Tree stem bases are sources of CH and N O in a tropical forest on upland soil during the dry to wet season transition.

Tropical forests on upland soils are assumed to be a methane (CH ) sink and a weak source of nitrous oxide (N O), but studies of wetland forests have demonstrated that tree stems can be a substantial source of CH , and recent evidence from temperate woodlands suggests that tree stems can also emit N O. Here, we measured CH and N O fluxes from the soil and from tree stems in a semi-evergreen tropical forest on upland soil. To examine the influence of seasonality, soil abiotic conditions and substrate availability (litter inputs) on trace greenhouse gas (GHG) fluxes, we conducted our study during the transition from the dry to the wet season in a long-term litter manipulation experiment in Panama, Central America. Trace GHG fluxes were measured from individual stem bases of two common tree species and from soils beneath the same trees. Soil CH fluxes varied from uptake in the dry season to minor emissions in the wet season. Soil N O fluxes were negligible during the dry season but increased markedly after the start of the wet season. By contrast, tree stem bases emitted CH and N O throughout the study. Although we observed no clear effect of litter manipulation on trace GHG fluxes, tree species and litter treatments interacted to influence CH fluxes from stems and N O fluxes from stems and soil, indicating complex relationships between tree species traits and decomposition processes that can influence trace GHG dynamics. Collectively, our results show that tropical trees can act as conduits for trace GHGs that most likely originate from deeper soil horizons, even when they are growing on upland soils. Coupled with the finding that the soils may be a weaker sink for CH than previously thought, our research highlights the need to reappraise trace gas budgets in tropical forests.