Environmentally-uncoupled tree stem methane fluxes from temperate, Mediterranean and tropical upland ecosystems

Trees can emit methane to the atmosphere through the stems. These fluxes might represent a large (and still unaccounted for) source of methane from forests to the atmosphere, but the uncertainties related with the spatial variability are still too large for properly estimating their contribution to the regional CH₄ budgets. The general understanding is that these emissions are microbial–produced (either originated in soils or in the heartwood of trees), and thus, they are assumed to be temperature-and-water dependent. However, this assumption has not been tested yet at large scales, from different and contrasted ecosystems and with multiple species. In this study, we measured stem CH₄ fluxes on more than 400 trees from 28 different species, spanning temperate, Mediterranean and tropical ecosystems. Our main goal was to distinguish between site-specific, species-specific, and environmental effects on controlling stem CH₄ fluxes. Preliminary results showed that species identity regulates stem CH₄ fluxes independently of environmental conditions, which might be due to wood properties providing a range of internal stem microhabitats for methanogenic communities or controlling gas diffusivity through the wood.