Distribution and Composition of Oak Savannas

The oak savannas occur in the transition between the more mesic oak forests of the East and the open prairie of the mid-West (Figure 1) (Haney and Apfelbaum 1993). The range of the oak savannas spans the entirety of the oak-prairie transition from Canada to Texas that historically encompassed 11-13 million hectares, and may have been more extensive in pre-colonial times (Nuzzo 1986). The pre-colonial distribution and present day range of oak savannas are fairly similar although the majority of these areas have been degraded due to fire suppression and agriculture conversion (Abrams 1992).

Oak savannas have been described as open plains with scattered trees and a largely graminoid understory layer (Rebertus and Burns 1997). The exact definition of a savanna is debatable although it is often defined as any stand with canopy openings with anywhere from 10-60% cover (Bray 1960). This definition is therefore inclusive of areas that have very few trees per acre up to stands with fairly dense canopy shading. The consensus is that oak savannas developed from forest that was exposed to a high fire frequency resulting in sparse trees and a prairie-like understory (Abrams 1992). Oak savannas could have also evolved from tall grass prairies that experienced a longer fire return interval resulting in increased oak regeneration and a more stable canopy (Bray 1960).

Fire regime, climatic changes, soil moisture and other contributing factors determine the spatial and temporal changes in vegetation, but no consensus has been reached on the driving factor behind oak savanna formation (Rebertus and Burns 1997). The prevalent fire regime produced a landscape with a mix of prairie, savannas, shrubland, and closed forests (Haney and Apfelbaum 1993, Rebertus and Burns 1997). Oak savannas developed on both xeric and mesic sites with a high fire frequency with species compositions that were specific to the underlying moisture gradient (Bray 1960, Haney and Apfelbaum 1993). This moisture gradient is also a result of increased shading from canopy trees (Bray 1960). Oak savannas have developed a number of different ecotypes ranging from xeric savannas to a more closed mesic forest as a result of varying climate conditions, fire frequency, and moisture gradients (Table 1: Dominant variations of the oak savanna in the Midwest) (Haney and Apfelbaum 1993, Rebertus and Burns 1997).

Oak Savanna Ecotypes

Oak savannahs have a low diversity overstory dominated mainly by Quercus species such as Quercus macrocarpa (bur oak), Quercus velutina (black oak), Quercus stellata (post oak), Quercus alba (white oak), Quercus marilandica (blackjack oak), and Quercus ellipsoidalis (northern pin oak) (Abrams 1992). Oak sites can be classed as either xeric or mesic and are further separated geographically (Haney and Apfelbaum 1993). The three xeric sites (Southern Oak Savanna, Northern Sand Savanna, and Eastern Sand Savanna) all occur in different geographic ranges and are dominated by fire resistant oaks and pyrophytic grasses. With frequent fires these systems would succeed to oak barrens. The Southern oak savanna, which is found from Indiana south into Texas, is dominated by Q. marilandica and Q. stellata. The Eastern and Northern Sand Savannas, which are dominated by Q. velutina and Q. ellipsoidalis, are found from Illinois up into Wisconsin and Michigan (Abrams 1992, Haney and Apfelbaum 1993). The more mesic savannas, also subdivided into three site tyoes, are located on more fertile clay soils making them predisposed to competition from more shade tolerant species (Abrams 1992, Haney and Apfelbaum 1993). The clay-loam savannas have higher plant diversity than other ecotypes with an overstory dominated by Q. macrocarpa. The flood plain savannas have an overstory composed of Q. macrocarpa or Q. alba and occur in alluvial soils. Mesic loam savannas can occupy dry sites but most consistently occur on well-drained loam soils. The mesic loam savanna is characterized by Q. alba, Q. rubra, Q. velutina, Q. macrocarpa, and Q. ellipsoidalis.

The understory of the oak savanna is composed of legumes, asters, and pyrophytic grasses that are usually associated with the adjacent grasslands (Bray 1960, Antonio and Masi 1993). Studies have documented up to 300 species across a mesic oak savanna (Bray 1960). The most common grass species are Schizachyrium, Sorghastrum, Andropogon, and Paspalum, as well as a number of introduced grasses (Abrams 1992). In the cooler Northern regions, other species such as Carex, Cyperus, and Stipa are found (Bray 1960, Smeins and Diamond 1986).