Radiation and temperature drive diurnal variation of aerobic methane emissions from Scots pine canopy

Abstract

Methane emissions from plant foliage may play an important role in the global methane cycle, but their size and the underlying source processes remain poorly understood. Here, we quantify methane fluxes from the shoots of Scots pine trees, a dominant tree species in boreal forests, to identify source processes and environmental drivers, and we evaluate whether these fluxes can be constrained at the ecosystem-level by eddy covariance flux measurements. We show that shoot-level measurements conducted in forest, garden, or greenhouse settings; on mature trees and saplings; manually and with an automated CO 2 -, temperature-, and water-controlled chamber system; and with multiple methane analyzers all resulted in comparable daytime fluxes (0.144 ± 0.019 to 0.375 ± 0.074 nmol CH 4 g −1 foliar d.w. h −1 ). We further find that these emissions exhibit a pronounced diurnal cycle that closely follows photosynthetically active radiation and is further modulated by temperature. These diurnal patterns indicate that methane production is associated with diurnal cycle of sunlight, indicating that this production is either a byproduct of photosynthesis-associated biochemical reactions (e.g., the methionine cycle) or produced through nonenzymatic photochemical reactions in plant biomass. Moreover, we identified a light-dependent component in stand-level methane fluxes, which showed order-of-magnitude agreement with shoot-level measurements (0.968 ± 0.031 nmol CH 4 g −1 h −1 ) and which provides an upper limit for shoot methane emissions.

Forests and methane: looking beyond carbon for nature-based climate solutions

“…In these ecosystems, methane exchange has been more difficult to disentangle chiefly due to the difficulty of making in situ measurements of very small fluxes, in either direction, and identifying their source from a range of processes at scale and from within relatively inaccessible (for chamber measurement campaigns) tree crowns and canopies. These sources include aerobic sources that have wide uncertainties (Keppler et al 2006, Kohl et al 2023. However, in Gauci et al (2024), woody stem surfaces of upland trees were shown to be a significant locus of atmospheric methane removal (AMR), potentially as large or larger than the well-understood global soil sink (Dunfield 2007).…”