Field evidence reveals conservative water use of poplars under high aerosol conditions in Beijing

<ol> <li><span>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 the leaf- and plant-level hinders our ability to understand how aerosols could affect plant water use. </span></li> <li><span>Aerosol concentrations in northern China fluctuates periodically over a wide range<a name="_Hlk47963537">.</a> Taking advantage of this unique natural experiment opportunity, we conducted a full series of supporting physiological and environmental measurements to explore aerosols' effect on leaf transpiration and sap flow in the field using poplar trees (<i>Populus </i>× <i>canadensis</i> Moench).</span></li> <li><span>We found that high aerosol concentrations suppressed sun-leaf transpiration by reducing leaf-to-air vapor pressure deficit (VPD_leaf) while not affecting shade-leaf transpiration, mainly because the negative effect of reduced VPD_leaf on transpiration offset the positive effects of the increased stomatal conductance (g_s). As aerosol concentration increases, the g_s of both sun and shade leaves decreased more rapidly with an increase in VPD_leaf, which caused their transpiration rates to become less sensitive to VPD_leaf. Similarly, aerosols reduced sap flow density and its sensitivity to VPD. </span></li> <li><span><i>Synthesis.</i> 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.</span></li> </ol>