Transpiration and canopy conductance variations of shelterbelt in an arid inland river basin of northwest China

Abstract:

The knowledge of plant water use characteristics under changing environmental conditions is essential for ecosystem management and water resources distribution in water-stressed environments. This study was conducted to quantify variations in transpiration and canopy conductance in a shelterbelt in the middle of the Heihe River Basin, China. Sap flow of eight Gansu Poplar trees (Populus gansuensis) with different diameters at breast height (DBH) was measured over three consecutive growing seasons (2012-2014). The evapotranspiration of groundwater via plant use was estimated by the White method, with diurnal water table fluctuations. Results showed that mean sap flow density varied between 30.62 ±11.44 and 101.88 ±28.98 kg m^-2 h^-1, and it increased linearly with the DBH. Variations of sap flow density were mainly controlled by meteorological factors in addition to water table depth. Average stand transpiration during the growing season was about 4.85 mm day^-1, and it had a logarithmic relationship with reference crop evapotranspiration. Precipitation increased stand transpiration, but not at a statistically significant level (p>0.05). The recharge of soil water by irrigation significantly accelerated stand transpiration (p<0.05). Stand transpiration and canopy conductance increased by 27% and 31%, respectively, when soil water conditions changed from dry to wet. Canopy conductance decreased logarithmically with vapor pressure deficit, whereas there was no apparent relationship between canopy conductance and solar radiation. The sensitivity of canopy conductance to vapor pressure deficit decreased under dry soil conditions. Groundwater evapotranspiration (0.6-7.1 mm day^-1) was linearly correlated with stand transpiration (1.1-6.5 mm day^-1) (R² = 0.71). During the drought period, approximately 80% of total stand transpiration came from groundwater evapotranspiration. This study highlighted the critical role of irrigation and groundwater for shelterbelts, and might provide the basis for the development of water requirement schemes for shelterbelt growth in arid inland river basins.