Spruce resins constitute a strong sink for methane (CH4)

Woody plants are known to emit methane (CH₄) as an important greenhouse gas into the atmosphere. Recent studies show that tree stems might be also sinks for CH₄; however, the mechanisms of CH₄ uptake and its fate are unknown. Norway spruce (Picea abies) is characterised as negligible CH₄ source in boreal forests. Even though spruce trees have been widely planted for its wood in large-scale monocultures in European temperate forests, no studies have focused on their CH₄ exchange potential in the temperate zone.

We determined stems of Norway spruce growing in a temperate zone aiming to find out whether the tree stems exchange CH₄ with the atmosphere and how they contribute to the forest trace gas exchange.

The measurements were performed at the experimental station of the ‘Kranzberg Forest Roof Experiment’ near Freising, Germany, in June 2019. Fluxes of CH₄ in mature tree stems were measured using non-steady-state stem chamber systems (n=32) installed in stem vertical profile approx. two weeks prior to measurements using a portable greenhouse gas analyser. Moreover, resins sampled from spruce stems were investigated for their CH₄ exchange potential. Control measurements were performed to ensure that the fluxes do not originate from used chamber materials, in particular silicones used for chamber installation.

Our preliminary results show that the spruce stems can be a strong sink for CH₄ (-0.288 ± 0.053 mg CH₄ m^-2 stem area h^-1, mean ± s.e.), even if a small amount of resin is present on the bark. The stems exuded resins to different extent (covering 4.8 ± 1.3% of the stem surface area in chambers), partly as a result of smoothening of rough surface layers of dead bark for chamber installation. However, even spruce stems without obvious “injuries” released small amounts of resins for unknown reasons (response to drought, bark-beetle attack, etc.?). The incubated resin samples consistently consumed CH₄ (-12.0 ± 1.7 mg CH₄ m^-2 resin area h^-1). Moreover, the detected stem CH₄ uptake negatively correlated with the resin occurrence in the stem chambers (R² = 0.884). After re-calculation of the stem fluxes to resin area, the CH₄ consumption rates of stems and resin samples were in the same order of magnitude at median level (-13.2 and -12.0 mg CH₄ m^-2 resin area h^-1, resp.).

Concluded, the spruce resins appear to be a very strong and until now undiscovered sink for CH₄. Even one small droplet of resins on bark can turn the known negligible CH₄ exchange of intact spruce stems into strong CH₄ sinks, having thus severe impact on the overall forest CH₄ balance. This consumption potential of fresh resins should be considered by estimation of forest ecosystem CH₄ balance especially in areas, where resin bleeding is widely spread or is to be expected (bark-beetle areas, drought events, tree harvest, clear-cutting).

 

Acknowledgement

This research was supported by the Czech Science Foundation (17-18112Y) and National Sustainability Program I (LO1415). We thank Prof. Thorsten Grams for all his kind support, and Jan Hrdlička and Thomas Feuerbach for their technical support.