Groundwater depth affects the daily course of gas exchange parameters of Populus euphratica in arid areas

In this case study, the diurnal courses of gas exchange in Populus euphratica at different groundwater depths in the lower reaches of Tarim River were investigated to understand the effects of soil hydrology on photosynthesis and water use efficiency (WUE) of vegetation in arid desert area. It was found that the photosynthetic rate (PN) was not sensitive to the change of groundwater depth (GD) within the range of 4.2–6.8 m. Compared to stomatal conductance (gs) and transpiration rate (E) of P. euphratica grown at GD 4.2, 5.6 and 5.8 m, gs and E at GD 6.8 m both markedly declined in June, suggesting that P. euphratica at deeper GD can avoid overall water loss by stomatal adjustment. The intrinsic water use efficiency of P. euphratica first decreases with the increasing GD, but when GD increased to 6.8 m, intrinsic WUE increased by 1.2–2.2 fold, compared with the WUE of P. euphratica at GD 4.2–5.8 m, indicating that intrinsic WUE of P. euphratica will increase when the plant suffers from moderate drought stress.