The forest floor of the upland boreal forest consists of organic matter resting on the mineral soil surface. The organic matter can be divided into two indistinct layers: litter and duff. The litter layer (L) consists of the loosely packed, largely unaltered dead remains of animals and plants usually recently cast. The duff has two layers: (1) an upper Flayer consisting of litter which has recently begun to decompose but with the particles still recognizable as to their origins and (2) a lower H layer which is made of welldecomposed organic matter which can not be recognized as to its origins.

In the upland boreal forest the thickness of the forest floor increases with colder soil and air temperatures (Figure 5.1). This leads to an increase in forest floor depth northward on similar sites but also locally on sites with different heat budgets. Generally, dry, warm sites with canopies of jack pine and white spruce have thinner forest floors and cooler, wetter sites with canopies of black spruce have thicker forest floors (Johnson 1981). The increasing forest floor depth is primarily caused by an increasing duff layer.

As we have already discussed in Chapter 3, forest fires spread mostly by flaming combustion in the surface litter layer because its large surface area and loose packing allow rapid drying. The duff, on the other hand, burns mostly by glowing combustion during and after the flame front passage, although some volatiles are evolved and ignite in flames. The glowing combustion is a result of decay which has left as fuel the less easily decomposed lignin.