Insect herbivory have significantly altered BVOC emissions, SOA concentration and radiative forcing over Fennoscandian birch forest

<p>Over Fennoscandian mountain birch forest region, there are increased attacks of geometrid moth larvae. These herbivores can change forests from a carbon sink to a carbon source. When moths start to chew on leaves, large quantities of biogenic volatile organic compounds (BVOCs) are released. Herbivory-induced BVOC emissions have been observed and quantified at a few sites over Fennoscandian mountain birch forest, but we know very little of their potential regional implications for atmospheric processes.</p> <p>In this work, we extracted birch defoliation information based on MODIS leaf area index (LAI) for an outbreak year 2012, and together with field-observed relationship between leaf defoliated level and changes in emissions, we modelled herbivory-induced BVOC emissions at regional scale using MEGAN. Taking a step further, we fed MEGAN-modelled BVOC emission data with or without considering herbivory impacts to a two-way coupled WRF-CMAQ system to dynamically assess the impacts of these emissions on the atmospheric chemistry and climate system .</p> <p>During the whole growing season of 2012, the defoliation at some MODIS grids can be as high as 90%, and the large defoliation mainly occurs in June and July. For <em>t</em>-<em>&#946;</em>-ocimene, Other Monoterpenes, Stress and Other compound groups, herbivory contributes to more than 30, 8, 5 and 16 times the increase in the seasonal sum for the defoliated regions. For terpenes, herbivory increased monthly emissions up to 3 times for June and July. The reduction of emissions caused by herbivory-caused decrease in LAI is much smaller than the herbivory-induced increase. We also found strong impacts of herbivory-induced BVOC emissions on downward shortwave radiation and cloud radiative forcing.</p> <p>This is the first time we can link all these components, i.e., satellite monitoring of leaf defoliation, in-situ observation, ecosystem and atmospheric modelling together to answer the research questions related to the regional importance of insect herbivory on atmospheric composition and climate.</p>