Forest canopy interactions with aerosols: important considerations in approaching future impacts and climate...

Aerosols influence forest ecosystems through changes in radiation and climate affecting plant physiology and structure. Conversely, forests also contribute to aerosol formation. They emit primary aerosol particles and volatile organic compounds, which promote secondary organic aerosol formation in the atmosphere. This forest-aerosol coupling is highly dynamic, influenced by temperature, radiation, humidity, and trace gases. Wildfires add further complexity via smoke plumes altering radiation and ecosystem functioning, tropospheric ozone levels and stratospheric chemistry. Aerosols modify the quantity, directionality, and composition of solar radiation. The type of diffuse light produced by aerosol particles is however strongly dissimilar to the one produced under clouds, and the relevance of the traditional diffuse/direct binary paradigm is discussed. Therefore, potential benefits from increased diffuse radiation are contingent on aerosol load, canopy structure, and prevailing environmental conditions. Beyond photosynthetic responses, aerosols alter forest water-use efficiency and microclimate, yet their long-term effects on plant development, architecture, and community composition remain poorly understood. This review highlights significant knowledge gaps and recent advances in understanding aerosol-forest interactions across temporal and spatial scales. We underline the urgent need for improved experiments with realistic diffuse shading, extensive in situ observations, mechanistic model intercomparison, and global validation to guide future research and policy.