Variation Of Hydraulic Function And Anatomical Structure With Tree Height In Two Species Of Taxodium

According to the hydraulic limitation hypothesis,gravity and path length increase with tree height,which makes it more difficult for tall trees to transport water to the upper leaves.The leaves at the upper part of the crown or the end of the branch are the most suitable for absorbing sunlight and have the greatest demand for water,but because they grow at the end of the transport path,water supply is difficult（water transportation efficiency）.Plant embolism（water transportation safety）is unavoidable when water supply is insufficient,and in severe cases,it will cause plant death.Therefore,hydraulic function of xylem（water transportation efficiency and water transportation safety）and its anatomical traits are closely related to the growth of tall trees.In addition,hydraulic efficiency will affect the photosynthetic capacity of leaves and further affect the carbon gain of plants.Studying the variation and coordination of branches and leaves of tall trees with height is helpful to deeply understand how tall trees keep the hydraulic integrity of long-distance water transportation path.In this paper,bald cypress（Taxodium distichum（L.）Rich.）and T.distichum var.imbricatum in Zhejiang Agriculture and Forestry University were taken as the research objects.Quantify and compare the hydraulic function of xylem（hydraulic efficiency and embolism resistance）,anatomical traits（tracheid diameter,tracheid density,double thickness of tracheid wall,thickness-to-span ratio of tracheid,pit membrane diameter,pit aperture diameter and xylem density）,water relation traits（midday leaf water potential and hydraulic safety margin）,foliar traits（leaf-to-sapwood area ratio and leaf mass per unit area）,and photosynthetic traits（maximum net photosynthetic rate,maximum stomatal conductance,maximum transpiration rate,water use efficiency）.The main results showed that:（1）The hydraulic efficiency of bald cypress（T.distichum（L.）Rich.）and T.distichum var.imbricatum were both positively correlated with tree height.The sapwood specific conductivity（p<0.05）and leaf specific conductance（p<0.01）of bald cypress（T.distichum（L.）Rich.）were positively correlated with tree height,and the leaf specific conductance（p<0.01）of T.distichum var.imbricatum was positively correlated with tree height.The variation of embolism resistance of bald cypress（T.distichum（L.）Rich.）and T.distichum var.imbricatum with tree height was different.Embolism resistance(P₅₀)was negatively correlated with tree height significantly in bald cypress（T.distichum（L.）Rich.）（p<0.05）,and decreased at first and then increased with tree height in T.distichum var.imbricatum.（2）In the upper crown,the leaves of bald cypress（T.distichum（L.）Rich.）and T.distichum var.imbricatum suffered more severe water deficit,but the carbon gain did not decrease accordingly:the midday leaf water potential was more negative with the increase of tree height,and the maximum net photosynthetic rate is positively correlated with tree height.Meanwhile,leaf morphology of upper crown showed more“xerophytic characteristics”:leaf area was decreased relatively to sapwood area（the leaf-to-sapwood area ratio was negatively correlated with tree height）,and leaf thickening（the leaf mass per unit area was positively correlated with tree height）.In addition,T.distichum var.imbricatum showed more“xerophytic characteristics”than bald cypress（T.distichum（L.）Rich.）.（3）There was no efficiency-safety trade-off and tracheid tapering in bald cypress（T.distichum（L.）Rich.）or T.distichum var.imbricatum.Hydraulic efficiency was positively correlated with tracheid diameter.No significant correlation was found between embolism resistance and tracheid diameter,thickness-to-span ratio of tracheid and double thickness of tracheid wall.Furthermore,the relationship between pit and hydraulic function of xylem was complex.（4）Coordination between branches and leaves:coordination between hydraulic efficiency and photosynthesis;coordination between water loss and water supply in bald cypress（T.distichum（L.）Rich.）;coordination between xylem embolism resistance and photosynthesis in T.distichum var.imbricatum.There is commonality in hydraulic transport efficiency of bald cypress（T.distichum（L.）Rich.）and T.distichum var.imbricatum,which is increasing hydraulic efficiency of the upper crown to ensure the water supply.The improvement of hydraulic efficiency of the upper crown is achieved by reducing the leaf area,thus increasing carbon gain.The safety of hydraulic transport is different:branches of the upper crown of bald cypress（T.distichum（L.）Rich.）and branches of the lower crown of T.distichum var.imbricatum faced more risk of mortality and will die first under water stress to maintain the survival of the trees.The death of branches of upper crown positions effectively lowers the height of the living part of the tree that can likely contribute to tree survival in drought-stressed conditions,while the shedding of lower leaves preventing further water loss and protecting the hydraulic integrity of the plant.