Phenology is the dominant control of methane emissions in a tropical non-forested wetland.

Tropical wetlands are a significant source of atmospheric methane (CH), but their importance to the global CH budget is uncertain due to a paucity of direct observations. Net wetland emissions result from complex interactions and co-variation between microbial production and oxidation in the soil, and transport to the atmosphere. Here we show that phenology is the overarching control of net CH emissions to the atmosphere from a permanent, vegetated tropical swamp in the Okavango Delta, Botswana, and we find that vegetative processes modulate net CH emissions at sub-daily to inter-annual timescales. Without considering the role played by papyrus on regulating the efflux of CH to the atmosphere, the annual budget for the entire Okavango Delta, would be under- or over-estimated by a factor of two. Our measurements demonstrate the importance of including vegetative processes such as phenological cycles into wetlands emission budgets of CH.