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

The transport processes of methane (CH 4 ) 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 4 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 4 flux in laboratory and field conditions were explained by the axial advective CH 4 transport from soil with xylem sap flow and the radial CH 4 diffusion through the stem conditions. Diffusion resistance caused by the bark permeability did not significantly affect gas transport or stem CH 4 flux in the laboratory experiments. The role of axial diffusion of CH 4 in trees was unresolved and requires further studies. Due to the transit time of CH 4 in the stem, the diurnal dynamics of stem CH 4 fluxes can deviate markedly from the diurnal dynamics of sap flow.