch coordinated the relationship between water loss and photosynthetic C assimilation in different ways.The conversion of starch to soluble sugar seems more important for ash to change the osmotic potential in response to water potential decline.However,larch had higher stomatal sensitivity and water potential corresponding to the 88%loss of photosynthesis(ΨA88),which restricted the C assimilation process.Meanwhile,larch could improve its water use efficiency(WUEinst)by reducing stomatal conductance(gs).Larch could increase WUEinst by 4.5 times as much as its value prior to drought stress(c.f.,only 1.7 times for ash).In addition,larch may maintain its basic physiological functions through its bark photosynthesis as an alternative C source.Overall,both species were more likely to suffer from hydraulic failure than C starvation as drought stress proceeded.In conclusion,as drought stress proceeded,ash and larch showed distinct hydraulic regulation and C assimilation strategies.Ash had relatively insensitive stomata to drought and adopted anisohydric behavior that could reduce water potential,and improved xylem embolism resistance to drought stress by converting starch to soluble sugars.Moreover,ash could maintain the hydraulic integrity by the vulnerability segmentation mechanism under extreme drought.On the contrary,larch had sensitive stomata to drought stress and adopted isohydric behavior to slow down the water potential decline and maintain the hydraulic function of leaves,but alleviated the C starvation caused by stomatal closure by increasing water use efficiency.These findings not only deepen the ecophysiological understanding of tree growth and survival under drought stress,but also provide a potential mechanistic explanation for the co-existence of tree species in temperate forests;they are of significance for the prediction of changes in forest ecosystem structure,function and distribution under global climate change scenarios. |
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