LB 24-255 - Widespread foliar methane uptake in upland forest trees

Background/Question/Methods:

Methane (CH₄) has received increasing attention in climate mitigation due to its rapid recent increase combined with relatively short residence in the atmosphere. Forest ecosystems on well-drained soils are an important global sink for CH₄. While it has been assumed that essentially all CH₄ uptake by forests is attributable to methanotrophic microorganisms in the upper soil horizons, the same taxa of methanotrophs have been detected in leaf tissues. Prior assessments of foliar methane flux have been few and ambiguous, with accuracy limited by available methods.



We conducted surveys of foliar CH₄ flux in both upland and lowland (periodically inundated) forest site in central Ontario using a gas analyzer based on off-axis integrated cavity output spectroscopy and a pressure equilibrated chamber system for measurements under a range of light conditions and matching foliar flux measurements to local soil flux. We predicted that foliar CH₄ uptake would be detectable in areas where soil was a CH₄ sink, ensuring that methanotrophic bacteria were present and that any foliar CH₄ sink would not be offset by dissolved CH₄ flux through the xylem stream. In addition, we expected that shading of leaves would reduce uptake, consistent with methanotrophic bacterial presence as endophytes within leaf tissue.

Results/Conclusions:

Foliar CH₄ uptake was pervasive in upland sites where soil was also a CH₄ sink. Estimated foliar CH₄ uptake in sunlit conditions average 0.54 nmol m^-2 s^-1, substantially exceeding the detection limit of the system (~0.02 nmol m^-2 s-^-1). In contrast in lowland sites with inundated soils showed leaves were a net source of CH₄. Foliar CH₄ flux varied substantially among species but was generally higher in broadleaf trees than conifers. Foliar CH₄ uptake was also reduced by shading of leaves and in nighttime measurements. An estimate of stand-level foliar uptake, made using local leaf area index estimates, suggests that foliar uptake is ~38% of total soil uptake. The results suggest that foliar CH₄ uptake is widespread and quantitatively important - and more broadly imply that the total terrestrial sink for atmospheric CH₄ may be substantially underestimated.