Transpiration and canopy conductance of Caragana korshinskii trees in response to soil moisture in sand land of China

Caragana korshinskii is widely used in vegetation reestablishment programs to stabilize the shifting of sand on the Loess Plateau. This sand shifting of sand exerts pressure on the limited soil water and vice versa. However, detailed transpiration and sap flow studies that focus on water use in these stands remain limited. The hourly sap flows, along with successive soil water balance and meteorological measurements, on three trees were measured using the thermal heat balance method during the growing season period. Results showed that the midday sap flow and stand transpiration were significantly lower during the stress period (from May to July) compared with those at the beginning of August concurrently with high soil water content. The heat balance measurements underestimated the transpiration obtained from the soil water balance method. When the tree conductance was fitted with climatic variables and soil water content, the canopy conductance increased exponentially with above-canopy radiation, whereas it decreased logarithmically with decreasing vapor pressure deficit. In the absence of water stress, the tree conductance was unchanged when the relative extractable water remained above 0.4, whereas the conductance decreased linearly after the wilting point. The model was generally well fitted to the measured transpiration data in terms of the response during the dry and rainy seasons, thus demonstrating the feasibility of developing a quantity schedule for C. korshinskii transpiration on the Loess Plateau in the presence of a soil water stress.