Effects Of Planted Shrub Encroachment On Evapotranspiration In Desert Steppe

The ecological engineering for combating desertification in northwestern China often plant a large number of shrubs such as Caragana intermedia in arid and semi-arid desert grassland areas,which will cause the phenomenon of shrub encroachment in desert steppe.Transformation will inevitably affect the material and energy cycle processes of the ecosystem,of which the impact on the ground-gas water vapor cycle process is the most important.Evapotranspiration(ET)is an important part of the ground-air water vapor cycle,including two physical processes of evaporation and transpiration.Studying the effects of planted shrub encroachment on ecosystem evapotranspiration(ET)in desert steppe can not only reveal the mechanism how human activities change the water cycle of ecosystem in semi-arid areas,but also guide local government to implement ecological restoration and construction.To this end,taking Yanchi County as an example,based on multi-source data such as remote sensing and meteorology,combined with micro-meteorological observations,ecosystem process model simulation and remote sensing spatio-temporal analysis methods,this article studies the effect of artificial shrub encroachment on desert grassland evapotranspiration from multiple spatiotemporal scale.The main work and conclusions are as follows:(1)Using Landsat remote sensing image data and field leaf area index(LAI)data as input data,the regression model was used to estimate the LAI from 1986 to 2019 in order to study the impact of artificial shrub encroachment on vegetation.The overall fluctuation of LAI increased from 1986 to 2019 in Yanchi County,and the 67.45%regional vegetation leaf area in the county was increasing,indicating that the ecological governance project increased the regional vegetation coverage and changed the original vegetation leaf area coverage structure in the desert grassland area of Yanchi.It may cause changes in the structure and function of the ecosystem,especially the regional ground-gas water vapor cycle.(2)Based on the MODIS ET and NDVI products from 2001 to 2018,trend analysis,correlation analysis and zoning statistics were used to study the effects of artificial vegetation reconstruction on regional ecosystem evapotranspiration,take desert steppe in Yanchi county as an example ecosystem.ET increased notably after large-scale artificial vegetation reconstruction in Yanchi county,and ET and NDVI showed a significant positive correlation.The correlation between the changes of ET and NDVI was 0.76(P<0.01).The vegetation structure change caused by vegetation reconstruction leads to greater evapotranspiration of the ecosystem,and more water consumed in the desert steppe area of Yanchi county,and the growth rate of ET is accelerated after artificial vegetation reconstruction.The artificial vegetation reconstruction of the desert steppe has increased the NDVI value and enhanced the ET of the regional ecosystem in Yanchi county.(3)The evapotranspiration of grassland and artificial shrub was obtained by micro-lysimeter and eddy covariance(EC)system,and the evapotranspiration difference between the two underlying surfaces was compared and analyzed.From December 2018 to November 2019,the evapotranspiration of grassland was 353.17mm,and the evapotranspiration of artificial shrub was 400.15mm.The main environmental driving factor for grassland evapotranspiration is photosynthetically active radiation,and the main environmental driving factor for artificial shrub evapotranspiration is air temperature.During the same period,the rainfall was 358.10 mm,which was almost the same as the total evapotranspiration of the grassland,and the evapotranspiration of artificial shrubs was significantly higher than that of atmospheric precipitation.In semi-arid regions where water is scarce,the persistent imbalance between artificial shrub evapotranspiration and precipitation will have an impact on the stability of the ecosystem.(4)Based on the physiological and ecological parameters of vegetation and the hydrothermal conditions of the desert steppe,the Biome Bio-Geochemical Cycles(Biome-BGC)model and the Breathing Earth System Simulator(BESS)model were combined to simulate the evapotranspiration changes and its components with the planted shrub encroachment occurring in the desert steppe ecosystem.The effects of the planted shrub encroachment on regional evapotranspiration was also been evaluated by the simulated data.The results showed that the planted shrub in grassland has changed the structure and characteristics of vegetation,and the annual maximum leaf area index(LAI)increased from 0.20 to 0.67,which changed the interannual and intra-annual characteristics of LAI in the desert steppe ecosystem.The annual average evapotranspiration of the ecosystem increased from 251.74 mm to 281.42 mm with the planted shrub encroachment occurred in the desert steppe of Yanchi county.Planted shrub encroaching significantly increased the ecosystem evapotranspiration in desert steppe during the growing season.The highest daily evapotranspiration was in August,the average daily evapotranspiration of desert steppe ecosystem increased from 1.27 mm/d to 1.56 mm/d in this month during the planted shrub encroaching.Planted shrub encroachment in desert steppe increased the transpiration of ecosystem by an average of 1.35 times and the evaporation by 1.06 times,which reduced the proportion of evaporation in evapotranspiration and increased the proportion of transpiration correspondingly,and therefore changed the structure of ecosystem evapotranspiration.