Field evidence reveals conservative water use of poplar saplings under high aerosol conditions

Anthropogenic aerosols could alter multiple meteorological processes such as radiation regime and air temperature, thereby modifying plant transpiration. However, the lack of field observations at leaf and plant level hinders our ability to understand how aerosols could affect plant water use. Aerosol concentrations in northern China fluctuate periodically over a wide range. Taking advantage of this unique natural experiment opportunity, we conducted a series of physiological and environmental measurements at different times of the day to explore diurnal aerosols' effect on leaf transpiration and sap flow of planted poplar saplings (Populus × canadensis Moench). We found that high aerosol concentrations suppressed sun leaf transpiration by reducing leaf‐to‐air vapour pressure deficit (VPDₗₑₐf), while had no effect on shade leaf transpiration mainly because the negative effect of reduced VPDₗₑₐf on transpiration offset the positive effects of the increased stomatal conductance (gₛ). As aerosol concentration increased, the gₛ of both sun and shade leaves decreased more rapidly with an increase in VPDₗₑₐf, which caused their transpiration rates to become less sensitive to VPDₗₑₐf. Similarly, aerosols reduced sap flow density and its sensitivity to VPD. Synthesis. Our study provided observational evidence on aerosols' effects on plant transpiration at the leaf and canopy scales. The reduced transpiration and stronger stomatal control indicated that plant water use becomes more conservative under elevated aerosol concentrations.