Transpiration Characteristic Of Dahurian Larch Trees Transplanted From A Latitudinal Gradient

Boreal and temperate forests occur in mid and high latitudes,which are sensitive to the ongoing climate change.Understanding tempo-spatial dynamics and controlling factors of the transpiration of these forests is essential to assessing and predicting the roles of the forests in the climatic system.Dahurian larch trees are the dominate tree species of the boreal forest in China.It is of great managerial and academic importance to research the tree-water relations on this tree species,since it is essential to evaluate and forecast the distribute patterns and the functional process changing of this species under the ongoing global climate change.5-year-old Dahurian larch plantation ecosystems(saplings,debris and the A and B layers of the soil) were transplanted(in 2004) to the southern boundary of its distribution area from four locations(Tahe,Songling,Sunwu and Dailing) according a latitudinal transect to constitute four experimental treatments,i.e.TH,SL,SW and DL,to examine tempo-spatial dynamics of tree-water relations and their controlling factors.Water physiological properties at leaf and individual scales,i.e.transpiration rate and sap flow,were measured by Li-6400 portable photosynthetic system and heat balance method,respectively.While,an automatic weather station with Campbell Scientific data loggers was used to measure associated environmental factors.The water-use characteristics of Dahurian larch saplings at leaf and individual scales were all significantly different between treatments.Transpiration rate(E) of the saplings from SL, SW and DL varied from 0.47 to 8.31 mmol m^-2s^-1 and the average value was 3.11 mmol m^-2s^-1 which was closed to that of boreal and temperate forest broadleaved trees.E of SL(3.95 mmol m^-2s^-1) was significantly higher than DL(2.97 mmol m^-2s^-1) and SW(2.73 mmol m^-2s^-1).There were significant differences between monthly E:August(6.08 mmol m^-2s^-1)＞September(2.99 mmol m^-2s^-1)＞July(1.65 mmol m^-2s^-1)≈June(1.50 mmol m^-2s^-1).E was significantly correlated to environment factors,such as air vapour pressure deficit(VPD) and soil volume water content(VWC) and physiological parameters,including specific leaf area(SLA), stomatal conductance(g_s),shoot water potential before sun rise and hydraulic conductance, among them,environment factors were the cause of monthly variance,while differences between treatments were due to the diversity of physiological parameters.Sapwood area-based sap flux(J_s) basically showed a bell-shaped diurnal pattern,the magnitude and lasted time of peaks were different during growing season.Peaks were higher and its lasted time was longer in the middle periods of growing season without water stress than that of other periods.Daily J_s of the saplings from TH,SL,SW and DL varied from 0.67 to 19.32 g cm^-2h^-1,which was closed to that of boreal and temperate forest broadleaved trees. Differences of daily J_s between treatments are obvious but not statistic significant due to huge individual variance(Coefficients of variance were as high as 70%) within treatment.When analyzed by paired samples T-test,seasonal J_s had significant differences between treatments, SL(11.19 g cm^-2h^-1)≈DL(10.82g cm^-2h^-1)＞TH(9.60g cm^-2h^-1)＞SW(8.86 g cm^-2h^-1). Individual daily water use of TH and SL were significantly lower than that of SW and DL.This course was corresponding to the individual sizes and the latitudes of the transplanted locations. There was detected sap-flow after the conifers had fallen(in non-growing season),but it should be taken caution when measured with heat balance method for the work hypothesis of which may be fault when air temperature was too low.The climatic factors influencing J_s,including PAR,VPD and Wind speed,changed with the growing periods and treatments,among which PAR was usually the key factor.When temporal scale extended,effects of meteorological factors became weaker and seasonal variance of J_s on sunny days was related to VWC.Otherwise,lower VWC could decrease the sensitivity of J_s to meteorological factors.Difference patterns of water use efficiency(WUE) between treatments at leaf and individual scales were distinct.Leaf-scale-WUE(WUE_leaf) of all treatments was insignificant differences,which of SW,DL and SL were 1.29,1.28 and 1.19μmolCO₂/mmolH₂O, respectively.WUE_leaf during the growing season varied from 0.24 to 1.89μmolCO₂/mmolH₂O and it is lower in June when severe water stress occurred and higher in September and July accompanying moderate VWC level,while,when VWC higher,the condition in August, WUE_leaf was moderate.Individual-scale-WUE(WUF_tree had significant differences(P＜0.001). With the increase of the latitude of transplanted location,WUE_tree decreased gradually,i.e.DL＞SW≈SL≈TH.Average value of all treatments was 2.403g kg^-1.By compared the diurnal courses between leaf and individual scales,time lags were found between leaf transpiration and stem sap flow.Thus,we concluded that,in our research with 9-year-old Dahurian larch saplings,water capacitance did exist in the tree body above the sap flow measuring location.Up-scaling processes from leaf to individual in trees were complex. By multiplied the measured E values of the long shoot tips and the leaf area,neglecting the variance within the canopy,we can get the whole tree water consumptions which were overestimated when compared with the sap flow.The representative of the sample shoots will have tremendous influences if neglecting the variance within the canopy.This study suggested that water relations of the Dahurian larch respond to environmental changes,which should be considered in climate change predictions.