Fire-driven changes in Alaskan black spruce forests: Regeneration sensitivity to surface fuel consumption

Fire is widespread across the boreal forest, but not all fires are created equal. Soils in black spruce forests often accumulate a thick layer of organic material on the soil surface, and variations in surface fuel consumption can have important effects on the ecological processes that govern post-fire recovery. This study investigates responses of tree recruitment to widespread fires of 2004 in Alaskan black spruce forests. Permanent sample plots were established in 92 forest stands across three major fire complexes in Alaska that covered a wide range of surface fuel consumption and site moisture conditions. Post-fire tree regeneration was monitored for two years after fire by recording natural seedling establishment and recruitment of artificially sown seed of five potential dominant tree species. Severely burned sites with exposed mineral soil and adequate moisture supply showed the highest rates of tree recruitment and strongly favored establishment of rapidly growing, deciduous tree species. Moist sites that retained thick organic layers after fire had recruitment dominated by spruce, and supported slower rates of vegetation growth. Dry sites showed generally low rates of tree recruitment, and severely burned sites with low moisture showed some of the slowest rates of vegetation establishment. These early patterns of post-fire tree regeneration suggest predictable variations in the sensitivity of black spruce forests to fire effects across landscape moisture gradients. As successful tree establishment is often limited to the first few years after fire, early impacts on tree recruitment are likely to have long-term consequences for future forest composition.