Soil moisture control of sap-flow response to biophysical factors in a desert-shrub species, &amp;lt;i&amp;gt;Artemisia ordosica&amp;lt;/i&amp;gt;

Tianshan Zha,Heli Peltola,Wei Feng,Charles P.-A Bourque,Bin Wu,Duo Qian,Xin Jia,Jingyong Ma,Yujie Bai,Yun Tian

doi:10.5194/bg-14-4533-2017

Abstract

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 (Js) to decline significantly, resulting in transpiration being 34 % lower in 2013 than in 2014. Sap flow per leaf area correlated positively with radiation (Rs), air temperature (T), and water vapor pressure deficit (VPD) when volumetric soil water content (VWC) was greater than 0.10 m3 m−3. Diurnal Js was generally ahead of Rs by as much as 6 hours. This time lag, however, decreased with increasing VWC. The relative response of Js to the environmental variables (i.e., Rs, T, and VPD) varied with VWC, Js 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 Js 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.

Highlights

This study provides a significant contribution to the understanding of acclimation processes in desert-shrub species to drought-associated stress in dryland ecosystems
When Rs peaked during mid-day (13:00–14:00) in summer, there was often insufficient soil water to meet the atmospheric demand, causing gs to be limited by available soil moisture and making Js more responsive to volumetric soil water content (VWC) at noon, but less responsive to Rs and T
Sap flow in Picea crassifolia peaked at noon (12:00 and 14:00) and decreased, heightening by increasing Rs, T, and vapor pressure deficit (VPD), when Rs < 800 W m−2, T < 18.0 ◦C, and VPD < 1.4 kPa (Chang et al, 2014); sap flow in Caragana korshinskii was significantly lower during the stress period, its conductance decreasing linearly after the wilting point (She et al, 2013)

Summary

Introduction

This study provides a significant contribution to the understanding of acclimation processes in desert-shrub species to drought-associated stress in dryland ecosystems. Due to the low amount of precipitation and high potential evapotranspiration in desert ecosystems, low soil water availability limits both plant water- and gas-exchange and, as a consequence, limits vegetation productivity (Razzaghi et al, 2011). Huang et al, 2011). This progression is predicted to continue under a changing climate (Houghton et al, 1999; Pacala et al, 2001; Asner et al, 2003). Studies have shown that desert shrubs are able to adapt to hot, dry environments as a result of their small plant surface area, thick epidermal hairs, and large root-to-shoot

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