Trunk Water Storage,Stem Hydraulic Safety And Foliar Water Uptake Of Woody Plants In Tropical Seasonal Rainforest And Savanna

There is a rich species diversity of plants and landform types in southwest China,including two distinct biological communities:tropical seasonal rainforest（TSF）and tropical savanna.However,Trees in tropical regions have been frequently threatened by extreme drought events resulting from global warming.Plants have evolved various water use strategies to improve their drought adaptation,such as trunk water storage,structural resistance and foliar water uptake.However,the relationship between trunk water storage capacity and physiological function of canopy branches and leaves in tropical habitats is still unclear.In this study,we selected dominant tree species from TSF and savanna to investigate how sapwood capacitance of trunks affects hydraulic safety of canopy branches.Additionally,we evaluated trunk water deficit using high-precision dendrometers and its relationship with canopy hydraulic safety during the dry season.At a regional scale,we assessed the difference of foliar water uptake of wood plants between two tropical habitats,and the relationship between foliar water uptake and drought adaptability during the dry season.The main findings are as follows:（1）In TSF and savanna,there was a tradeoff between the sapwood capacitance of trunks and xylem embolism resistance(P₅₀ and P₈₈)of canopy branches.Additionally,sapwood capacitance of trunks was positively correlated with the theoretical hydraulic conductivity of a trunk(K_th)and the specific hydraulic conductivity of canopy branches（K_s）（R²=0.30,P<0.01;R²=0.32,P<0.01）.It suggests that high sapwood capacitance of trunk was coordinated with efficient water transport efficiency.Furthermore,high sapwood capacitance of trunks effectively mitigated water stress of canopy branches.Higher sapwood capacitance was associated with higher photosynthetic rate and stomatal conductance of canopy leaves,thereby improving primary productivity of tropical forests.Capacitance was closely related to the fiber lumen fraction（R²=0.56,P<0.001）and fiber wall reinforcement（R²=0.68,P<0.001）,not to the axial and ray parenchyma fractions.Trunk sapwood capacitance of tropical trees plays a crucial role in determining drought adaptability and maintaining physiological and ecological functions of canopy branches and leaves.（2）The patterns of stem expansion and contraction differed between TSF and savanna trees.At TSF site,stems of trees showed shrinkage during the dry season and growth during the rainy season,while trees in savanna exhibited frequent stem expansion and contraction in all months.Additionally,relative humidity and precipitation were important environmental factors that affected tree water deficit（TWD）in both habitats.By constructing logistic regression between relative tree water deficit and the midday water potential of canopy branches(Ψ_md),we predicted the dynamics ofΨ_md throughout the year.The results showed that plants in savanna experienced vessel embolism of 50%in canopy branches during dry season,while the trees in TSF hardly experienced embolism.By comparing the D₅₀（midday water potential at 50%depletion of water in internal storage tissues）of tree species in two habitats,the results showed that internal storage water depletion of savanna species was slower than that of TSF species（TSF:-1.50±0.09 MPa;Savanna:-2.26±0.12 MPa）.Besides,there was positive correlation between D₅₀ and P₅₀,but no correlation with sapwood capacitance among all studied tree species.（3）Terminal leaves of species in TSF and savanna were capable of absorbing water during dry season,and foliar water uptake（FWU）differed between two study sites,savanna species had higher foliar water uptake compared to those in TSF(TSF:0.476±0.095 mg cm^-2;Savanna:1.661±0.266 mg cm^-2).Plants in savanna can absorb water through their leaves to maintain a higher minimum leaf water potential during the dry season.Besides,savanna species have a lower leaf water potential at turgor loss point(Ψ_tlp)compared with TSF species（TSF:-1.39±0.08 MPa;Savanna:-2.60±0.14MPa）.Species with greater FWU exhibited a higher leaf water potential at turgor loss point in savanna,but no similar finding was observed in TSF.Furthermore,a negative correlation was found between FWU and total cuticle thickness in savanna plants,revealing the structural determinant of foliar water uptake during a drought.However,no similar result was observed in TSF.These results provide insights into understanding the relationship between foliar water uptake and drought adaptability in tropical savanna.In conclusion,plants in tropical seasonal rainforest and savanna can make use of the water stored in their trunks to mitigate water stress,thereby maintaining a higher photosynthetic rate of canopy leaves and enhancing the primary productivity of tropical forest.There was a tradeoff between trunk sapwood capacitance and xylem embolism resistance of canopy branches.Moreover,trees in TSF and savanna exhibited distinct patterns of stem radial growth and contraction.The midday water potential predicted by logistic regression showed that trees of savanna experienced xylem embolism,but not in TSF trees.Apart from trunk water storage,plants in both TSF and savanna were able to absorb water through their leaves.This implies the crucial role of leaf water absorption for maintaining canopy water status and improving drought adaptability among plants from different tropical regions.