Response of boreal ecosystems to varying modes of permafrost degradation

Abstract

Permafrost degradation associated with a warming climate is second only to wildfires as a major disturbance to boreal forests. Permafrost temperatures have risen to 4 °C since the “Little Ice Age”, resulting in widespread thawing of permafrost. The mode of permafrost degradation is highly variable, and its topographic and ecological consequences depend on the interaction of slope position, soil texture, hydrology, and ice content. We partitioned this variability into 16 primary modes: (1) thermokarst lakes from lateral thermomechanical erosion; (2) thermokarst basins after lake drainage; (3) thaw sinks from subsurface drainage of lakes; (4) glacial thermokarst of ice-cored moraines; (5) linear collapse-scar fens associated with shallow groundwater movement; (6) round isolated collapse-scar bogs from slow lateral degradation; (7) small round isolated thermokarst pits from surface thawing; (8) polygonal thermokarst mounds from advanced ice-wedge degradation; (9) mixed thermokarst pits and polygons from initial ice-wedge degradation; (10) irregular thermokarst mounds from thawing of ice-poor silty soils; (11) sinkholes and pipes resulting from groundwater flow; (12) thermokarst gullies and water tracks from surface-water flow; (13) thaw slumps related to slope failure and thawing; (14) thermo-erosional niches from water undercutting of ice-rich shores; (15) collapsed pingos from thawing of massive ice in pingos; and (16) nonpatterned ground from thawing of ice-poor soils. These modes greatly influence how thermokarst changes or disrupts the ground surface, ecosystems, human activities, infrastructure, and the fluxes of energy, moisture, and gases across the land–air interface.

Transformation of terrestrial organic matter along thermokarst-affected permafrost coasts in the Arctic

“…Besides lower C/N-ratios and δ 13 C-DOC values an increase of DIN (mainly consisting of NH 4 -N) accompanied by a decrease of TN in the mudpool indicates active degradation of OM accompanied by the liberation of NH 4 -N (Ladd and Jackson, 1982). A major portion of NH 4 -N is however provided by thawing permafrost, where it was preserved in a frozen state (Jorgenson and Osterkamp, 2005;Mikan et al, 2002;Wilhelm et al, 2012). After thaw, this formerly preserved NH 4 -N is diluted with melted massive ice and accumulates in the mudpool, where it mixes with freshly mineralized NH 4 -N after permafrost thaw.…”

“…Retrogressive thaw slumps (RTS) are one of the most widespread thermokarst forms (Jorgenson and Osterkamp, 2005;Kokelj and Jorgenson, 2013;Krieger, 2012). Along with other degradation landforms, like active layer detachments and gullies, RTS can affect up to 1 to 2% of continuous permafrost areas (Krieger, 2012).…”

Thermal state of permafrost in Russia

“…Recent assessments of the permafrost thermal state point to an ongoing warming over large areas (ACIA, 2004;Romanovsky et al, 2007Romanovsky et al, , 2008Brown and Romanovsky, 2008;Oberman, 2008;Osterkamp, 2008), and local evidence of permafrost degradation is apparent in parts of Alaska, Europe, Siberia and Canada (Jorgenson et al, 2001;Payette et al, 2004;Jorgenson and Osterkamp, 2005;Oberman and Mazhitova, 2001;Oberman, 2008). Despite the importance of the roles of permafrost in the geological, ecological, engineering and climate change sciences, observations of permafrost have, for the most part, remained the domain of individuals and small groups of scientists.…”

Sources and sinks of carbon in boreal ecosystems of interior Alaska: A review

“…Permafrost, defined as any substrate remaining below 0°C for more than two consecutive years, can persist where mean annual air temperatures are as high as +2°C. This is due to localized differences in the soil thermal regime, which is inf luenced by topography, slope, aspect, hydrology, winter snowfall, ground ice content, soil texture, plant cover, and fire history (Osterkamp and Romanovsky, 1999;Hinzman et al, 2003a;Jorgenson and Osterkamp, 2005;Myers-Smith et al, 2008). Regional temperatures are not low enough to sustain permafrost everywhere .…”