Methane Stem Fluxes Under Elevated CO2

Tree stems can exchange CH₄ with the atmosphere at rates that can strongly affect GHG budgets at regional scales. However, we do not know those fluxes’ sensitivity to different components of climate change, such as the increase in concentrations of atmospheric CO₂. An increase in CO₂ concentrations might result in a change of water use efficiency, reducing transpiration fluxes, which may enhance soil methanogenesis due to an associated increase in soil water content. Additionally, an increase of CO₂ could also stimulate net primary production, increasing the supply of fresh carbohydrates to the rhizosphere, and thus stimulating CH₄ production in soil anaerobic microsites which may themselves become larger or more numerous due to the additional oxygen demand placed by the fresh carbohydrate on the soil atmosphere. Given the positive relation between soil and stem CH₄ exchange processes, any increase in soil CH₄ production may result in higher stem emissions. However, that effect of soil production on stem fluxes might decrease with stem height, with lower fluxes or even CH₄ uptake at higher stem heights. In this study, we present preliminary data on spatial and temporal variability of stem CH₄ fluxes measured in mature oak trees growing under both control and elevated CO₂ concentrations (~150 ppm above atmospheric concentrations) in a FACE experiment (free air CO₂ enrichment; BIFoR-FACE). These data may be crucial for informing processed based models on how forests GHG fluxes might behave under predicted future climate conditions.