Fire Regime and Other Disturbances in Oak Savannas

Oak savannas have been maintained by frequent disturbance processes. However, the range of oak savannas has been constantly shifting over the past 10,000 years due to changes in climate, cultural influence, and fire regimes (Figure 3) (Anderson 1990).  This constant flux makes identifying areas of past and existing oak savanna very difficult and is further complicated by the conversion of lands into agriculture or pastureland.  Understanding the disturbance regimes, particularly, the fire history is important when considering management and restoration of oak savannas (Dey et al. 2004).   

Fire has been a driving factor in the formation of oak savannas. There are conflicting opinions of what or who was responsible for these landscape scale fires. Natural fires are not uncommon, occurring during thunderstorms that originate in the Mid-West. However, rarely do these storms produce lightning strike ignitions as they occur in the wetter growing season (Guyette et al. 2002, Guyette and Spetich 2003). In the oak savannas, humans were considered a main ignition source typically overwhelming any natural ignitions (Cutter et al. 1999, Williams 2000). Anthropogenic ignitions prior to settlement resulted in low intensity widespread fire unimpeded by artificial fire breaks (Cutter and Guyette 1994, Guyette and Spetich 2003). 

The fire regime has been impacted over time by fluctuations in population density and changing cultural influences (Williams 2000). The period prior to European settlement dominated by Native Americans is referred to as the “ignition dependent phase” (Guyette et al. 2002). During this time Native Americans not only survived but prospered in the oak savannas using fire for hunting, producing food, managing pests, clearing riparian areas, tree felling, and home fireproofing (Williams 2000). Fire scar analysis shows that fire return interval in the “ignition dependent phase” varied across the oak savanna range from 2.3-45 years (Table 2) (Williams 2000, Guyette et al. 2002). In the Boston Mountains of Arkansas, the mean fire return interval was 11.2 years and in the White Ranch State Forest in Missouri the mean was 3.7 (Table 2) (Guyette et al. 2002, Guyette and Spetich 2003, Dey et al. 2004). The probability of an area burning was dependent on its proximity to a human ignition source.  Thus, fire frequency was positively correlated with human population density (Guyette et al. 2002, Guyette and Spetich 2003).   Because Native Americans frequently moved, the mean fire interval was non-stationary (Williams 2000).

Following European settlement, the oak savannas entered a “period of fuel limitation” that was the result of a high fire frequency (1.5-6.8 years) (Table 2). During this time, the population continued to grow and fire frequencies increased to a point where the driving factor was fuel availability rather than the probability of human ignition (Guyette et al. 2002).  Around 1890, fire occurrence and extent began to decrease because of fire breaks such as roads, railroads, and other improvements to infrastructure. This was the “fuel-fragmentation stage” in which population density increased dramatically and probability of burning became negatively correlated with the growth (Guyette et al. 2002, Dey et al. 2004). The range of the fire return interval during this time was 1.7-19 years with a mean of 5.8 years (Guyette et al. 2002). Fire return intervals continued to increase well into the era of fire suppression when the dominant public perception was that fire should be excluded to protect the forest and public property. This period was also dominated by extensive timber harvest, further exacerbating the historic fire regime. The fire return interval increased to a mean of greater than 20 years (Table 2) and a range of 6.8-50 years (Guyette et al. 2002). During fire suppression, vast areas of oak savanna succeeded into closed canopy oak forests (Guyette et al. 2002). 

Although the fire return interval is important in determining the present fire regime, fire intensity and severity are equally important in the oak savannas (Faber-Langendoen 1995). It is surmised that additional low intensity fires also occurred throughout the oak savanna range in the past, but were undetectable in fire scar records (Guyette et al. 2002). This stresses the importance of managing fire intensity as well as fire frequency. Overall climate and periodic drought have also been a big determinant in the range of the oak savannas (Figure 3) (Anderson 1990, Faber-Langendoen 1995). Fire has been suggested by some to be less important than the shifting climate regime that has determined the optimal location of the oak savannas across the Midwest (Anderson 1990). Drought also impacts fire intensity with reduced fuel moisture leading to high severity fires and a heavy impact on vegetation. Rainfall can also increase or decrease fuel loadings and fire intensity in the oak savannas (Figure 4) (Anderson 1990). Although closed canopy forests can have higher fuel loadings, fire ignition and spread are decreased due to increased moisture. 

Other disturbances have played a role in shaping the oak savannas and the adjacent prairies.  Large ungulates such as bison interacted with disturbances such as drought, fire, and soil nutrition (Collins 1990, Hartnett et al. 1996). The migratory patterns of large groups of ungulates such as bison and elk had a major impact on species composition. They created spatial heterogeneity in the understory community through intense soil disturbance thereby altering fuel loads and continuity (Hartnett et al. 1996). Impacts of bison also increased during periods of intense drought leading to greater impacts on oak savannas.