A multiple-temporal scale analysis of biophysical control of sap flow in Salix psammophila growing in a semiarid shrubland ecosystem of northwest China

Abstract

With increased usage of plantations in the control of desertification and water shortages in the Mu Us Desert, water use by desert-dwelling shrubs, such as Salix psammophila has become a serious topic in the management of ecologically-important plants. However, our understanding of the unevenness in water consumption across multiple timescales is still limited for many desert shrub species. Interannual variation in water consumption and its environmental controls are scarcely documented. To attend to this deficit, sap-flow measurements were continuously collected in a region of the Mu Us Desert from 2012 to 2017 to investigate the biophysical controls on sap-flow and water-consumption dynamics in S. psammophila. Diurnally, sap flow per leaf area (Js) was shown to be controlled by shortwave radiation (Rs), peaking in summer about two hours ahead of Rs, and about three to five hours ahead of air temperature (Ta) and vapor pressure deficit (VPD), respectively. Seasonal patterns in Js varied with precipitation and subsequent changes in soil volumetric water content (VWC). Soil volumetric water content was observed to modify the influence of the other environmental variables on Js (i.e., Ta, Rs, and VPD), with Js being more responsive to the site variables, when VWC > 0.09 m3 m−3, and less responsive, when <0.09 m3 m−3. Furthermore, Ta and VPD were responsible for most of the variation observed in Js during the leaf-expansion stage. Meanwhile, VWC and leaf area index (LAI) became more important during the leaf-expanded stage, and Rs during the leaf-defoliation stage. Interannually, LAI and meteoric water input to soils was responsible for most of the variation observed in Js. Reduction in Js sensitivity to biophysical factors under water-stressed conditions across temporal scales and the hysteresis effect point to species acclimatization to dry soil conditions. This study has the potential to help: (i) clarify the temporal dynamics in Js for S. psammophila across multiple timescales; and (ii) contribute to the sustainable management and control of desertification in the region under future climate change and projected increases in severe drought.