Studies show that the variation in depth of burning of the surface organic layer and site moisture are critical factors in altering post-fire succession in the black spruce forests (see Figures 13.4 and 13.5). Black spruce can grow under a wide range of temperature and moisture conditions, and the presence of even a shallow (4—6 cm) surface organic layer restricts the invasion and survival of deciduous tree species (see Figure 13.4a, 13.4b and 13.5b). Thus, even in a warming climate, it is unlikely that the distribution of black spruce will be reduced unless deep burning of the surface organic layer occurs. Fire severity studies show that in sites with surface organic layers deeper than about 20 cm, the seasonal thawing of the ground layer controls the moisture of the deeper surface organic layers. Because of this, sites with deeper surface organic layers are only vulnerable to deep burning during fires that occur late in the growing season (i.e. after 1 August).
Recent changes to the fire regime in interior Alaska (see Figure 13.6) indicate that black spruce forests have experienced an increase in late season fires (see Figure 13.7). In addition, warming of permafrost since the beginning of the 1960s means that the upper soil layers of sites underlain by permafrost have experienced a deeper thawing and are therefore better drained late in the growing season. This would indicate that, at the same time Alaska's boreal forests have experienced an increase in late season burning, the sites have also experienced lower duff moisture later in the growing season because of warming permafrost. Thus, changes to the fire regime and a warming of permafrost have most likely increased the vulnerability of Alaska's black spruce forests to deep burning fires, and have combined to accelerate the rate of change.
Was this article helpful?