Historic Fire Regimes of Northern Hardwoods

Lightning fires seem to be rather infrequent in the Southern Appalachians and most often occur on mid-elevation ridges and upper slopes with south, southwestern or western aspect (Barden & Woods 1974; Harmon 1982; Runkle 1985; Bratton & Meier 1998). Northern hardwoods, however, are typically found on higher elevation north-facing slopes rendering natural fires even more unlikely. Due to the mesic conditions in northern hardwood forests fires are not easily ignited. The thick and moist litter layer usually causes fires to smolder and move slowly (Parshall & Foster 2002; Frelich & Lorimer 1991). These circumstances lead to fire return intervals between 200 and 400 years (Leenhouts 1998) and even longer for stand replacement fire regimes (300 to 500 years) (Stanturf et al. 2002). These values, especially the fire return interval of 200 to 400 years, are valid for the conterminous United States and include numbers of Northeastern northern hardwood forests. Due to the rough topography of the Southern Appalachians and the fact that Appalachian northern hardwood forests are interspersed with other, possibly fire prone communities, the actual fire return intervals might be shorter. Unfortunately, only little is known about actual fire regimes in Southern Appalachian high elevation forests.

In comparison to other forest types, northern hardwoods take intermediate positions regarding the influence of fire. Fahey & Reiners (1981) compared modern fire records of different forest types in Maine and New Hampshire. Their results show that northern hardwoods burn less often than pine forests but more often than spruce-fir forests.  The percentage of area burned is smallest in spruce-fir forests followed by northern hardwoods, while it is largest in pine and birch-aspen forests. Charcoal records indicate that this is not only a phenomenon of modern times: fire was very uncommon in northern hardwoods at least during the past 1000 years (Parshall & Foster 2002). Since there are interactions between climate, vegetation, and fire frequency, charcoal data cannot ultimately tell us whether a change in fire frequency induces vegetation change or if altering vegetation (and therefore fuel load) causes changes in fire frequency.  However, charcoal records show that fire frequency declined with the expansion of hardwood taxa and that there were no fires during the time of hardwood dominance (Clark et al. 1996).

Nevertheless, fires do occur in northern hardwood forests. Especially in the Southern Appalachians, an area with diverse plant communities across short distances, fires burning in a neighboring community can creep into northern hardwoods. Northeastern northern hardwoods also show evidence of fire, but those fires have been linked to human activities like land-clearing operations or logging (Lorimer 1977; Frelich & Lorimer 1991). In New Hampshire the highest burning percentages in northern hardwood forests were closely associated with highest population densities (Fahey & Reiners 1981) and in Canada charcoal data suggest that even in prehistoric times fire occurrence and Indian occupation coincided (Clark & Royall 1995; see also A History of People and Fire).