Transpiration Characteristics And Driving Factors Of Betula Platyphylla Secondary Forest In The Greater Hinggan Mountains

Stand transpiration is an important component of the hydrological cycle in forest ecosystems,which not only regulates the regional water cycle and sustainable utilization of water resources,but also an important component of forest evapotranspiration.In recent years,due to factors such as fires and logging,a large area of secondary Betula platyphylla forests has emerged in the Greater Hinggan Mountains region,accounting for 41.15%of the total forest area in the region.In order to explore the water consumption patterns of transpiration in the secondary forest of Betula platyphylla,this study used the thermal diffusion probe method to observe the trunk sap flow from May to September 2021.The changes in sap flow density and transpiration of individual Betula platyphylla were analyzed,and the water consumption of transpiration in the secondary forest of Betula platyphylla was calculated on a scale.The characteristics and driving factors of transpiration changes in the secondary forest of Betula platyphylla were explored.The main research results are as follows:（1）At the hourly scale,the trunk sap flow of a single Betula platyphylla shows a trend of high in the day and low in the night.The fluctuation of sap flow is significant on sunny days,while it is relatively small on rainy days.The start time and peak time of trunk sap flow on rainy days are delayed by about 1 hour compared to sunny days.The average sap flow density of dominant tree is 1.14 times that of intermediate tree and 1.91 times that of suppressed tree.The average sap flow of Betula platyphylla shows a trend of first increasing and then decreasing with the change of months,with the highest sap flow in July,with a maximum value of 194.42 g·cm^-2·d^-1,and smaller in May and September.In addition,different rainfall levels significantly affect the sap flow of Betula platyphylla,with moderate rain having the greatest impact.Correlation analysis shows that potential evapotranspiration and vapor pressure deficit have the most significant impact on the sap flow of Betula platyphylla,while soil moisture and soil temperature are not important factors affecting the sap flow of Betula platyphylla.（2）The hourly transpiration of a single Betula platyphylla also shows a characteristic of high diurnal and low nighttime.On sunny days,the transpiration of Betula platyphylla shows a single peak curve with a significant diurnal variation,while on rainy days,it shows a slightly fluctuating broad flat curve.The peak transpiration on sunny days is 2.5 times that on rainy days.And the higher the differentiation level of Betula platyphylla,the earlier the start time of transpiration and the later the end time.As the months change,the average transpiration of white birch shows a trend of first increasing and then decreasing,with the peak transpiration in July.（3）During the observation period from May to September,the daily average transpiration of the secondary Betula platyphylla forest was 1.47 mm·d^-1.Moreover,the stand transpiration shows a trend of first increasing and then decreasing with monthly changes.The total transpiration of the secondary Betula platyphylla forest during the observation period was 224.54 mm.The stand transpiration increased from 19.84 mm in May to 65.08 mm in July,and then decreased to 22.92 mm in September.In addition,during the entire observation period,the total transpiration of dominant trees in the secondary forest of Betula platyphylla reached 140.67 mm,which was 1.95 times and 12.05 times that of intermediate trees（72.17 mm）and suppressed trees（11.67 mm）,respectively.（4）At the hourly scale,air temperature and vapor pressure deficit are the dominant factors driving stand transpiration;On a daily scale,potential evapotranspiration,leaf area index,vapor pressure deficit,and air temperature are the dominant factors;On a monthly scale,air temperature is the dominant factor.