Quantifying stand water use of a multi-species afforestation site through sap flow and groundwater measurements

Afforestation efforts on degraded croplands characterized by rising groundwater tables and saline soils need to rely on well-adapted tree species. We estimated stand water-use (SWU) of a ~2.5 ha afforestation site established in 2003 with Elaeagnus angustifolia, Populus euphratica, and Ulmus pumila. Sap flow was recorded by heat dissipation sensors in two trees per species during the growing seasons (March-October) in 2012 and 2013. Simultaneously, the SWU was quantified from observations of diurnal groundwater oscillations monitored at each species plot. The SWU interpreted from the heat dissipation sensors ranged from 3 to 551 mm a^-1 whereas groundwater-based estimates ranged between 63 and 1,383 mm a^-1. The SWU peaked in P. euphratica irrespective of the evaluation method, suggesting that this species should be recommended cautiously for large-scale afforestation efforts in drylands. The results scaled up from sap flux measurements tend to underestimate the SWU whereas those calculated from groundwater oscillations might be overestimations.