Sandy soils in the forests of northern Michigan (USA) as methane sinks: Impacts of tree species composition and winter snowpacks

Upland forest soils are one the largest terrestrial methane (CH₄) sinks. The strength of this CH₄ sink, however, is sensitive to the influence of forest composition and other environmental factors on the activity of CH₄ oxidizing bacteria and archaea (methanotrophs). Climate change projections for the Great Lakes region (USA) point to expansion of deciduous broadleaf species at the expense of conifers, as well as decreased winter snowpack thicknesses. To assess the sensitivity of the soil CH₄ sink to forest composition, winter snowpack thicknesses, and soil frost, we conducted a 2-year snow removal experiment in sandy soils. The sites were dominated by either sugar maple (Acer saccharum) or red pine (Pinus resinosa) in northern Michigan, USA. We measured in-situ CH₄ uptake using static chambers in plots where snowpacks were either left undisturbed or removed. Our results indicate that annual soil CH₄ uptake is 25–30 % higher under sugar maple than under red pine, suggesting that predicted broadleaf forest expansion will increase the strength of the soil CH4 sink. Nonetheless, our estimates of annual CH₄ budgets for these stands also indicate that this process may only contribute 2–3 % of the annual climate mitigation services in these forests. We observed no differences in rates of wintertime CH₄ uptake between treatments, and no carry-forward effects of snow removal on rates of growing season CH₄ uptake. Depth trends in soil CH₄ concentration indicate that wintertime CH₄ uptake in our control treatment was limited primarily by the rate of diffusion of atmospheric CH₄ through the snowpack and into the soil, whereas it appears that CH₄ uptake in our removal treatments was limited by soil freezing. This implies that the response of the soil CH₄ sink in these forests to climate change will depend on the balance between snowpack thicknesses, as it impacts soil freezing.