Mechanism Research On The Forest Evapotranspiration In The Earth-rock-Mixed Mountainous Region Of North China

The earth-rock-mixed mountainous forest is the most important water conservation area in North China.It has some significant specificities in hydro-climatic conditions,bedrock properties,edaphic features,vegetation conditions,etc.,leading to the complex hydrologic process,and the strong interactions and feedbacks between hydrologic process and ecological process.Evapotranspiration is the key link in the hydrologic process of the earth-rock-mixed mountainous forest in North China,and it also has some significant specificities.In this study,an experimental watershed was established in a representative area of the earth-rock-mixed mountainous forest in North China,and the long-term systematic field measurements of forest evapotranspiration were then conducted in the experimental watershed.Based on the field measurements,a sap flow upscaling procedure was established,and the comprehensive evaluations were made for the applicabilities of the several methods for determining evapotranspiration and its components in the earth-rock-mixed mountainous forest in North China.The mechanisms of the environmental and physiological controls on canopy transpiration were explored in depth,and an empirical formula was established for canopy transpiration estimation.The quantitative analyses were conducted on the physiological mechanism and controlling factors of the diurnal time lag effect between sap flow and canopy transpiration.Among the estimates of evapotranspiration and its components using several different methods in the earth-rock-mixed mountainous forest in North China,the estimate using the eddy covariance method is relatively accurate at multiple temporal scales;the estimate using the sap flow upscaling procedure established in this study,accounting for the diversities and spatial variabilities of forest types and tree species,is also relatively accurate,and has high spatial representativeness at regional scale;the catchment water balance method,without the considerations of bedrock infiltration and groundwater flow,significantly overestimates annual evapotranspiration,probably because of the widely distributed fractured bedrocks under the ground in the earth-rockmixed mountainous forest in North China,which may lead to unnegligible bedrock infiltration and groundwater flow;the estimate using the soil water budget method at daily scale can principally reflect the seasonal trend and daily dynamics of evapotranspiration.In the earth-rock-mixed mountainous forest in North China,the canopy transpiration is determined by the combined effect of environmental and physiological factors;the leaf area index determines the range and upper bound of canopy transpiration significantly,and the control of leaf area index shows a threshold behavior,while the environmental factors lead to the fluctuations in canopy transpiration within the upper bound;photosynthetically active radiation is identified as the key environmental factor controlling canopy transpiration.With the combination of the dominant controlling factors of canopy transpiration,photosynthetically active radiation and leaf area index,an empirical formula was established at daily scale for individual tree transpiration estimation,which can explain 89 % of the variation in transpiration.There is a significant diurnal time lag effect between sap flow and canopy transpiration for aspen(Populus davidiana)in the earth-rock-mixed mountainous forest in North China;the internal water storage and hydraulic conductivity of tree stem,as well as tree growth status and phenology,significantly control this diurnal time lag effect,and the control of the internal water storage of tree stem is dominant;the obvious difference between the tall tree,aspen and the low shrub,cotton(Gossypium hirsutum L.)in the diurnal time lag effect may also result from the difference in the internal water storage of tree stem;the environmental factors also,to some extent,control the diurnal time lag effect,while they may not be the essential controls at the level of mechanism.