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

Abstract

Upland trees can have the capacity to emit CH4 through the stems. Tree-derived emissions can result from soil source or be produced in tree tissues by methanogenic archaea inhabiting the trees. However, there is still limited information on how the different origins depend on different species or environmental conditions. No studies of tree CH4 emissions have been done in water-limited ecosystems such as Mediterranean ones. Here we present a study on stem derived CH4 emissions from cork oak (Quercus suber), a species well distributed across the Mediterranean basis. The bask of this species (cork) is commonly extracted for business, since it has insulation characteristics. We assessed the effect of cork removal (peeling) on the potential stem emissions, since cork may be acting as a physical barrier for methane diffusion from the stem to the atmosphere. We measured CH4 stem emissions of peeled and unpeeled trees at two different heights, one on the cork extraction zone (bottom part of the stem) and the other above it (unpeeled zone). Measures were made on five campaigns around peeling season. We correlated these emissions with diameter at breast height and soil volumetric water content (VWC). We also took tree cores to assess their capacity to produce CH4 under laboratory anaerobic incubations. Our results prove that cork oaks were not only capable of emitting CH4 but also had high fluxes compared to other studies (59.83 µmol m-2 h -1 on average), positively correlated with DBH. Fluxes had a very strong vertical pattern through the stem, being higher on the base of the tree. This could be due to the composition and physical properties of the cork at the lower part of the tree, since all trees in our study were peeled 12-14 years ago. The extracted layer regenerates to a thinner and more fractured cork. We suggest that the regenerated layer may be more permeable than the original one, being less resistant to gas diffusivity. Surprisingly, no differences were found between peeled and unpeeled trees, not even three months after extracting the cork, suggesting that the regenerated cork layer may have almost the same resistance to CH4 flux as the absence of the layer. VWC did not influence the flux either, suggesting that soil production was unlikely to be the methane source. Moreover, CH4 production was found in all three tissues for all trees (with no difference between peeled and unpeeled trees), suggesting that the emitted CH4 was produced by tree tissues