Studies Of Platycladus Orientalis Stomatal Conductance And Its Regulation On Canopy Transpiration In Beijing Mountainous Area

In this study,the typical conifer species of Platycladus orientalis in Beijing mountainous area was taken as the research object.The different water control conditions plots(no precipitation,half precipitation,normal precipitation and double precipitation)and the corresponding plot were established.We modified and verified the canopy stomatal conductance(Gs)model to explore the change process of Gs in Platycladus orientalis,and the response of Gs to external environmental factors and internal hydraulic factors.The process of canopy transpiration was obtained by sap flow system and the relationships between canopy transpiration and environmental factors were studied.According to the coupling relationships between Gs and environment and hydraulic factors,as well as the comprehensive responses,the regulation mechanism of Gs on canopy transpiration process was explored.The main conclusions were stated as follows:(1)The mechanism model of canopy stomatal conductance was modified,and the model(?)was obtained.The prediction of the model was good.The variation of Gs showed a bimodal curve or a growth-flat-decline trend.Canopy stoma had the largest opening in summer and the smallest in winter.Thus,the canopy stomatal conductance of Platycladus orientalis under different water control gradients in the experimental group had little interference with each other.(2)The relationship between Gs and forest air temperature increased first,and reduced like parabolic trend,which was affected by the relative humidity.The response of Gs to photosynthetically active radiation was interval,mainly affected by water vapor pressure deficit.When vapor pressure deficit<1kPa,photosynthetically active radiation was within the range of 0?200 W·m-2·s-1,and the response of the two showed a logarithmic relationship.The response of Gs to vapor pressure deficit had a negative logarithmic relationship.The response of Gs to soil moisture content is parabolic.The soil moisture content corresponding to the highest canopy stomatal conductance under half precipitation,normal precipitation and double precipitation were 8.5%,12.5%and 18.5%,respectively.Gs increased with the increasing of hydraulic conductivity,and there was a threshold phenomenon,the threshold was generally within the range of 0.1-0.15 mol·m-2·s-1·mPa-1.The better the soil moisture status was,the stronger the response of Gs to environment and hydraulic factors were,and the better the fitting effect was.(3)Diurnal variation of transpiration in the canopy of Platycladus orientalis was characterized by a single peak curve or a growth-flat-decline trend.The daily transpiration was the highest in July and the lowest in January.The average daily canopy transpiration under double precipitation was the highest,while that was the lowest under half precipitation.Month average transpiration was highest in August,and lowest in January or December.As for annual transpiration,it was the largest(369.25mm)under the condition of double precipitation,followed by the normal precipitation(350.68mm)and the minimum(296.33mm)under no precipitation.(4)The sensitivity of Gs to VPD(8)could be used as a coupling point to connect environmental and hydraulic factors.For natural precipitation and double precipitation,the?/Gsref was slightly larger than 0.6,while was equal to 0.6 for no and half precipitation.Vapor pressure deficit,photosynthetically active radiation and hydraulic conductivity on Gs are mainly concentrated on the daily scale,while atmospheric hydrothermal conditions and soil moisture content are the seasonal scale factors.There was a logarithmic relationship between stomatal conductance and transpiration intensity.When Gs was 0-100 mmol·m-2 ·s-1,transpiration intensity changed rapidly with the increase of Gs.When the soil water condition was good,Gs could regulate transpiration more rapidly.