Fire and Ice

Were it not for forest fires, it is likely that there would be more permafrost in the valleys of interior Alaskan and corresponding regions of northern Canada. It is not that forest fires melt frozen ground, rather that the fires create a long-lasting condition that prevents permafrost from forming.

In regions where the mean annual temperature hovers near the freezing point, permafrost is intermittent. It forms only where the ground surface is insulated or otherwise protected from summer sun's warmth. For instance, it is reported that permafrost is now forming in soil on the north sides of buildings placed on previously frost-free sites in the Fairbanks area. A heavy layer of moss on the forest floor can also lower the year-around ground temperature enough to produce perennially frozen soil.

When new soil is laid down by river flooding or meandering, a succession of northern forest growth starts. Two to five years afterwards the soil supports an open growth of willow and alder shrubs. The shrubs build a closed cover during the next five years, but then balsam poplar trees grow and become tall enough to begin destructive shading of the shrubs below. By the end of the first century following soil deposition, a forest of mature balsam poplar has developed, beneath which some alder still exists and young white spruce trees are growing. The next two centuries see the development of a mature white spruce forest floor with a deep moss layer.

Measurements reported by Keith Van Cleve, Ted Dyrness and Les Viereck, associated with the University of Alaska or the Institute of Northern Forestry at Fairbanks, show that soil temperature decreases as the moss layer in the white spruce forest thickens through the years. Consequent development of permafrost and cold water-logged soil eventually can cause the white spruce to be replaced by the less-stately black spruce which can tolerate these soil conditions.

But that usually does not happen. Natural forest fires occur frequently enough to prevent completion of the full cycle. Almost every non-coastal forest area is burned over at least once each 200 years or so. Each time, the fiery death of the insulating moss layer opens the soil to the sun's rays once again, and a new cycle of life commences.