Plants are recognized sources of methane (CH4) but plant-mediated CH4 emissions have mostly been studied on herbaceous species, although also trees are known to emit CH4. Emissions from tree canopies likely mostly derive from an aerobic, abiotic process. Aerobic CH4 production from trees has not been thoroughly studied, leaving uncertainties to the global source strength estimates, which vary from 0 to 240 Tg yr-1. Even less is known about how aerobic emissions affect the CH4 cycles in boreal forest ecosystems, as the environmental and physiological drivers are not fully understood.
In this study, shoot-level CH4 fluxes of boreal conifer trees were measured outdoors and in the greenhouse, to investigate the environmental and physiological drivers and regulators of shoot-level CH4 fluxes. Most of the measurements were done from saplings of Pinus sylvestris L. (Scots pine), one of the most important tree species of the boreal region of the Eurasian continent, by using chamber enclosure methods with spectral, online greenhouse gas analysers. The measurements were conducted either manually, or with an automated measurement system, developed to overcome issues related to manual measurement techniques.
The shoots of Scots pine showed small but significant emissions of CH4 in all experimental setups, the emissions were driven by light, enhanced by elevated temperature, and occurred independently from drought and photosynthesis. Solar radiation was a more significant driver of these CH4 emissions than artificial light with UV-A. These results show that Scots pine canopies have the potential to produce CH4 in a similar process that has been described before for the foliage of herbaceous plants, but these emissions are smaller than the initial estimates of the aerobic CH4 source from vegetation. The boreal forest canopies are sources of CH4 and have the capacity to decrease the CH4 sink strength of boreal upland forests by ~ 5 %.