Fire Regimes of Shrub Bogs

Shrub bogs are embedded within upland communities that historically burned frequently. Frequent fires that swept through surrounding uplands are thought to have affected at least the outer portions of shrub bogs regularly (Weakley and Schafale 1991, Robertson et al. 1998). A large range of values has been reported in the literature for fire return intervals for shrub bogs. Basin shrub bogs may have burned as often as every 3 to 8 years (FNAI 1990) or every 15 to 30 years (Sutter and Kral 1994). For titi swamps values range from 5-10 years and 20 – 50 years (citations in Wolfe 1988). Variable fire frequencies are also reported for streamhead shrub bogs ranging from 20 to 50 years (FNAI 1990) to 1 to 5 years with fire frequencies decreasing toward the center with an increasing moisture gradient (Robertson et al. 1998). Vegetation stature is likely related to time since last fire (Robertson et al. 1998). In the absence of fires, bogs are thought to become bay forests.

Fires are thought to have periodically swept through shrub bogs during prolonged summer droughts when water tables dropped. Under conditions when entire shrub bogs ignite, fires are intense. Most above ground fuel is consumed and peat is sometimes consumed (Wolfe et al. 1988). Fuel loads are high. Depending on burn history, fuel loads in small shrub bogs are likely similar to those found in pocosins (6 – 12 or more tons/acre) (Wade and Ward 1973).

(See Shrubs of Pocosins and Shrub Bogs: Adaptations to Fire)

Human Alteration of Fire Regimes

Like other systems in the southeast the natural fire regime of shrub bogs has been altered. Like most wetlands in the southeast, the fire frequency of shrub bogs has decreased since presettlement times. Shrub bogs have not typically been the primary focus of historical prescribed fire programs so when they do burn it is a consequence of the surrounding communities burning and fire sweeping downhill into shrub bogs. Thus, any changes in the frequency and season of surrounding communities would be reflected in the fire regime of shrub bogs. Further, many shrub bogs have purposely been “protected” from fire by the construction of firebreaks.

As a consequence of decreased fire frequency, shrub bogs vegetation may be of a larger stature than under the natural fire regime or may have succeeded toward bay forest. Decreases in fire frequency and shifts from growing season to dormant season have lead to an expansion of shrub bogs into areas formerly dominated by herbs (Bridges and Orzell 1989). This process has negatively affected numerous rare herb species including carnivorous plants (Harper et al. 1998).