Resolving the aerobic methane emissions from Scots pine shoots

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Release of CH4 from shoots remains the least understood and most enigmatic process of tree-mediated CH4 fluxes. While stem emissions of trees derive from transported or local, biotic pathways, CH4 emissions from shoots likely originate dominantly from abiotic, aerobic production within the canopies. The estimates of the global source strength of aerobic CH4 emissions suffer from large uncertainties, due to insufficient understanding of the source processes. In this contribution, we show the environmental drivers, temporal patterns, and physiological determinants of the aerobic CH4 emissions from the shoots of Scots pines and discuss the contribution of aerobic canopy emissions to the boreal forest CH4 cycles. We present shoot-level CH4 fluxes from saplings and mature Scots pine trees, measured in various settings, outdoors and in the greenhouse. We used chamber enclosure methods with online greenhouse gas analysers in both manual and automated measurement settings. For the automated measurements we built a custom measurement system that allowed continuous measurements of greenhouse gas fluxes. The results from CH4 flux measurements under different light sources indicate that aerobic CH4 emissions are to the most part determined by the intensity and spectral composition of light, and that the emissions are most prominent under direct solar radiation. Hence, the diurnal variations exhibited by these emissions are associated with the diurnal cycle of sunlight, but also vary depending on the cloud conditions. By exposing Scots pine saplings to drought, we further distinguished that the light-driven CH4 emissions from shoots are not a byproduct of photosynthesis-related biochemical reactions. Rather, these emissions result from abiotic thermal and photodegradation of plant compounds. In ambient conditions, we show median aerobic CH4 emissions of 5.41 ng CH4 g-1 DW h-1 under direct sunlight and 2.52 ng CH4 g-1 DW h-1 during variable cloudiness. These emissions are 1-2 % of the emission factor used in most of the global upscale estimates of aerobic CH4 emissions from vegetation. Therefore, Scots pine canopies in boreal climates are likely a CH4 source of only minor importance on a global scale. These emissions may, however, decrease the sink strength of the boreal upland forest soils: our conservative estimate is that the canopy emissions of CH4 may decrease this soil sink strength by 2.1 – 4.6 %. This estimate may yet underestimate the significance of canopy fluxes on the ecosystem scale due to the high spatiotemporal variation of both the canopy CH4 emissions and the CH4 uptake rates of boreal upland soils. To further refine the estimates of the source strength of aerobic CH4 emissions of tree canopies it is, therefore, important to gain more data of shoot-level CH4 fluxes from field measurements.