Aerobic 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 Scots pine trees, a dominant tree species in boreal forests, identify source processes and environmental drivers, and evaluate the potential of leaf emissions to constrain methane emissions at the ecosystem-level 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 CO2 -, temperature-, and water-controlled chamber system; and with multiple methane analysers all resulted in comparable daytime flux rates (0.094±0.035 to 0.241±0.090 nmol CH4 g-1 foliar dw h-1). We find that methane emissions from Scots pine shoots exhibit a pronounced diurnal cycle that closely follows photosynthetically active radiation (PAR) and is further modulated by temperature. These diurnal patterns indicate that methane production is associated with diurnal cycle of sunlight, suggesting that methane is either a byproduct of photosynthesis-associated biochemical reactions (e.g. the methionine cycle) or produced through non-enzymatic photochemical reactions in plant biomass. Moreover, we identified a light-dependent component in stand-level methane fluxes, which showed an order-of-magnitude agreement with shoot-level measurements (0.968±0.031 nmol CH4 g-1 h-1), and which provides an upper limit to shoot methane emissions.

added missing datapoints in garden measurements