Anthropogenic activities have induced a dramatic increase in atmospheric aerosol. Although aerosol reduces total radiation, it increases the fraction of diffuse radiation and therefore could increase canopy photosynthesis. High aerosol pollution is often concurrent with higher air humidity and lower air temperature. However, the effect of the concomitant meteorological conditions on plant carbon assimilation under elevated aerosol is difficult to disentangle and their contributions remains unquantified.
The large temporal variation in aerosol level in Northern China provides unique opportunity for studying how photosynthesis and plant growth respond to different aerosol loadings. We measured aspen sunlit and shade leaf photosynthesis rates, stomatal conductance (Gs), transpiration rates (Tr) and daily stem growth under different aerosol loadings for three growing seasons. We aimed to (1) assess how aerosol induced radiation changes affect sunlit, shade leaf photosynthesis capacity and tree growth; (2) disentangle the relative impacts of the accompanying meteorological conditions on the response of plant carbon assimilation to aerosols.
Results/Conclusions
Our experiment found that photosynthesis of sunlit leaves, shade leaves and daily stem growth increased linearly with increase in aerosol loading. For sunlit leaves, the positive response was partially attributed to aerosols-induced decreasing total radiation, whereas for shade leaves, the increase in photosynthesis was mainly due to the increase in diffuse radiation. In addition, we found that concurrent changes in air humidity and temperature play an important role in modulating plant response to aerosols through stimulating stomatal conductance and relieving the depression by midday high-temperature. Our experiment provides direct field evidences on aerosol’s positive effect on leaf photosynthesis and plant growth. Our findings also demonstrate that the concurrent changes in meteorological conditions alter the response sensitivity of photosynthesis to aerosols, and these effects could be as important as the diffuse radiation fertilization effect induced by aerosol.