Oak woodlands and savanna: Historic and futuristic perspectives

Background/Question/Methods

I examine the historic distribution of oak savannas and woodlands, the role of climate change and human activities in the origin of these vegetation types, factors involved in the decline of oak savannas and woodlands, and possible changes in these communities resulting from future climate change and human activities. In mid-continent North America, oak savannas and woodlands were best developed along a North-South gradient from Minnesota to southeastern Texas where eastern deciduous forests and grasslands meet.  Extensive occurrence of these communities was of relatively recent origin and was associated with a warming and drying trend (Hypsithermal), which began about 8 kyBP (thousand years before present), peaked near the middle of the Holocene (5-6 kyBP), and ended 3.5-5 kyBP, and concurrent fires set by Native Americans.  Cooler and presumably moister conditions following the Hypsithermal would have favored conversion of oak savanna and woodlands to closed forest dominated by mesophytic trees species on most sites.  However, fires set by Native Americans as a management tool to maintain needed resources, including oak and hickory mast and species of wildlife hunted for food, maintained oak savannas and woodlands.  While future climate change and invasive species will have negative effects on remaining remnants of these communities, most of the historic oak savannas and woodlands were lost during a relatively short period of time, from the middle of the 1800’s to the present time.  For example, in Midwestern United States, fire suppression in the previous two centuries, habitat fragmentation, and agricultural and urban development reduced these communities to less than 0.02% of the 11-13 million hectares they historically occupied.  In addition, the variety and diversity of these communities has declined as well as the area they historically occupied.  For example, on many mesic and dry-mesic sites, successional invasion by native mesophytic trees, and subsequent mesophication, has all but eliminated possibilities for restoration of degraded remnants of oak savanna and woodlands. 

Results/Conclusions
Despite extensive loss of savanna and woodland habitat reasonably large areas with potential for restoration occur on low competition, low nutrient xeric sites.  On these sites, oak savannas and woodlands may persist with appropriate management despite climate change.  Nevertheless, as was the case with historic oak savannas and woodlands, intervention by humans, including fire management and efforts to reduce influx of invasive species or their removal once established, will be necessary to maintain these communities.