Response of Populus euphratica Oliv. sap flow to environmental variables for a desert riparian forest in the Heihe River Basin, Northwest China

Being an important desert riparian forest in the lower reaches of the Heihe River Basin, Populus euphratica Oliv. forest functions as a natural barrier in maintaining and preserving the stability of local oases. Accordingly, accurately estimating the water use of P. euphratica is important for vegetation protection and water resource allocation. To date, little data are available for evaluating the hysteretic effects between sap flow and environmental variables, and for estimating the water use of desert riparian forest. In this study, we measured the sap flow velocity (V-s ) of P. euphratica using the heat ratio method during the growing season of 2012. Based on the response of V-s to solar radiation (R-s ) and vapor pressure deficit (VPD), we estimated the hourly V-s and daily V-s using the multivariable linear regression and a modified Jarvis-Stewart (JS) model, respectively. Hysteretic response of R-s to environmental variables was then evaluated using a sap flow model. We found the thresholds of R-s responses to R-s and VPD at both hourly and daily scales during the growing season, and successfully estimated the seasonal variations of hourly R-s and daily R-s using the JS model. At an hourly scale, the maximum R-s occurred earlier than the maximum VPD by approximately 0.5 h but later than the maximum R-s by approximate 1.0 h. At a daily scale, the maximum R-s lagged behind the maximum VPD by approximately 2.5 h while occurred earlier than the maximum R-s by approximately 2 h. However, hysteretic response of V-s was weakened when R-s and VPD were measured together using the JS model at both hourly and daily scales. Consequently, short-term and intensive field campaigns, where V-s and environmental variables can be measured, may be used to estimate short-run sap flow and stand transpiration using only two environmental variables.