Water Relations And Carbon Dynamics Under Drought Stress And Mechanisms Underpinning Drought-induced Tree Mortality In Four Tree Species

Drought-driven tree die-offs are occurring globally and are affecting the forest community structure and ecosystem function.However,the underlying physioecological mechanisms are poorly understood.Under extreme precipitation pattern in the future,elucidating the physiological mechanisms under drought-induced mortality can contribute to prediction accuracy about the response of forest to future climate changes.In this study,one-year-old seedlings(Syzygium rehderianum,Castanopsis chinensis,Schima superba and Ardisia lindleyana)were planted individually in pots and exposed to two intensity of drought(extreme drought and moderate drought).We compared the differences of water relations and carbon dynamics under difference intensity of drought and tried to find out the mechanisms underlying drought-induced die-off.The main findings are as follows:1.The pre-dawn leaf water potential,midday water potential,net photosynthetic rate and stomatal conductance of Syzygium rehderianum,Castanopsis chinensis,Schima superba and Ardisia lindleyana reduced significantly in both two drought treatments,and they reduced more quickly and steeply in extreme drought than moderate drought.2.Under the moderate drought treatment,the wood density decreased significantly in Syzygium rehderianum and Schima superba.3.Whole-plant dry mass decreased in Syzygium rehderianum,Castanopsis chinensis and Ardisia lindleyana under two drought treatment,but patterns of biomass allocation in organs were different among treatments and tree species.In the extreme drought treatment,leaf biomass reduced significantly in Syzygium rehderianum,and root biomass decreased in Castanopsis chinensis and Ardisia lindleyana.Moderate drought reduced the biomass of all organs in all species.4.Nonstructural carbohydrate(NSC)concentration changed significantly under two drought treatments.NSC concentration was changed on leaf in extreme drought-induced mortality and on root under in moderate drought-induced mortality.5.“Hydraulic failure” occurred during tree die-off in all tree species and drought treatments.“Carbon starvation” didn't occur in extreme drought-induced die-offs of Syzygium rehderianum,Castanopsis chinensis and Ardisia lindleyana according to NSC concentration didn't decrease in all organs.At moderate drought-induced mortality,root NSC concentrations of Syzygium rehderianum,Castanopsis chinensis and Ardisia lindleyana were significantly decreased and they probably occurred “Carbon starvation”.