The Transpiration Characteristics Of Pinus Tabulaeforims Plantation In The Loess Hilly Region

Water shortage is regarded as one of the most limiting factor for the success of revegetation in the Loess Plateau,vegetation transpiration is the main component of evapotranspiration and hydrological cycle in terrestrial ecosystem.Pinus tabulaeformis is a widely distributed tree species in the hilly region of Chinese Loess Plateau.The main purpose of its plantation is to control serious soil erosion and restore the degraded ecosystem function in this region.Terracing measures can directly change and reshape the micro-geomorphic pattern of slope and watershed,significantly affect the water balance as well as runoff and sediment processes.Terracing measures have significant ecological,as well as social and economic benefits.In this study,Pinus tabulaeforimis plantation were selected in Longtan catchment in the loess hilly region.We measured sap flow,meteorological factors,soil moisture and vegetation parameters,and the mainly objectives of this study were to: quantify transpiration dynamics and its main influencing factors at different time scales,explore response of soil moisture and sap flow to rainfall pulse,and simulate transpiration based on Hydrus-1D model.This study aims to provide scientific basis and theoretical reference for the choose suitable tree species,assess the impact of vegetation on water resources,and predict the response of vegetation to drought and environmental change.The main results are as follows:（1）Continuous observation of the sap flow rate and meteorological factors of Pinus tabulaeformis plantation during 2015-2020 growing season.The factors that impacting transpiration of Pinus tabulaeformis plantation were varied at different temporal scales.On the daily scale,transpiration was significantly positive correlated with solar radiation,vapor pressure deficit,air temperature and wind speed,and significantly negative correlated with relative humidity.It is obvious that these factors have a combined effect on hourly transpiration.On the daily scale,the response of Pinus tabulaeformis plantation transpiration to solar radiation and vapor pressure deficit is nonlinear.The relationship between them can be explained by the explained by the exponential growth model of saturation.The transpiration of Pinus tabulaeformis plantation was unevenly distributed within and between years,during the growing seasons of 2015 to 2020 was 147.0,95.2,174.0,216.2,213.9,and 169.7 mm,respectively.On the monthly and annual scales,rainfall was the most important factor affecting dynamics changes in transpiration of Pinus tabulaeformis plantation.（2）The response of soil moisture and sap flow to rainfall events and rainfall levels in Pinus tabulaeformis plantation was investigated by using observation data during the growing season of 2015-2016.After a single small rainfall event（<5 mm）,the surface soil water content（0-20 cm）did not increase significantly.Continuous small rainfall events can cause a significant increase in the surface soil water content of the Pinus tabulaeformis plantation.The sap flow peak time frequency distribution of Pinus tabulaeformis before and after exists obvious difference,because of replenishment of soil moisture after rainfall.The amount of change in sap flow of Pinus tabulaeformis showed a gradually increasing trend with the increase of rainfall.Threshold-Delay（T-D）model showed that the minimum rainfall threshold that can cause a significant increase in transpiration of Pinus tabulaeformis is 4.6 mm,and the response lag is 1.67 d,which indicated small rainfall was very important for the growth and survival of plants,and the response had hysteresis.（3）The Hydrus-1D model was used to simulate the dynamic changes of transpiration water consumption of Pinus tabulaeformis plantation,and the sensitivity analysis of the input parameters was performed.The results show that the water content of the 20 cm soil layer is most susceptible to environmental factors;the second is the water content of the 40 cm soil layer.Hydrus-1D model has good simulation accuracy for simulating soil moisture of Pinus tabulaeformis plantation with different terrace measures.In addition,from the simulation results of different soil depths,the simulation accuracy of surface soil moisture is lower than that of deep soil moisture.The Hydrus-1D model assumes that both the fish-scale pit and the zig terrace can intercept all rainfall,which makes the model runoff underestimated by 41.7 mm and 11.6 mm,respectively.The model simulates the transpiration of Pinus tabulaeformis plantation on the natural slope,fish scale pit,and zig terrace in the 2019 growing season to be 229.15,217.35,and 221.77 mm,respectively.