Transpiration Conductance And Simulation Of Canopy-Atmosphere Interface Of Platycladus Orientalis Plantation

Canopy stomatal conductance was often used as a main indicator to measure transpiration resistance in study of forest canopy-atmosphere interface water vapor output resistance.But it was easy to ignore the effect of canopy boundary layer aerodynamic conductance on water vapor output,which resulting in errors.In order to quantify the canopy-atmospheric water vapor output resistance,we took Platycladus orientalis plantation as the research object and worked basedon the meteorological data of PAR,saturated water vapor pressure deficit(VPD),temperature(T),wind speed(u)and other meteorological factors(fromthe capital circle forest ecosystem station of National Forestry and Grass Administration in 2017).we used the thermal diffusion probe method(TDP)to continuously observe the trunk fluid density(Js)and calculated the canopy transpiration.Kostner’s simplified formula and Monteith-Obukhov theory were respectively applied to calculate the Platycladus orientalis canopy conductance under canopy scale and above.We constructed a transpiration conductance model of simulating canopy conductance and aerodynamics conductance combined with controlling the moisture output at the canopy-atmosphere interface,analyzed the spatio-temporal variation characteristics and the responses of environmental factors of the three conductivities,and constructed Jarvis models about canopy conductance and transpiration conductance(based on the environmental factors and the responses of canopy conductance and transpiration conductance).This study provided theoretical basis for revealing the influence mechanism of water vapor output resistance at the can’vas-atmosphere interface.The main results were as follows:(1)Canopy conductance was higher in growth season than that in the non-growing season.Its daily average was highest in August(209.12 mol·m-2·s-1)and lowest in May(63.63 mol·m-2·s-1),and showed a double peak trend in August and November,while a single peak trend appeared in other months.The annual variation of canopy conductance ranged from 3.28 mol·m-2·s-1 to 499.93 mol·m-2·s-1.Aerodynamic conductance of Platycladus orientalis showed that the month with large diurnal temperature difference was larger than that with small,which was highest in June(300.71 mol·m-2·s-1)and lowest in September(75.33 mol·m-2·s-1),and its daily average aerodynamic conductance showed a single peak trend during the whole year.Noteworthy,aerodynamic conductance diurnal variation was more intense in the non-growing season and the growing season from March to June.The annual aerodynamic conductance variation ranged from 15.82 mol·m-2·s-1 to 377.67 mol·m-2·s-1.Transpiration conductance of Platycladus orientalis were basically the same as that of canopy conductance in non-growing season.However,transpiration conductance in the growth season was obviously affected by aerodynamic conductance.Average high level of transpiration conductivity in daytime was only about 30-40%compared to canopy conductance.The daily average transpiration conductance was highest in January(96.70 mol·m-2·s-1)and lowest in June(36.12 mol·m-2·s-1).The annual transpiration conductance varied from 2.98 mol·m-2·s-1～192.37 mol·m-2·s-1.It can be concluded that:canopy conductance were mainly controlled by air temperature during the summer and winter,accrroding to all conductance and major environmental factors.But air dynamics conductance was regulated indirectly by the instantaneous temperature difference rate.Variation trend of transpiration conductance and canopy conductance in winter were basically the same,while transpiration conductance variation characteristic was similar to aerodynamic conductance in other seasons..(2)Correlation analysis(P<0.05)showed that the influence of each environmental factor on the canopy conductivity of Platycladus orientalis in the non-growing season ranked as:T(r=-0.213)>VPD(r=0.151)>u(r=-0.079)>PAR(r=-0.058),the influence of each environmental factor on the aerodynamic conductivity listd as:PAR(r=0.426)>VPD(r=0.230)>T(r=-0.056),and the influence of each environmental factor on transpiration conductance was T(r=-0.198)>PAR(r=-0.074)>u(r=-0.050)>VPD(r=-0.049),which indicated that T had largest impact on canopy conductance and transpiration conductance in non-growth season,while VPD affected aerodynamic conductance most.In the growing season,the influence of various environmental factors on the canopy conductance listed as:VPD(r=-0.511)>T(r=0.352)>PAR(r=0.308)>u(r=0.144),the influence of academic conductance ranked as:VPD(r=0.368)>PAR(r=0.222)>T(r=-0.144),and the influence of transpiration conductance 0as:u(r=0.488)>VPD(r=-0.299)>PAR(r=0.228)>T(r=0.114),which showed VPD had the greatest influence on the canopy conductance and aerodynamic conductance during the growing season,and u had the greatest influence on transpiration conductance.Compared with non-growing season,the positive and negative feedback relationships of VPD,PAR,T and u were inconsistent with canopy conductance,and transpiration conductance was only inconsistent with the positive and negative feedback relationships of PAR,but,but the positive and negative feedback relationships of all environmental factors were basically consistent with aerodynamics conductance.The influence of canopy conductance and aerodynamics conductance on transpiration conductance(r=0.749)was larger than that of aerodynamics conductance(r=-0.127)in non-growing season,and the influence of canopy conductance(r=0.574)and aerodynamics conductance(r=0.544)on transpiration conductance was basically the same in growing season.(3)Through the simulation analysis of Jarvis model,the simulated canopy conductance showed the same change trend as that calculated by trunk fluid flow.When aerodynamic conductance was greater than the canopy conductance,the results of the transpiration conductance simulation and the calculated value fitted well.And transpiration conductance simulation value had a better convergence via cross validation.