Cloud cover, tree size and species modulate the positive effects of aerosols on tree growth

Abstract Atmospheric aerosols can influence plant growth and terrestrial carbon cycling by altering solar radiation regimes and microclimate conditions. However, accurately quantifying their net effects requires a better understanding of how aerosol-growth relationships are modulated by cloud cover conditions and plant traits, for which empirical evidence remains limited. To address this, we conducted a six-year field campaign to monitor PM2.5 concentrations (as a proxy for aerosol loading), meteorological variables, and daily stem growth in two temperate tree species: aspen (Populus euramericana Neva.) and pine (Pinus tabuliformis Carr.). We systematically evaluated how aerosols affect tree growth and how these effects are regulated by cloud cover, tree size, and species. Our findings revealed that aerosols generally promoted the stem growth of aspen, primarily through their diffuse radiation fertilization effect and concomitant improvements in air humidity (i.e. lower vapor pressure deficit). This positive effect was highly dependent on sky conditions: growth increased linearly under clear skies but exhibited a saturating-to-declining trend under overcast skies. Additionally, tree size influenced aspen’s growth response to aerosols, with smaller trees showing a lower absolute enhancement in stem cross-sectional area but a greater relative stem growth rate under high aerosol conditions. In contrast to the fast-growing aspen, the slow-growing pine showed no significant stem growth response to aerosols. These field-based findings propose a mechanistic framework for studying aerosol-ecosystem interactions and provide empirical benchmarks essential for improving predictions of plant growth under realistic environmental conditions.