Biophysical control on nighttime sap flow in Salix psammophila in a semiarid shrubland ecosystem

The impact of extreme desert conditions on long-term nighttime sap flow dynamics in desert-dwelling shrubs is poorly understood. Based on heat-balance measurements in a semiarid area of northwest China, we investigated the nighttime sap flow (transpiration) dynamics in Salix psammophila as affected by changes in site biophysical variables over a six-year period (2012-2017). Nocturnally, sap flow rate per leaf area ( J s n ) formed a positive linear relationship with air temperature (Ta), water vapor pressure deficit (VPD) and stomatal conductance (Gs), when Ta J s n during wet and dry years, respectively. Interannually, Ta and Gs were able to explain a significant portion of observed variation in J s n , individually explaining about 74 and 83% of that variation. Interannual control on J s n also varied with SWC, Ta and Gs explaining 84% of the variation. The diminished sensitivity of J s n to both Ta and VPD and increased sensitivity to SWC during dry years, suggested that the shrubs had a physiological capacity to adapt to dry soil conditions. Daytime sap flow rates ( J s d ) were significantly greater than those at night (i.e., J s n ), with an interannual mean difference of 0.15 ± 0.04 g cm−2 d−1 and coefficient of variation of 27%. In addition, J s n formed a positive linear association with J s d during the growing season. Mean J s n contributed to the daily total sap flow (i.e., J s n /Js24-h × 100) by 8-14% over the six years, with an interannual mean of 11%. Our findings underscore the importance of nighttime sap flow in S. psammophila and the importance of temperature and SWC in controlling nighttime water losses through transpiration. It is suggested that the proportion of nighttime sap flow could increase during plant acclimatization in response to increasing temperature and water shortages with future climate change.