Fire Trends in the Alaskan Boreal Forest

Fire is ubiquitous throughout the global boreal forest (Wein 1983, Payette 1992, Goldammer and Furyaev 1996, Kasischke and Stocks 2000). The inter- and intra-annual patterns of fire in this biome depend on several interrelated factors, including the quantity and quality of fuel, fuel moisture, and sources of ignition. Fire cycles in different boreal forest types vary between 25 and >200 years (Heinselman 1981, Yarie 1981, Payette 1992, Conard and Ivanova 1998). Although the increased presence of humans in some regions of boreal forest has undoubtedly changed the fire regime (DeWilde 2003), natural fire is still a dominant factor in ecosystem processes throughout this biome. Boreal forest fires are similar to those of other forests in that they vary between surface and crown fires, depending on forest type and climatic factors. Surface fires kill and consume most of the understory vegetation, as well as portions of the litter or duff lying on the forest floor, resulting in varying degrees of mortality of canopy and subcanopy trees. Crown fires consume large amounts of the smaller plant parts (or fuels) present as leaves, needles, twigs, and small branches and kill all trees. These fires are important in initiating secondary succession (Lutz 1956, Heinselman 1981, Van Cleve and Viereck 1981, Van Cleve et al. 1986, Viereck 1983, Viereck et al. 1986). Unlike fires in other forest types, smoldering ground fires in the boreal forest can combust a significant fraction of the deep organic (fibric and humic) soils in forests overlying permafrost (Dyrness and Norum 1983, Landhauesser and Wein 1993, Kasischke et al. 2000a, Miyanishi and Johnson 2003). During periods of drought, when water tables are low, or prior to spring thaw, organic soils in peatlands can become dry enough to burn, as well (Zoltai et al. 1998, Turetsky and Wieder 2001, Turetsky et al. 2002).