Substantial Stem Methane Emissions From Rainforest and Cacao Agroforest Partly Negate Soil Uptake in the Congo Basin

Abstract Most stem CH 4 emission investigations were conducted in wetland forests, and it remains unknown whether trees in tropical rainforests on heavily weathered soils are substantial contributors to ecosystem CH 4 fluxes. Additionally, Africa remains the continent with the lowest numbers of published field studies on soil greenhouse gas fluxes. Over one year, we measured monthly stem and soil CH 4 fluxes with forest conversion to traditional cacao agroforestry in the Congo Basin, Cameroon. All the measured trees emitted CH 4 throughout the year. The annual total (soil + stem) CH 4 fluxes were −2.62 ± 0.37 kg C ha −1  yr −1 from the forest and −3.23 ± 0.45 kg C ha −1  yr −1 for the cacao agroforestry, with stem CH 4 emissions equating to 4%–21% of the soil sink in the forest, and 3%–27% of the soil sink in cacao agroforestry. 2% of our measured trees were “hot spots” of CH 4 emission, with two orders of magnitude larger emissions than the remaining trees in our sites. Correlations of stem CH 4 emissions and soil drivers suggest a potential belowground source of stem CH 4 emissions, although this pathway may be working simultaneously with a within‐tree microbial CH 4 source. Overall, forest conversion to traditional, mature cacao agroforestry did not affect stem and soil CH 4 fluxes. Our results demonstrate that tropical trees on well‐drained, highly weathered soils represent potential CH 4 emission pathways that have largely been ignored, and provides evidence that upland ecosystems may be weaker sinks of atmospheric CH 4 than previously estimated.