Sap flow characteristics of four typical species in desert shelter forest and their responses to environmental factors

Sap flows of four typical species, Populus. russkii Jabl., Populus euphratica Olive., Ulmus pumila L., and Elaeagnus angustifolia L., of artificial shelter forest in a desert area were monitored in all-weather conditions using SF300 Sap flow Meter based on the theory of thermal compensation. Meanwhile, air temperature (T (a)), leaf temperature (T (l)), soil temperature (T (s)), relative humidity (RH), and wind velocity (V (w)) were simultaneously recorded by an automatic weather station. The results indicate that (1) the diurnal processes of stem sap flows of P. russkii Jabl., P. euphratica Olive., and U. pumila L., but not E. angustifolia L., show an obvious circadian rhythm. Significant differences of stem sap flow rates were found among species, but not genus. The average sap flow rate of P. russkii Jabl. is 13.8-fold of that of E. angustifolia L. The order of sap flux density (SFD) from the largest to the smallest is P. russkii Jabl., P. euphratica Olive, U. pumila L. and E. angustifolia L.; (2) compared with 373 mm, 747 mm irrigation can induce microenvironmental changes that result in the suppression of photosynthesis and transpiration and the decline of stem sap flow rates of the above four species, indicating 373 mm irrigation meets the growth needs of the above species during experiment; (3) sap flow rates are different at different stem positions: the flow rates of P. russkii Jabl., U. pumila L. and E. angustifolia L., but not P. euphratica Olive, decline gradually from cambium to pith; (4) the correlation analysis indicates that stem sap flow is negatively correlated with RH and T (s) and positively correlated with T (a), T (l) and saturation vapor pressure deficit (VPD). The sap flow rate of P. russkii Jabl. is significantly affected by V (w) due to its large size and height. In addition, a model was established by stepwise regression analysis to estimate the relationship between the environmental factors and stem sap flows of the above four typical species of shelter forest in the desert area.