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

Abstract

Most stem CH4 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 CH4 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 CH4 fluxes with forest conversion to traditional cacao agroforestry in the Congo Basin, Cameroon. All the measured trees emitted CH4 throughout the year. The annual total (soil + stem) CH4 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 CH4 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 CH4 emission, with two orders of magnitude larger emissions than the remaining trees in our sites. Correlations of stem CH4 emissions and soil drivers suggest a potential belowground source of stem CH4 emissions, although this pathway may be working simultaneously with a within-tree microbial CH4 source. Overall, forest conversion to traditional, mature cacao agroforestry did not affect stem and soil CH4 fluxes. Our results demonstrate that tropical trees on well-drained, highly weathered soils represent potential CH4 emission pathways that have largely been ignored, and provides evidence that upland ecosystems may be weaker sinks of atmospheric CH4 than previously estimated.