Local methanogenesis drives significant methane emissions from upper tree trunks in a cool-temperate upland forest

Area of Science:

• Forest Ecology
• Biogeochemistry
• Environmental Science

Background:

• Quantifying methane (CH4) emissions from tree trunks is crucial for understanding forest carbon cycling.
• Emissions from the upper parts of tree trunks have been largely overlooked, leading to potential underestimation of total emissions.
• The vertical patterns of CH4 emissions and the underlying biological and physical processes within tree trunks are not well understood.

Purpose of the Study:

• To investigate the vertical patterns of CH4 emissions from tree trunks.
• To determine the in-situ CH4 production and transport mechanisms within tree trunks.
• To assess the contribution of upper trunk sections to overall tree CH4 emissions.

Main Methods:

• Measured CH4 emissions, internal CH4 concentration, and the abundance of the methanogenic archaeal gene mcrA at various heights (up to 12m) on six trees across three species.
• Analyzed CH4 production rates within trunk tissues.
• Modeled CH4 transport within the trunk using a diffusion-advection equation.

Main Results:

• A substantial portion (44-89%) of total CH4 emissions originated from the trunk sections above 3m, with peak emissions observed between 4-6m.
• CH4 production and the presence of mcrA were detected up to 12m, indicating active methanogenesis in the upper trunk.
• CH4 transport simulations suggested limited long-distance transport from lower to upper trunk sections, implying local production is a key source.

Conclusions:

• The upper parts of tree trunks are a significant source of methane emissions, often produced at the same height.
• Current methods may underestimate total tree CH4 emissions by not accounting for upper trunk contributions.
• Future CH4 emission inventories should incorporate measurements from the entire tree trunk, including upper sections.