The Sap Flow Dynamics and Response of Hedysarum scoparium to Environmental Factors in Semiarid Northwestern China

Hedysarum scopariumis an important, fast-growing and drought-resistant shrub that has been extensively used for grassland restoration and preventing desertification in semiarid regions of northwestern China. The primary objective of this study was to investigate the diurnal and seasonal variations in stem sap flow (J(s)) and its relation to environmental factors. The stem heat balance method was applied to plants that were approximately 17 years old (with diameters of 25, 16, 13, and 9 mm at ground level and heights of 3.1, 1.8, 1.7 and 1.4 m) and growing under natural conditions. The vertical soil temperature profile (ST), soil surface heat flux (SoilG), volumetric soil moisture content (SWC) and meteorological variables such as solar radiation (R-n), air temperature (T-a), vapour pressure deficit (VPD), wind speed (W-s) relative humidity (RH) and precipitation (P) were simultaneously measured at a meteorological station on site. Results indicated that J(s) varied regularly during the diurnal and seasonal term. The nocturnal J(s) was substantial, with a seasonal variation similar to the patterns of daytime J(s). The magnitude of J(s) changed considerably between sunny and rainy days. Redundancy (RDA) and Kendall's tau analysis suggested that daily J(s) in large plants was more sensitive to environmental factors, and the variation in daily J(s) during the growing season could be described by a multiple linear regression against environmental variables including T-a, VPD, Ws, RH, ST, and SoilG. While the nocturnal J(s) in smaller plants was more sensitive to meteorological factors. T-a, VPD, and W-s were significantly correlated with nighttime J(s). The hourly nighttime sap flow rate of H. scoparium corresponded closely to T-a and VPD following a non-linear pattern. The results of this study can be used to estimate the transpiration of H. scoparium.