Physiognomy and Structure: Structure in this community ranges from large open-grown trees 10-12 m tall, to gnarled trees 2-4 m tall. Shrub cover can become significant, and even predominant, over graminoid cover. Smaller gnarled trees tend to be found on drier soils which preclude even moderate tree growth. Canopy cover ranges from 10-30%, with higher canopy cover more likely to form in areas of slightly lower fire frequency, areas with greater microtopographic variability that allows for more fire protection, and areas under less grazing pressure.

Floristics: These savannas exhibit a mosaic of physiognomies, with smaller patches of grassland and forest interdigitating in complex patterns. Typically, there are two main structural layers, tree canopy and herbaceous, though shrubs could be scattered or clumped. Tree densities for stems greater than 10 cm dbh may historically have been between 1 and 40 stems per hectare (Wing 1937, Cottam 1949, Curtis 1959, Brewer and Kitler 1989). Structure in this community ranges from large open-grown trees 10-12 m tall, to gnarled trees 2-4 m tall. Shrub cover can become significant, and even predominant, over graminoid cover. Smaller gnarled trees tend to be found on drier soils which preclude even moderate tree growth. Canopy cover ranges from 10-30%, with higher canopy cover more likely to form in areas of slightly lower fire frequency, areas with greater microtopographic variability that allows for more fire protection, and areas under less grazing pressure. The tree layer is composed of Quercus macrocarpa, Quercus alba, and Quercus velutina, with some Carya ovata. Quercus velutina is more common on sandier soils. The ground layer is dominated by graminoids, especially Andropogon gerardii and Schizachyrium scoparium, accompanied by a large diversity of forbs. Shrub cover can range widely from 0-100%, with Ceanothus americanus and Amorpha canescens being two of the most important shrub species. Understory species typical of oak openings require some direct sunlight throughout the growing season and are quickly reduced when heavily shaded by either tree or shrub canopy. Common shrubs in this community are adapted to the moderate fire frequencies found in oak openings, and resprout vigorously after burning.

Macroclimate, as expressed in relative moisture stress levels, is thought to play an important role in species composition variation from east to west in the Oak Openings region. Although an oak opening in Iowa and Ohio may have soils of the same water-holding capacity, plants in the Iowa site will experience more drought stress than plants in the more humid climate of Ohio. Thus, species that compete best in droughty situations will require a more open canopy to compete in an eastern oak opening than they will in a western one of similar soil type. Conversely, species that compete best in sunny but moist situations will require a less open canopy to compete in an eastern oak opening than they will in a western one of similar soil type.

Dynamics:  The abundance of oak openings has historically shifted back and forth from east to west over the past 10,000 years (Anderson 1983, Grimm 1984). Pollen records indicate that deciduous forests dominated the western Midwest 9000 years before present (BP). The climate became drier about 8300 BP, with graminoid-dominated communities (prairie, savanna, and oak woodland) becoming predominant. About 5000 BP, the climate started becoming more moist again and forest development renewed. It is thought that a much greater proportion of the forest-prairie transition zone would have grown up to forest during the last few thousand years if it were not for repeated burning of the region by Native Americans.

Following European settlement fire was almost entirely eliminated from the landscape. Observers in the mid to late 1800s noted that nearly all of the former prairies and oak openings which had not yet been converted to agriculture were growing up into oak brush and eventually oak woodlands due to lack of fire. Most of the oak forests extant today in the Upper Midwest were oak openings or oak woodlands during the early European settlement era.

Oak openings have been able to persist only where fires have been allowed to occur. One such site is railroad rights-of-way. Another is formerly burned woodlots. Burning of sparse oak openings to provide pasture was a common practice in parts of the Upper Midwest (notably the Driftless Area) before World War II (Pruka 1994). However, since the popularization of Smokey the Bear, total fire suppression has become standard and the dry-mesic oak opening remnants which survived as sparsely wooded pastures until the early 1940s have since grown up into oak woodlands and forests. Some of these sites may still be recoverable as oak openings, especially those on shallow-to-bedrock soils. Such soils prevent fast canopy regrowth rates. This delays the time before full canopy closure and thereby increases the period of persistence for oak opening understory flora.

Fire and grazing are the two important natural disturbances which encourage savanna formation. Fire appears to be the most important of these two, since woody growth release was not commonly noted until fire suppression began in the early European-settlement period, well after the period of massive reduction of bison and elk herds (Pruka 1994). Fire eliminates intolerant tree species. Fires in areas of oak opening were infrequent enough to allow certain species (Quercus alba, Quercus macrocarpa, Quercus velutina, and occasionally Carya spp.) to reach a fire-resistant size. Quercus alba, being less fire-tolerant than Quercus macrocarpa, is limited mostly to more hilly, dry and dry-mesic areas, since these less fertile soils support less grass fuel (and hence less intense/frequent fires) and are less prone to fire. In flatter, mesic areas, with their higher fuel loads, only Quercus macrocarpa was able to persist. Quercus velutina is less fire-tolerant (but more drought-tolerant) than Quercus alba and is favored on the most sandy/least fertile soils (Anderson 1991).

Before European settlement, fires were frequently and intentionally set by Native Americans to create conditions favorable for travel and for supporting large game. This may have led to larger expanses of savanna than would have occurred without human presence. It also probably encouraged savanna formation farther into the humid east than would have occurred without human-set fire (Pyne 1982). Several authors have suggested that Native American fires were most often set in the fall (Chapman 1984, Schwegman and McClain 1985, Swink and Wilhelm 1994). Grimm (1984) noted that several early European settlers recorded summer burns ignited by lightning.