High aerosol loading decreases the transpiration of poplars both in the day- and night-time

Aerosols alter plant photosynthesis and ecosystem carbon uptake through radiative effects. These effects inevi-tably change plant transpiration and ecosystem water use as CO2 and water exchange couple through leaf sto-matal conductance. However, because of lacking field observations, we know poorly about how elevated aerosols could affect plant transpiration, which greatly hinders our confidence in projecting aerosol's climate impacts. In this study, taking the advantage of aerosol loading in northern China fluctuated periodically over a wide range, we conducted five year-site observations of the stem sap flow of poplar saplings (Populus euramericana Neva.), and simultaneously monitored environmental factors such as PM2.5 concentration, total and diffuse radiation, air temperature, humidity, and soil water content. These observations enabled us to comprehensively explore how aerosols affect the daily dynamics of plant canopy transpiration in the field. We found that high aerosol loading significantly decreased the sap flow density of poplars both in the day-and night-time, indicating that aerosols decreased canopy transpiration. Furthermore, during the day-time, a reduction in total solar radiation and moisture demand (vapor pressure deficit, VPD) dominated the decrease of sap flow density, but an increase in diffuse radiation had an insignificant impact. In the night-time, VPD significantly decreased under high aerosol loading conditions, thereby reducing nocturnal sap flow density. Our in-situ observations revealed that different from the positive response of canopy photosynthesis associated with the diffuse fertilization effect, aerosols inhibit canopy transpiration mainly because of the declined VPD. Our findings highlighted that plant carbon and water fluxes are governed by different meteorological factors under elevated aerosol.