Dominance?caused Differences In Sap Flow Characteristics Of Larix Principisrupprechtii And Their Influence On Forest Transpiration Estimation

Understanding the change characteristics of individual tree sap flow and its influencing factors is a prerequisite for accurate estimation of forest transpiration.Larix principis-rupprechtii Mayr.is the most important plantation resources in the Liupan Mountain in Ningxia.To this end,in this study,during the experimental days of 2018 and 2019,a typical plot of Larix principis-rupprechtii was selected in the Xiangshui River watershed in the semi-humid area of Liupan Mountain in Ningxia.We divided the 12sample trees into three classes:dominant tree,intermediate tree and surpassed tree,using thermal diffusion technology to monitor the real-time sap flux density of 12 sample trees,and meteorological factors and soil moisture characteristics were simultaneously monitored in the plot.We analyzed the characteristics of sap flux density and its environmental factors with different classification,and established individual sap flux model based on environmental factors and forest spatial structure to provide a scientific basis for accurate estimation of forest transpiration and fine forest water management.The main results are as follows:?1?On diurnal scale,the extend of sap flow usually appear as dominant tree>intermediate tree>surpassed tree.On the daily scale,the sap flow exhibit as dominant tree>intermediate tree>surpassed tree under the three different dominance classifications if month difference is not considered.?2?On the diurnal scale,the sap flow is mainly affected by solar radiation and vapor pressure deficit,and there is a difference in tree dominance,dominant trees are more sensitive to changes in solar radiation than surpassed trees.On the daily scale,the sap flow is mainly affected by reference crop evapotranspiration(ET_ref)and relative extract water?REW?.Surpassed trees are more sensitive to changes in REW than dominant trees.?3?The individual sap flow is comprehensively affected by the spatial structure of trees and environmental factors.As far as the spatial structure of trees is concerned,its impact can be simplified as the effects of relative tree height?H_R?at the vertical level and Hegyi competition index?CI?at the horizontal level,in terms of environmental factors,its impact can be simplified to the effect of ET_refand REW.Sap flow has an exponential growth function relationship with the H_R,a quadratic polynomial function relationship with CI and ET_ref,and an exponential growth relationship with REW tending to saturation.Based on this,a single factor response function was coupled to establish a composite model of individual tree sap flow,the simulation effect of the model in the construction and verification stage is good?R²=0.813,NSE=0.81;R² after verification=0.799,NSE=0.793?.?4?Based on the model,the estimated forest transpiration during the 2019 study period is 82.6 mm,which is 2.44%lower than the method without considering the spatial differences of trees.The range of individual plant transpiration is between 2.38-22.5L,and the coefficient of variation is 0.61.When using relative tree height to classify the dominance of stand trees,the contribution to forest transpiration of dominant tree,intermediate tree and surpassed tree accounted for 44.6%,46.3%,and 9.1%,respectively.When using Hegyi competition index to classify the dominance of stand trees,the contribution to forest transpiration of dominant tree,intermediate tree and surpassed tree accounted for 50.3%,46.6%and 3.1%,respectively.