Transpiration of Pinus sylvestris var. mongolica trees at different positions of sand dunes in a semiarid sandy region of Northeast China

Key message Mongolian pine transpiration was driven by soil water at the top and vapor pressure deficit at the bottom of sand dunes; this was modulated by interannual variability in water availability. Mongolian pine (Pinus sylvestris var. mongolica) plantations have been suffering frequent diebacks in years of extreme drought due to water deficit in semiarid sandy regions of Northeast China. However, the transpiration dynamics of Mongolian pine trees at different positions of sand dunes and their responses to environmental variables remain unknown. Here, canopy transpiration and conductance of 38-year-old Mongolian pine trees at top (TS) and bottom (BS) of sand dunes were quantified using sap flow method from 2018 to 2019. Canopy transpiration per unit leaf area (E-L) was, respectively, 0.45 +/- 0.13 and 0.53 +/- 0.17 mm d(-1) at TS and BS in 2018, with relatively low soil moisture, and it was, respectively, 0.51 +/- 0.15 and 0.59 +/- 0.19 mm d(-1) in 2019, with relatively high soil moisture. Moreover, E-L variability explained by climate variables was greater at BS than at TS during 2018, but the trend was reversed during 2019. In addition, canopy conductance at BS was 11.5% higher than that at TS, but magnitude of increase from 2018 to 2019 was higher at TS. Stomatal sensitivity to vapor pressure deficit was higher at BS than at TS in 2018, but comparable in 2019. Consistently, stomatal sensitivity to soil moisture was higher at TS than at BS in 2018, but the trend was reversed in 2019. Taken together, these findings indicate that Mongolian pine trees at TS exhibit lower transpiration, which is primarily driven by soil moisture, than those at BS, presenting greater susceptibility to dieback under extreme drought.