Water-use response to climate factors at whole tree and branch scale for a dominant desert species in central Asia: Haloxylon ammodendron

Water use of a dominant desert species, Haloxylon ammodendron, growing in the southern edge of Gurbantunggut Desert in northwest China, was measured using sap flow technologies at both tree and branch scales. Daily sap flow at whole-tree scale showed a good relationship with atmospheric evaporation demand (P<0.001), whereas daily sap flow at branch scale showed good relationships with the variable of transpiration rate (P<0.01). Hysteresis in hourly sap flow at both scales was observed as a function of micro-meteorological variables, in which the hysteresis between sap flow and vapour pressure deficit was clockwise rotation, suggesting that the water use of H.ammodendron was related to not just the stomatal behaviour but also use of stored plant water. The hysteresis between sap flow and photosynthetic photon flux density was counter-clockwise after precipitation, but turned to be clockwise under dry conditions, which was ever clearer at whole-tree scale. Different responses of sap flows at branch and whole-tree scale to climate factors were identified, which strongly indicated that H.ammodendron had some mechanisms to reduce whole-tree water use at high transpiration demand, while maintaining part of canopy under relatively high water use, which resulted in canopy scale patchiness. Copyright (c) 2012 John Wiley & Sons, Ltd.