Fire effects on methane emissions from a larch forest in Northeastern Siberia

Abstract:

Understanding how boreal forest fires affect the fate of soil carbon in northern permafrost regions is critical to our understanding of feedbacks from Arctic ecosystems on global climate change. The frequency and intensity of fires have been increasing across the northern boreal and tundra region. Fire makes permafrost vulnerable because it removes the insulating plant and organic layers. The removal of these insulating layers in Siberian larch forests underlain by ice and carbon rich permafrost can lead to ground subsidence and saturate soils. Saturated and anoxic soils are ideal conditions for the production of methane, which is ~30x more effective at trapping heat than carbon dioxide. Most boreal ecosystems are currently considered to be sinks for methane, but not much research has been done to study how fire may affect methane production in these regions. We predict that fires will increase methane production in boreal ecosystems underlain by permafrost due to increases in thaw depth, ice wedge thawing, and ground subsidence. This study focused on a ten-year old burn site composed of mainly larch trees and tussocks located near the bank of the Kolyma River in Northeastern Siberia. The ground of the burn site was substantially more subsided and had larger areas of surface water and saturated soils than the nearby unburned forest. We investigated the flux of methane from the surfaces of small ponds that formed over thawing ice wedges and in the subsided depressions. While previous studies have reported low dissolved organic carbon concentrations in streams affected by fire in permafrost regions, we found high DOC concentrations in pond water (21-27 mg/L). Methane fluxes from ice wedge ponds ranged from 20 to 180 mg CH₄ m^-2 d^-1. These values are comparable to fluxes from other permafrost ecosystems including bogs, wet tundra, and fens that are considered globally significant sources of CH₄. Additionally, the burned forest contained some subsided areas that were extreme CH₄ hotspots, one of which released between 290 and 950 mg CH₄ m^-2 d^-1. These results show that fires in boreal forests underlain by permafrost may cause substantial changes in topography, leading to increased areas of anoxic environments that promote methanogenesis and increase CH₄ emissions, possibly turning Siberian forests from a CH₄ sink to a source.