Temperate Forest Methane Sink Diminished by Tree Emissions

Abstract:

Global budgets ascribe 4-10% of atmospheric CH₄ sinks to upland soils and assume that soils are the sole surface for CH₄ exchange between upland forests and the atmosphere. The prevailing dogma that upland forests are sinks of atmospheric CH₄ was challenged a decade ago by large discrepancies in bottom-up versus top-down models of CH₄ concentrations over upland forests that are still unexplained. Evidence of a novel abiotic mechanism for CH₄ production from plant tissue is too small to explain the discrepancy. Alternative hypotheses for this observation have been proposed, but not tested. Here we demonstrate that CH₄ is emitted from the stems of dominant tree species in an upland forest. Tree emissions occur throughout the growing season while soils adjacent to the trees are consuming CH₄, challenging the concept that forests are uniform sinks of CH₄. Scaling by stem surface area showed the forest to be a net CH₄ source during a wet sample in June and a reduced CH₄ sink by 5% annually. High frequency measurements revealed diurnal cycling in the rate of CH₄ emissions, pointing to soils as the CH₄ source and transpiration as the most likely pathway for CH₄ transport. We propose the forests are smaller CH₄ sinks than previously estimated due to stem emissions. Stem emissions may be particularly important in upland tropical forests characterized by high rainfall and transpiration, resolving differences between models and measurements.