Field Evidence Reveals Conservative Water Use of Poplar Saplings under High Aerosol Conditions

Aerosols could alter the plant photosynthesis and transpiration through its radiative and climatic effects, and consequently influence ecosystem carbon and water cycles. While many concerns have been focused on how aerosols affect photosynthesis, few have paid attentions to aerosol effects on transpiration. In particular, field observations of plant water use in leaf and plant level under varied aerosol loadings are still lacking, which greatly hinders our understanding of the underlying mechanisms. Here, we conducted a series of physiological and environmental measurements at different time of the day in northern China to explore aerosol effects on leaf transpiration and sap flow of planted poplar saplings (Populus × canadensis Moench). We found that high aerosol loading suppressed sun leaf transpiration by reducing leaf-to-air vapor pressure deficit (VPDleaf), but had no effects on shade leaf mainly because the negative effects of reduced VPDleaf on transpiration were offset by the positive effects of increased stomatal conductance (gs). At the conditions with high aerosol loading, the gs of both sun and shade leaves decreased more rapidly with increase in VPDleaf, leading to transpiration rates less sensitive to the changes of VPDleaf. Similarly, aerosols reduced sap flow density and its sensitivity to VPD. The reduced transpiration and stronger stomatal control indicated that plant water use becomes more conservative under elevated aerosol concentrations.