Temporary reductions of stem CO2 efflux during rainfall events across tree species

Stem respiration is an important component of forest carbon cycling, yet its accurate quantification remains challenging. While stem respiration is commonly measured as CO 2 efflux from the stem surface (E A ), several processes can decouple measured fluxes from actual respiratory activity. This study investigated the influence of rainfall events on E A across five temperate tree species during five growing seasons and assessed the potential consequences for seasonal carbon budget estimates. Continuous automated chamber measurements revealed distinct short-term decreases in E A coinciding with rain events. Time-lapse camera records confirmed that stem surfaces were frequently covered by a water film during rainfall, suggesting that wet bark temporarily restricted CO 2 diffusion to the atmosphere. Rain-related decreases in E A accounted for 2 – 13% of the available seasonal data (May–October) and lead to a 0.3 – 5.3% underestimation of the seasonal sums of measured E A , with the effect varying among tree species. The largest impact was observed in beech, followed by hornbeam and spruce, whereas oak and ash showed the smallest underestimation. Our results demonstrate that rainfall can systematically bias E A measurements and lead to underestimation of seasonal and annual carbon fluxes from tree stems. Recognizing and accounting for these temporary but repeated events is therefore essential for improving the accuracy of forest carbon budget assessments under both current and future precipitation regimes.