Unexpectedly high aldehyde emissions from stems of six major European tree species

• Tree stems emitted diverse BVOCs, with aldehydes dominating across six species. • Aldehydes accounted for ≥ 60% of total stem BVOC fluxes in all species. • Conifers showed higher stem monoterpene emissions than broadleaved species. • Stem BVOC emission patterns were species-specific and partly temperature-driven. • Upscaled stem sabinene and limonene fluxes contributed ∼0.2% of foliar fluxes. Tree stems represent an important but usually overlooked ecosystem component contributing to the budget of biogenic VOCs (BVOCs) in forests. Here, we quantified stem BVOC fluxes from six major European tree species – Quercus robur, Fagus sylvatica, Larix decidua, Pseudotsuga menziesii, Abies alba and Picea abies – in a mixed temperate forest in southwestern Germany. BVOC flux rates were determined in late spring and early summer 2025. Across all tree species studied, 25 BVOCs were consistently exchanged between stems and the atmosphere. BVOC composition of all trees was dominated by aldehydes (≥ 60% of total BVOCs exchanged), followed by monoterpenes, aromatics and alkanes. In contrast to deciduous trees, conifers showed considerably higher monoterpene emission. Multivariate analyses revealed species-specific emission patterns particularly for Q. robur and F. sylvatica . To compare tree stem to leaf BVOC emissions at the ecosystem scale, we upscaled sabinene and limonene emissions based on total stem surface area and leaf area index (LAI), respectively. Based on the approximation that the total stem surface area amounts to ca. 29% of the total leaf area, stem sabinene and limonene fluxes from F. sylvatica accounted for approximately 0.19–0.21% of corresponding foliar emissions. However, given the much higher proportion of aldehyde emissions, stems could be non-negligible and should be considered when quantifying total BVOC fluxes in temperate forests, underscoring the need to incorporate species-specific stem fluxes into ecosystem-scale emission models.