Model of methane transport in tree stems: Case study of sap flow and radial diffusion.

The transport processes of methane (CH ) in tree stems remain largely unknown, although they are critical in assessing the whole-forest CH4 dynamics. We used a physically based dynamic model to study the spatial and diurnal dynamics of stem CH transport and fluxes. We parameterised the model using data from laboratory experiments with Pinus sylvestris and Betula pendula and compared the model to experimental data from a field study. Stem CH flux in laboratory and field conditions were explained by the axial advective CH transport from soil with xylem sap flow and the radial CH diffusion through the stem conditions. Diffusion resistance caused by the bark permeability did not significantly affect gas transport or stem CH flux in the laboratory experiments. The role of axial diffusion of CH in trees was unresolved and requires further studies. Due to the transit time of CH in the stem, the diurnal dynamics of stem CH fluxes can deviate markedly from the diurnal dynamics of sap flow.