Methane emissions and production from tree stems of Quercus suber in a Mediterranean forest

Upland trees can exchange CH₄ with the atmosphere through the stems. Stem emissions could be produced in the soils and transported through the roots, or could be produced in the hardwood by methanogenic archaea inhabiting the trees. However, there is still limited information on how the different origins depend on different species or environmental conditions. There is the general understanding that stem CH₄ emissions are controlled by soil moisture conditions, and therefore, trees from water-limited ecosystems might present little (if any) emissions. However, this hypothesis has not been tested yet in water-limited ecosystems, such as Mediterranean ones. Here we present a study on stem CH₄ emissions from cork oak (Quercus suber), a drought-adapted species from the Mediterranean basin. The bark of this species (cork) is commonly extracted for business, since it has insulation characteristics. We assessed the effect of cork removal (peeling) on stem emissions, since cork may act as a physical barrier for methane diffusion from the stem to the atmosphere.

We measured CH₄ stem emissions from peeled and unpeeled trees at two stem heights, one on the cork extraction zone (bottom part of the stem) and the other above it (unpeeled zone). Additionally, we performed wood anaerobic incubations to assess the CH₄ production capacity, and analysed the microbial community composition in the hardwood, sapwood and cork tissues.

Our results showed that cork oaks emitted high CH₄ rates (59.83 μmol m^-2h^-1 on average), which were positively correlated with DBH. Surprisingly, we did not see any effect of cork peeling in CH₄ emissions, not even in the measurements performed immediately after the cork removal. We observed, however, a strong vertical pattern for all trees and campaigns, with emissions being higher on the base of the trees. Despite this vertical pattern, usually associated with soil CH₄ origin, significant CH₄ production in the tree cores, and a positive correlation between stem CH₄ fluxes and the abundance of methanogenic-related genes suggest an internal stem origin of CH₄. These results suggest that stem internal conditions might be more important controlling stem CH₄ emissions than soil or atmospheric environmental conditions.