Transpiration and water use sources of poplar (Populus × xiaozhuanica) plantations with 7-years vs. 20-years old in a semiarid sandy region of Northeast China

Determination of transpiration and water use sources of trees in arid and semiarid areas is crucial for forest management under climate change scenes. Here, seasonal variations in transpiration and water use sources of the 7– (1167 trees ha⁻¹) and 20 (475 trees ha⁻¹)–year–old poplar (Populus × xiaozhuanica) plantations were determined based on thermal dissipation and stable isotope methods. Results showed that transpiration per unit leaf area averaged 1.6 and 1.7 mm d⁻¹ for the 7– and 20–year–old plantations during the measurement period, respectively, whereas stand transpiration (E_s) was 1.5 and 1.4 mm d⁻¹. Sensitivity of E_s to climatic variables decreased with reduction of soil moisture for both plantations, but larger decrease for 7–year–old plantation indicated higher sensitivity to drought. E_s of both plantations significantly decreased with decline of groundwater levels, but E_s of 7–year–old plantation was more limited by decline of groundwater level under drought conditions. During the measurement period, accumulated E_s occupied 45 % and 44 % of precipitation for 7– and 20–year–old plantations, respectively, indicating that water consumption of plantation did not increase with increasing stand age. Isotope data indicated that both 7– and 20–year–old trees switched water sources from 0 to 100 cm soil layer and groundwater in spring to 0–200 cm soil water and groundwater in summer as soil moisture and groundwater level decreased, but contribution of groundwater decreased from 47 % to 32.3 % and from 39.6 % to 37 % for 7– and 20–year–old plantation, respectively. These findings indicated that transpiration of poplar plantations did not vary with stand age, but a decline of groundwater level reduced contribution of groundwater to transpiration for 7–year–old poplar plantations and thus exacerbated negative effect of soil drought on transpiration compared with 20–year–old poplar plantation.