Seasonal dynamics of the land surface energy balance of a boreal forest-peatland landscape affected by degrading permafrost in the Taiga Plains, Canada

Abstract:

Northern boreal ecosystems along the southern limit of permafrost comprise a mosaic of forests with permafrost, and permafrost-free peatland and lake ecosystems. The proportion of permafrost-free areas has rapidly increased over the last decades due to increasingly warmer air temperatures. This change in land cover causes changes in vegetation composition and structure affecting land surface characteristics such as albedo and surface roughness with important implications for the land surface energy balance and thus regional climate. For example, a decrease in sensible heat flux potentially cools the atmosphere and thus constitutes a negative feedback to the climate system. Changes in latent heat fluxes alter regional water vapour dynamics and thus may affect precipitation patterns. To better understand the land surface energy balance under the influence of degrading permafrost, we measured sensible and latent heat fluxes with two eddy covariance systems, one at 15 m and one at 2 m above the ground surface, along with net radiation and soil heat flux at Scotty Creek, a watershed in the discontinuous permafrost zone in the southern part of the Northwest Territories, Canada. The flux footprint of the 15 m-eddy covariance system covers an area equally covered by black spruce forests and permafrost-free, treeless peatlands whereas the flux footprint of the adjacent 2 m-eddy covariance system covers a single bog within the footprint of the 15 m system.

Peak sensible heat fluxes at the bog were up to 200 W m^-2 smaller than the landscape-scale fluxes between April and July 2014. During the snow free period, peak latent heat fluxes at the wet bog were about 50 W m^-2 higher than the landscape-scale fluxes. Albedo of the forest was generally smaller compared to the bog except for the immediate post-melt period when the bog was affected by widespread surface flooding. This difference in albedo leads to higher net radiation at the forest site, particularly during the snow cover period. Our nested tower measurements indicate that with further permafrost degradation sensible heat and latent heat fluxes are likely to decrease and increase, respectively. This shift in the land surface energy balance could attenuate the regional positive air temperature trend and favour increased summer precipitation in the northern boreal zone.