Seasonal variation in vegetation-climate interactions shape the CO₂ exchange in a degraded raised bog

Abstract. Pristine peatlands act as natural carbon sinks, but disturbance — mostly anthropogenic drainage — turns them into CO₂ sources, responsible for 2–5 % of annual greenhouse gas (GHG) emissions globally. Complex interactions between vegetation, soil, climate, and hydrology produce highly variable CO₂ budgets on different types of peatlands and between years. Abandoned drained peatlands are considered low-hanging fruit for rewetting due to expected high GHG emissions and low resistance to repurposing yet remain underrepresented in research. To close this gap in literature we measured three years (2023–2025) of CO₂ and CH₄ fluxes alongside meteorological and hydrological conditions in a drained shrub-dominated ombrotrophic raised bog in NW Germany, investigating carbon flux budgets and the main seasonal drivers of fluxes. Methane fluxes were negligible throughout, likely due to consistently deep water tables (>15 cm). Annual CO₂ budgets were highly variable: the site was a considerable source in 2023 and 2025 (131 and 86 gC m^-2 a^-1) but near-neutral in 2024. An anomalously warm spring in 2024 triggered earlier vegetation greening and substantially increased CO₂ uptake capacity from April through June. In contrast, warming later in the growing season increased CO₂ emissions due to a stronger reaction of respiration than of photosynthesis to warming — highlighting how the timing of climate anomalies matters. Partitioning the effects of high air temperature (TA) and vapor pressure deficit (VPD) revealed that high VPD suppressed carbon fluxes in the first half of the growing season but not the second, while extreme TA did not limit GPP or ecosystem respiration the way extreme VPD did. TA and solar radiation were the dominant daily flux drivers; water table depth did not govern daily or interannual carbon flux variability. Together, our results demonstrate that the timing of TA and VPD anomalies — mediated through vegetation responses — decisively shapes their impact on the carbon balance. These results will become increasingly relevant as climate extremes intensify with ongoing global warming.