Impact of permafrost thaw on methane fluxes across a heterogeneous boreal landscape

Permafrost thaw in the southern Taiga Plains of northwestern Canada causes widespread forest loss at the expense of increasing wetland coverage. Permafrost soils in boreal forests store large amounts of frozen organic carbon, which may become available for anoxic microbial decomposition upon thaw in saturated wetland soils. Increased anoxic decomposition of organic matter leads to enhanced methane (CH₄) production potentially affecting net CH₄ fluxes (F_CH4) between soil and atmosphere. However, the magnitude of thaw-induced boreal forest loss on landscape-scale F_CH4 is still unknown.

In this study, we use a nested eddy covariance tower setup at Scotty Creek near Fort Simpson, Northwest Territories, Canada, flux footprint modelling, and spectral decomposition (i.e. singular spectrum analysis [SSA]) of F_CH4 time series to assess the impact of permafrost-thaw induced boreal forest loss on growing season F_CH4. The nested tower setup consists of a 2-m tower measuring F_CH4 over a permafrost-free wetland and a 15-m landscape tower with F_CH4 flux footprints originating from a heterogeneous boreal forest-wetland landscape. During the study period (May – October 2014 & 2015), wetlands contributed 51 % to the landscape-tower flux footprints while drier, raised forested permafrost peatlands contributed 46 %. The remaining contributing 3 % were attributed to a shallow, permafrost-free lake.

We find that with 13.0 g CH₄ m^-2 the total growing season wetland F_CH4 in 2014 and 2015 (May – October) were about twice as large as the emissions over the heterogeneous boreal forest-wetland landscape (6.3 g CH₄ m^-2). Maximum monthly F_CH4 were observed in August with 1.8 g CH₄ m^-2 (2014) and 2.3 g CH₄ m^-2 (2015) at the landscape tower and with 3.4 g CH₄ mm^-2 (2014) at the wetland tower.

Using SSA, we find that the seasonal FCH4 signal (i.e. 10-day window) accounted for 42 % and for 74 % of the total variance in F_CH4 at the landscape and wetland tower, respectively. With 21 %, the sub-weekly F_CH4 signal (i.e. 6h to 7-day window) accounted for a larger part of the total variance in F_CH4 at the landscape tower than at the wetland tower (i.e. 9 %), likely a result of a more pronounced footprint heterogeneity at the landscape tower. The seasonal F_CH4 signal closely followed an exponential function of the soil temperature in the wetland explaining 80 % and 75 % of the seasonal F_CH4 signal at the landscape and wetland tower, respectively. Adding water level as explanatory variable to the model only increased the coefficient of determination slightly by 6 % and 1 % for the landscape and the wetland tower, respectively. Wetland contributions to landscape tower fluxes showed a positive relationship with the sub-weekly, landscape F_CH4 signal and explained on average 23 % of its variance, supporting the observed larger F_CH4 at the wetland tower compared to the landscape tower.

Our results indicate that permafrost-free wetlands represent a significant growing season source of F_CH4 to the atmosphere, while CH₄ of the forested permafrost peatlands seems to be negligible. A recent study has shown that wetlands at Scotty Creek have increased in coverage by about 0.25 % per year since 1977. Combining these change rates and our eddy covariance observations, we conclude that the landscape-scale growing season source may have grown accordingly by ~0.03 g per m² per year over the past 40 years as a response to continued forest loss. Using a simple model of atmospheric CH₄ and CO₂ pools, we show that a concurrent continuous CO2 sink of ~250 g CO₂ per m² per year is required to neutralise the positive radiative forcing of these increasing CH₄ emissions within a time frame of 50 years. Widespread permafrost thaw-induced forest loss in the southern Taiga Plains and other lowland boreal forest regions may therefore enhance the radiative forcing of terrestrial CH₄ emissions and may be partly responsible for the observed positive trends in global atmospheric CH₄ concentrations.