Responses of canopy transpiration and conductance to different drought levels in Mongolian pine plantations in a semiarid urban environment of China

Although tree transpiration and conductance responding to soil drought are increasingly investigated in urban settings, the increasingly important effects of atmospheric drought (i.e., high vapor pressure deficit, VPD) and combined drought (high VPD and soil drought) on urban tree water use remain unclear. These drought events are becoming more frequent and severe with climate change, posting a greater threat to urban trees, especially in arid regions. Therefore, we continuously measured canopy transpiration (Ec) of Mongolian pine (Pinus sylvestris var. mongolica) trees, soil moisture (VWC), and meteorological factors located in a semiarid urban environment in northern China from May to September in 2017-2019 and 2021. Seasonally, Ec and canopy conductance (Gc) were lower in May and June because of the low precipitation. With increased precipitation and VWC since July, Ec and Gc increased significantly. Interannually, Ec was significantly higher in dry years (2017 and 2021) than in wet years (2018 and 2019). Overall, due to high soil water availability and transpirational pull, Ec under atmospheric drought (1.03 mm d-1) was significantly higher than non-drought (0.74 mm d-1), combined drought (0.67 mm d-1), and soil drought (0.48 mm d-1). The Ec and Gc were dominantly influenced by solar radiation (Rs) under non-drought conditions, while Rs and air temperature had larger effects on Ec under atmospheric drought conditions. The Gc was significantly inhibited by VPD when VPD was high. During soil and combined droughts, Ec and Gc were mainly affected by VWC. The Gc increased with decreasing VPD and low VPD still promoted Ec under soil drought conditions. Our study confirmed that different drought levels have significantly different effects on Ec and Gc. As future climate changes, atmospheric drought and combined drought will accelerate the water loss of Mongolian pine and aggravate tree water stress in urban environments.