Physiognomy and Structure: Structure in this community ranges from large open-grown trees 10-12 m tall covering 30-80% of the canopy. In the years following occasional catastrophic fires, characteristic canopy dominants may exist only as oak grubs 1-2 m tall. Shrubs similarly reach peak height of 1-3 m after several years without fire, but average less than 1 m tall for the first few years following a fire. Shrubs dominate the ground layer, with forbs and graminoids still remaining as important understory components. Many of the predominant graminoids are sedges.

Floristics: There are three main structural layers: tree canopy, tall-shrub and herbaceous. Tree densities for stems greater than 10 cm dbh may historically have been between 50-200 stems per hectare (Bowles et al. 1994). The tree layer is composed predominantly of Quercus alba and Quercus macrocarpa, with Quercus ellipsoidalis or Quercus velutina more common on dry sites, and occasional codominance by Quercus rubra, Carya spp., and Populus spp. (MNNHP 1993). Sassafras albidum can become an important tree layer component in the eastern end of the oak woodland range.

This community may include portions of stands also described as brushland (Grimm 1984, MNNHP 1993) or barrens (Bowles and McBride 1994, White 1994a). Shrub cover is usually high (30-50%) with some stands forming impenetrable thickets. Typical shrubs include Corylus americana and Cornus foemina. Throughout the range of the type, dominant herbs include Andropogon gerardii, Carex pensylvanica, Desmodium glutinosum, Geranium maculatum, and many others. These brushlands are dominated by woody species capable of reaching overstory size, but nearly all woody individuals are found in a shrubby state because intense fires are frequent enough to preclude them from reaching overstory size. Other barrens have been described to have scattered woody individuals of canopy size (White 1994a).

This community may also include stands described as "closed savanna" or "closed woodland" (Packard 1988, Swink and Wilhelm 1994). These stands experience low-intensity but moderately frequent fires that preclude the establishment of fire-sensitive tree saplings of species such as Acer saccharum, Tilia americana and Fagus grandifolia, but allow the establishment and growth to canopy size of moderately fire-tolerant tree saplings such as Quercus rubra and even Acer nigrum. Shrub cover is quite low in this subtype due to the high frequency of low-intensity fire. Fuel loads for fires in this community are provided by several cool-season sedges and grasses such as Carex rosea, Carex pensylvanica, Elymus villosus, and Elymus hystrix (Wilhelm 1991).

Macroclimate, as expressed in relative moisture stress levels, is hypothesized to play an important role in species composition variation from east to west in the Oak Woodlands region. Although an oak woodland in Minnesota and Michigan may have soils of the same water-holding capacity, plants in the Minnesota site will experience more drought stress due to macroclimate than the Michigan site. Thus, species that compete best in high-moisture stress situations will require a more open canopy to compete in an eastern oak woodland than they will in a western oak woodland of similar soil type. Conversely, species that compete best in lower moisture stress situations will require a more closed canopy to compete in a western oak woodland than they will in an eastern oak woodland. For example, Hamamelis virginiana, a species which competes well only under low moisture stress conditions, readily invades unburned woodland in Michigan, but is found only in more shaded ravine situations in Wisconsin (B. Pruka pers. obs.) and Minnesota (MNNHP 1993).

Quercus alba is the most common overstory tree in dry-mesic oak woodland (Bowles et al. 1994). Quercus rubra becomes important in more mesic soils where it can successfully compete with Quercus alba (Nowacki et al. 1990). Quercus rubra is a successful gap-colonizer species, and woodlands codominated by it are more likely to be located adjacent to nearby dry-mesic and mesic forests (Kline and Cottam 1979). The catastrophic nature of many woodland fires promotes the presence of woody invaders such as Populus tremuloides, Populus grandidentata, and Fraxinus americana (Kline and Cottam 1979, MNNHP 1993).

Dynamics:  Occasional, but often intense, fire is the important natural disturbance which encourages woodland formation. Fires must be infrequent enough to allow tree species to occasionally reach tree size yet frequent enough to kill fire-tolerant tree saplings as well as prevent canopy closure. Because fires are rather infrequent, a large buildup of woody fuels occurs between fires. Woodland understories, shaded by heavier tree and shrub canopies, do not dry out during spring and fall as quickly as prairie and savanna (Stearns 1974) and so are not always dry enough to burn during these seasons. Additionally, these site are often topographically protected from fire by streams or lakes (Bowles et al. 1994). Yet when they do burn, woodlands often burn so intensely that many of the adult trees die, and woodland overstory canopy is temporarily reduced to that of a brushland or brushy oak savanna. With vigorous resprouting of oak grubs and an extended period before another catastrophic fire, these temporal brushlands can recover to a woodland overstory. Fires were intentionally set by Native Americans in the pre-European settlement era. This may have led to larger expanses of oak woodland than would have occurred without human presence, particularly in the eastern end of the oak woodland range where fire prevented the development of closed forests composed of shade-tolerant tree species (Grimm 1984).