Forest Fire Effects on Soil

The soils of Alaska and Yukon typically differ from those of more southern climes where chemical weathering proceeds more rapidly. Due to cooler temperatures in the north, chemical weathering is slower, the result being shallow soil layers with low clay content.

Instead of being mixed into the soil, organic matter of forest land tends to lie in a mantle atop the soil. Farther south, earthworms and other creepy-crawlies foster the mixing of humus into the soil, but in boreal soils these animals are largely lacking. The unincorporated humus mantle is usually several inches thick under birch, aspen and white spruce forests and as much as a foot thick under black spruce forests. Usually it is moderately acidic.

A forest fire usually destroys the humus mantle and in so doing creates several secondary effects that, at least temporarily, help speed up plant growth. One significant effect is the removal of insulation provided by the humus mantle. The soil below then can be warmed more in summer so that a greater depth of annual thaw results. Not only is the soil made warmer in summer, nutrients locked up in near-surface permafrost are temporarily made available to new plant growth.

Destruction of the humus layer by fire makes the remaining soil layer less acidic, and it releases nutrients from the humus to the soil. The amount of usable nitrogen, calcium, potassium, and phosphorus seems to increase immediately following a fire because the heat breaks up proteins and other large molecules into simpler compounds or into elemental forms that plants can assimilate.

The combined effects of fire on the soil clearly play a big role in determining the succession of plants that will follow the fire. Just the change in soil temperature and chemistry alone could lead to a temporary plant succession quite different from the forest cover before the fire.