Soil moisture control of sap-flow response to biophysical factors in a desert-shrub species, Artemisia ordosica

Abstract. The current understanding of acclimation processes in desert-shrub species to drought stress in dryland ecosystems is still incomplete. In this study, we measured sap flow in Artemisia ordosica and associated environmental variables throughout the growing seasons of 2013 and 2014 (May–September period of each year) to better understand the environmental controls on the temporal dynamics of sap flow. We found that the occurrence of drought in the dry year of 2013 during the leaf-expansion and leaf-expanded periods caused sap flow per leaf area (J_s) to decline significantly, resulting in transpiration being 34 % lower in 2013 than in 2014. Sap flow per leaf area correlated positively with radiation (R_s), air temperature (T), and water vapor pressure deficit (VPD) when volumetric soil water content (VWC) was greater than 0.10 m³ m⁻³. Diurnal J_s was generally ahead of R_s by as much as 6 hours. This time lag, however, decreased with increasing VWC. The relative response of J_s to the environmental variables (i.e., R_s, T, and VPD) varied with VWC, J_s being more strongly controlled by plant-physiological processes during periods of dryness indicated by a low decoupling coefficient and low sensitivity to the environmental variables. According to this study, soil moisture is shown to control sap-flow (and, therefore, plant-transpiration) response in Artemisia ordosica to diurnal variations in biophysical factors. This species escaped (acclimated to) water limitations by invoking a water-conservation strategy with the regulation of stomatal conductance and advancement of J_s peaking time, manifesting in a hysteresis effect. The findings of this study add to the knowledge of acclimation processes in desert-shrub species under drought-associated stress. This knowledge is essential in modeling desert-shrub-ecosystem functioning under changing climatic conditions.