In the taiga of Alaska, permafrost and vegetation are closely related. In areas underlain by permafrost, the nature of the vegetation is important in determining the thickness of the active layer. In a black spruce stand, the active layer is normally 30-60 cm thick. Flooding has several effects on the vegetation-permafrost relationship on floodplain forest stand. Flooding and water table rise by warm water can quickly thaw existing permafrost or cause higher soil temperatures over at least the upper 150 cm of the substrate. Siltation during flooding results in the compaction and death of the moss layers, thus reducing their insulating value in summer which results in higher soil temperatures and an increase in thickness of the active layer. The results of thawing of frozen layers heavily laden with ice can be surface subsistence, tipping of trees, and eventually the formation of thaw ponds. In some cases, flooding over permafrost results in a separation of the organic layer at the permafrost boundary and a compression and rolling of the organic layer into peat mounds. Fire in forest types underlain by permafrost results in a temporary thickening of the active layer. For the first 15 years after fire, thaw is more than 1 M; return to preborn thaw levels takes about 50 years.