The biosphere and atmosphere are intricately linked, with atmospheric changes impacting biodiversity and vice-versa. Plants release BVOCs, whose atmospheric oxidation leads to BSOA formation. BSOA significantly influences Earth's radiative balance and cloud formation. Climate change and biodiversity loss are known to affect BVOC and BSOA, yet concerted measurements across diversity gradients are lacking. This study addresses this gap by proposing a conceptual framework detailing the relationship between biodiversity, BVOC emissions, and BSOA formation. The framework considers three key hypotheses: 1) Increased biodiversity enhances plant biomass and thus BVOC emissions; 2) Diverse communities mitigate abiotic stress, reducing BVOC emissions; and 3) Reduced biotic stress in diverse communities decreases BVOC emissions. The framework is partially tested through a case study using a tree diversity experiment, aiming to establish a link between biodiversity and BVOC/BSOA formation and guide future research on the coupled crises of biodiversity loss and climate change.

Existing literature highlights the strong interconnection between the biosphere and atmosphere, with atmospheric drivers impacting biosphere integrity and biodiversity. Conversely, the biosphere influences the atmosphere through BVOC releases from plants and soil. The oxidation of these BVOCs leads to BSOA formation, impacting Earth's radiative balance and cloud formation. Studies show that climate extremes and biodiversity changes affect plant BVOC emissions and subsequently, BSOA formation. However, a critical knowledge gap exists: few studies have simultaneously measured BVOCs and BSOAs along diversity gradients of plant species. This gap is particularly concerning given human preference for monoculture plantations and the urgent need for effective reforestation strategies in the face of climate change. Previous research emphasizes species-specific BVOC emissions, the role of BVOCs in plant interactions (communication, defense), and the correlation between forest biomass and BVOC emissions. The effects of biotic and abiotic stress on BVOC emissions are well-documented, highlighting the need to investigate how biodiversity influences these factors.

This study investigated the effects of tree diversity on BVOC emissions and BSOA formation using the MyDiv tree diversity experiment in Germany. Ten plots with varying tree diversity (monocultures, two-species, and four-species mixtures of *Acer pseudoplatanus*, *Fraxinus excelsior*, *Prunus avium*, and *Sorbus aucuparia*) were selected. BVOC and BSOA were measured simultaneously using custom-made sampling boxes installed in the upper canopy. Gas-phase sampling utilized Carbotrap 300 and Tenax cartridges, while particulate matter sampling used a PM10 impactor. Samples were collected for four hours daily over 13 days, excluding rainy days. BVOCs were analyzed via gas chromatography-mass spectrometry (GC/MS) with thermodesorption. BSOA compounds were extracted from quartz filters and analyzed using ultra-high-performance liquid chromatography coupled with high-resolution Orbitrap mass spectrometry. Meteorological parameters (temperature, humidity, pressure, wind speed, UV radiation) and atmospheric oxidants (NO3 and ozone) were also measured. Wood volume and annual wood productivity were calculated for each plot using existing MyDiv inventory data. Statistical analyses included one-way ANOVA, Tukey's HSD post-hoc test, t-tests, and standardized mean difference (SMD) analysis to assess the impact of diversity on BVOC and BSOA. Pearson's correlation coefficients examined associations between atmospheric radicals, α-pinene, and BSOA compounds.

Nine BVOCs (α-pinene, camphene, β-pinene, 3-carene, *p*-cymene, limonene, α-terpinene, isophorone, acetophenone) and fifteen BSOA compounds were quantified. Analysis showed that most BVOCs did not significantly differ between monocultures and mixtures, except for limonene and acetophenone, which were higher in monocultures. Comparing observed and expected BVOC amounts in mixtures revealed that four-species mixtures produced significantly lower amounts of limonene and acetophenone than expected. Standardized mean difference analysis showed that increasing tree diversity significantly decreased the overall concentration of BVOCs, particularly limonene, with marginal significance for β-pinene, *p*-cymene, α-terpinene, and acetophenone. The negative correlation between these BVOCs and annual wood productivity contradicted the initial hypothesis. For BSOA, no significant differences were found between monocultures and mixtures, though some compounds showed trends (succinic and sebacic acids higher in monocultures; pinonic and terebic acids higher in mixtures). The overall effect of diversity on BSOA was mixed and non-significant. Correlations between atmospheric radicals and α-pinene with BSOA showed limited significant associations due to the small number of measured plots.

The findings support hypotheses 2 and 3, suggesting that reduced abiotic and biotic stress in mixed forests contributes to lower BVOC emissions. The decrease in BVOCs with increasing biodiversity can be attributed to reduced resource competition and stress amelioration in mixed stands. Mixed results for BSOA likely stem from the influence of regional air masses on BVOC-to-BSOA conversion, highlighting the need for local and regional-scale models in future work. The complex patterns observed in BSOA formation underscore the role of source precursors, atmospheric oxidants, and other factors (e.g., gas-phase oxidation mechanisms, climate variability) influencing the type and amount of BSOA produced. These factors emphasize the need for more comprehensive measurements across larger diversity gradients.

This study demonstrates a general relationship between biodiversity and BVOC emissions, but further research is needed to fully elucidate the complex interactions affecting BSOA formation. Future studies should incorporate detailed microclimate investigations, monitoring of above- and below-ground biotic and abiotic stresses, manipulation of stress conditions in long-term experiments, and incorporate seasonal variations in photosynthetic activity. Local and regional-scale modeling combined with field and chamber measurements are crucial for advancing the understanding of biosphere-atmosphere interactions and the reciprocal effects of biodiversity and climate change.

The relatively small number of plots in the case study limits the statistical power, especially for BSOA analysis. Regional-scale influences on BSOA formation could not be fully accounted for. The study focused on a specific time period, preventing generalizations about seasonal effects. Extrapolation to other forest ecosystems or biodiversity scenarios may require caution, as local climate and species composition can influence BVOC and BSOA dynamics.