| Forest water transport is an important link to maintain the balance of water and energy,and to explore regional hydrological cycle and climate change.Jinyun Mountain at Chongqing is an important ecological barrier area at the end of the Three Gorges Reservoir area.However,the characteristics of forest water transport and the response mechanism to environmental factors in this region are still unclear.In order to solve the above problems,this study used thermal diffusion probe technology and Eddy-covariance method to explore the water transport characteristics of a coniferous-broadleaved mixed forest in Jinyun Mountain at four scales(inner tree,single tree,stand and forest).Based on the quantitative relationships of water transport rates at different depths of tree trunk,the transformation at four different study scales was realized,and the accuracy of transformation results was verified by Eddy-covariance method.Based on the analysis of water transport characteristics at different scales,the water transport system of coniferous-broadleaved mixed forest was constructed,and the dynamic process of water transport was clarified.Finally,on the basis of traditional meteorological factors,two influence factors,energy and ecological competitiveness.are added to explore the response mechanism of water transport to various environmental factors at different scales.The main research results are as fol ows:(1)Water transport at inner tree scale mainly occured in the daytime,and the diurnal variation of sap flux density was characterized by an inverted U-shape distributon(a high rate at noon and a low rate in the morning and evening).Sap flux density at nighttime was weak;The strongly water transport mainly occurred at rainy season and extremely dry season;Due to the "photosynthetic noon break",the sap flux density was declined at noon;The sap flux density at different depths of tree trunk showed significant differences;The distribution patern of sap flux density at different depths was single-peaked;The order of water transport ability was Pinus massioniana>Cunninghamia lanceolata>Symplocas setchuensis.(2)Based on the radial difference of sap flux density in tree trunk,single tree transpiration was estimated;Consistent with sap flux density,the diurnal variation of single tree transpiration was also characterized by an inverted U-shape distributon;The monthly variation of single tree transpiration changed as sinusoidal distribution pattern and the transpiration mainly occurred from April to October;The amount of seasonal transpiration at single tree scale was:extremely dry season>rainy season>dry season;There were significant differences of transpiration between individual trees.DBH can explain some of the transpiration differences between individual trees.(3)Based on the radial difference of sap flux density in tree trunk,the amount of transpiration in stand scale was 812.44mm,accounting for 26.78%of the rainfall;On the daily scale,the variation of stand transpiration occured behind that of rainfall;On the monthly scale,the trend of stand transpiration variation was consistent with rainfall;On the seasonal scale,the amount of stand transpiration ordered as:extremely dry season>rainy season>dry season;The contribution rate of transpiraiton of three species changed with time.From April to August,Pinus massoniana was the main contributor to stand transpiraiton,and from December to March,Sympplocas setchuensis was the main contributor.(4)Forest land evapotranspiration estimed by Eddy-covariance method was 13.86%higher than that estimed by considering the radial difference of sap flux density in tree trunk.The daily variation trend of forest land evapotranspiration estimed by those two methods was consistent,and a significant correlated relationship was shown(R2=0.64).This result indicates the accuracy of transformation results which estimed by considering the radial difference of sap flux density in tree trunk;The amount of forest land evapotranspiration estimed by considering the radial difference of sap flux density in tree trunk was 1115.83mm,accounting for 64.37%of the total rainfall.In the rainy season,the water content of forest land was generally input,and in the extremely dry season and dry season,it was consumed.The forest land evapotranspiration is dominated by forest tree transpiration,followed by canopy interception evaporation,and finally by forest litter and soil evaporation,There were significant differences of evapotranspiration contribution at different season.In rainy season,canopy interception evaporation was the main contributor to forest land evapotranspiration,while stand transpiration and soil and litter evaporation were the main contributors in dry season,and stand transpiration was the main contributor in extremely dry season.(5)The response mechanism of water transport to its environmental factors is obviously different at different scales.On the inner tree scale,the spatial structure and competitiveness of single trees are important factors affecting sap flux densitiy(Fd).Besides,PAR and VPD are the main meteorological factors controlling Fd,Ta,RH and Vwind are the secondary meteorological factors controlling Fd;On the single tree scale,the spatial structure and competitiveness of single trees are also important factors affecting single tree transpiration(Q).Q presented a linear growth trend with the increase of PAR and Ta,a logarithmic growth trend with the increase of VPD,and a linear decrease trend with the increase of RH.Competitiveness is the reason that affects the response degree of Q to PAR and leads to the Q differences among individuals;On the stand scale,stand transpiration(Et)increased with the increase of PAR,VPD and Ta,and decreased with the increase of RH.In different seasons,the response degree of Et to meteorological factors is shown as:rainy season>extremely dry season>dry season.There was no correlation between Et and soil water content,mainly because of the time-delay effect;On the forest land scale,net radiation(Rn),evaporation rate(LE/Rn)and Bowen ratio(β)are important factors controlling forest land evapotranspiration(ET).The main meteorological factors controlling ET were VPD and Ta in rainy season,PAR,VPD and RH in extremely dry season,and PAR and Ta in dry season.The variation of ET consistented with soil water content.ET was only related to the soil water content at 70cm depth.This result suggested that,the water deficit in the extremely dry season may lead to the consumption of deep soil water. |
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