Transpiration from three dominant shrub species in a desert-oasis ecotone of arid regions of Northwestern China

Conservation management for the water dependent desert-oasis ecotone in arid northwest China requires information on the water use of the dominant species. However, no studies have quantified their combined water use or linked species composition to ecotone transpiration. Here, the water use of three dominant shelterbelt shrubs (Haloxylon ammodendron, Nitraria tangutorum, and Calligonum mongolicum) within an ecotone was measured throughout the full leaf-out period for three shrub species from 30 May to 16 October 2014, with sap flow gauges using the stem heat balance approach. Species-specific transpiration was estimated by scaling up sap flow velocities measured in individual stems, to stand area level, using the frequency distribution of stem diameter and assuming a constant proportionality between sap flow velocity and basal cross-sectional area for all stems. The mean peak sap flux densities (J_sn) for H. ammodendron, N. tangutorum, and C. mongolicum, were 40.12 g cm⁻² h⁻¹, 71.33 g cm⁻² h⁻¹, and 60.34 g cm⁻² h⁻¹, respectively, and the mean estimated daily area-averaged transpiration rates (T_daily) for the same species were 0.56 mm day⁻¹, 0.34 mm day⁻¹, and 0.11 mm day⁻¹. The accumulative stand transpiration was approximately 140.8 mm throughout the measurement period, exceeding precipitation by as much as 42.1 mm. Furthermore, T_daily of these shrubs appeared to be much less sensitive to soil moisture as compared to atmospheric drivers, and the relationship between J_sn and atmospheric drivers was likely uninfluenced by soil moisture regimes in the whole profile (to 1-m depth), especially for H. ammodendron and C. mongolicum. Results indicate that these shrubs may use deep soil water recharged by capillary rise, or may directly access shallow groundwater. This study provides quantitative data offering important implications for ecotone conservation and water and land resource management.