Decline of Oak Savannas

The oak savanna ecosystem once covered an extensive part of the prairie-oak transition totaling more than 32 million acres. Now they are quite rare and cover only about 0.02% of their pre-settlement range (Nuzzo 1986). A number of factors have contributed to this decline, but fire suppression is the primary cause. Fire suppression was considered beneficial for humans and forested landscapes that appeared to be destroyed by wildfires. When instituted, the ecology of certain landscapes was not fully understood and it was unknown that some forests had developed under a frequent fire regime. Other changes that could have contributed to the decline in oak savannas include removal of Native Americans shitfting landuse and anthropogenic fire and loss of large migratory ungulates.

Fire suppression beginning in the early 1920s, which lasted for nearly 60 years, was a main factor contributing to the decline of the oak savannas (Anderson and Brown 1983). Fire suppression has led to increases in basal area, density and canopy cover leading to the transition of oak savannas to closed canopy forests (Abrams 1992, Faber-Langendoen 1995, Peterson and Reich 2001). Savannas in central Oklahoma have transitioned to closed post oak/blackjack oak forests and gallery oak savannas in east Kansas have also become closed forests with low grass cover after only 50 years of fire suppression (Hartman 2001, Abrams 1992). These closed canopy forests have decreased grass cover, reduced native savanna flora and fauna and eventually will lose the dominant canopy oak species (Faber-Langendoen 1995). Xeric system oaks are more tolerant of fire disturbance due to physical characteristics, but are incapable of adapting to low light levels found in closed canopies (Johnson et al. 2002). Oak regeneration is reduced in these low light regimes and will eventually be replaced by more shade tolerant species in the absence of frequent disturbance (Abrams 1992, Soucy et al. 2004). 

Oak savannas support relatively frequent low intensity ground fires due to a continuous grass fuelbed. With fire suppression, canopy closure occurs, the herbaceous fuels are shaded out, and fine fuel is reduced. These changes lower the ignition probability and if a fire does start it is less likely to spread because of reduced fuel continuity. There is also a significant impact on the composition of oak savannahs if fire is excluded from the system. Plant species can be reduced from 300 to fewer than 25 in the transition from open to closed canopy forest (Bray 1960). Wildlife can also be impacted as in the Ozark Highlands in Missouri, where 64 bird species occur in oak savannas while only 39 occur in mature oak-hickory forests (Evans and Kirkman 1981). 

Oak savannas have been directly impacted by landscape fragmentation and land use change, but fragmentation has also indirectly impacted them by reducing the number of large scale fires (Abrams 1992, Faber-Langendoen 1995, Peterson and Reich 2001). Land conversion has reduced the existing oak savannas as well as minimized the amount of areas available for restoration. The creation of infrastructure such as roads, railroads, and urbanization has introduced artificial firebreaks. The once landscape level fires which dominated the oak savannas are no longer possible when such barriers are present and fire has to become more of a restoration and active management tool. A major issue in oak savanna restoration is the use of fire in the wildland urban interface that often dictates management techniques. Educating the public can help to alleviate negative perceptions about prescribed fire. 

Agricultural use has also been a leading cause of decline in oak savannahs through either conversion or degradation (Auclair 1976). Agricultural practices and timber removal have resulted in significant erosion and sometimes irrevocable soil loss (Hartman 2001). Free range livestock and other grazing animals can reduce oak seedling density and in some cases increase the dominance of unwanted woody vegetation in the understory, especially when coupled with fire suppression (McPherson 1997). Increased presence of non-native species, primarily grasses introduced for livestock grazing, also provides unwanted competition for native pyrogenic grasses.

The oaks historically existed in a transitional zone and thus have always been in a state of flux due to fire regimes, climate and perhaps large ungulate migration. Climatic shifts and the extirpation of bison migratory routes could have impacted the system. The extent of this impact on the formation of oak savannas is still highly debated and much uncertainty exists. The removal of buffalo coupled with fire suppression could have created a rapid transition in oak savannas to closed canopy forests.