Can tree stem and shoot emissions close the gap in the methane budget of a boreal Scots pine forest during the summer months?

The role of boreal upland forests in the global methane cycle remains poorly constrained. While chamber-based measurements clearly show that the soils of upland forest act as methane sinks, micrometeorological measurements indicate that the same forests are methane-neutral at the ecosystem level. We conducted a measurement campaign covering soil, tree stem, tree shoot, and ecosystem-level flux measurements to test whether upscaled methane fluxes from tree stems and shoots can close the observed gap between the soil and ecosystem fluxes.

The campaign was conducted in a Scots pine dominated upland forest in southern Finland at the SMEAR II Hyytiälä research station between July 1 - Aug 15 2017. It included weekly measurement of methane fluxes at 15 soil locations, 47 stem chambers at the three tree species (Pinus sylvestris, Picea abies, Betula sp.), and 6 shoot chambers, as well as micrometeorological measurement of methane fluxes at 23 m height with two methods, eddy covariance (EC) and true eddy accumulation (TEA). Soil and stem methane fluxes were further upscaled based on a topographical statistical model (Vainio et al., 2021).

Our results show a persistent gap between chamber- and micrometeorological flux measurements. While the soil acted as a moderate methane sink (-1.71 nmol m^-2 s^-1 ,95% confidence interval -2.03 to -1.39), micrometeorological measurements indicated that the forest was near methane neutral (EC: -0.29±0.24 nmol m^-2 s^-1; TEA: -0.25±0.16 nmol m^-2 s^-1). Spatial heterogeneity was a significant factor for soil methane uptake, as the median methane location in the tower footprint showed an approximately 0.5 nmol m^-2 s^-1 greater uptake than the footprint average. Methane exchange from stems (-0.035 to 0.083 nmol m^-2 ground area s^-1) and shoots (0.025 to 0.075 nmol m^-2 ground area s^-1) were at least an order of magnitude smaller than the gap between the soil and ecosystem measurements. While these estimates are associated with significant uncertainties primarily stemming from the upscaling model, it is unlikely that the stem and shoot fluxes act as the missing methane source in this ecosystem.

Overall, results indicate that the gap between soil and ecosystem fluxes results either from a systematic error in micrometeorological flux measurements or from too high uncertainties related to measured fluxes very close to the detection limit of the EC/TEA system. It is also possible that an unidentified methane source exists in these forests. We were, for example, not able to conduct shoot flux measurements at moist sites within the flux tower footprint. We further note that our campaign was conducted during the peak summer months when stem and soil fluxes are expected to be relatively small due to low soil moisture. Nevertheless, our data suggests that a difference between trace gas fluxes at the soil and ecosystem level are not necessarily indicative of stem or canopy processes, and that such differences need to be interpreted with great care.

References:

Vainio, E., Peltola, O., Kasurinen, V., Kieloaho, A.-J., Tuittila, E.-S., Pihlatie, M.: Topography-based statistical modelling reveals high spatial variability and seasonal emission patches in forest floor methane flux, Biogeosciences, 18, 2003–2025, https://doi.org/10.5194/bg-18-2003-2021, 2021.