Thermokarst terrain may recover from the effects of thermokarst activity if permafrost re-aggrades and incorporates excess ice or peat. Permafrost aggradation results from reduced surface energy inputs due to factors such as vegetation growth, peat accumulation, improved drainage or climate cooling (Table 13.1). Regrowth of vegetation after a forest fire may lead to active-layer thinning and incorporation into permafrost of ice lenses formed at the bottom of the active layer (Mackay 1995), thus heaving the ground surface. Terrain recovery is also manifest in areas where lake basins have drained at different times. On the coastal plain of northern Alaska, permafrost re-aggrades beneath the floors of drained basins, allowing excess ice to build an ice-rich layer in near-surface permafrost; hence the oldest drained basins tend to be the most ice-rich and therefore show the greatest recovery (Sellmann et al. 1975). Recovery of the ground surface from thermokarst subsidence or thermal erosion in ice-wedge terrain also occurs where peat preferentially accumulates in wet ice-wedge troughs or in the centres of low-centred polygons.
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