Oak woodland is a type of savanna that is intermediate between more open oak opening and more closed canopied oak forests. It tends to be dominated by members of white oak group, especially white oak, sometimes with bur oak, and locally in southwestern Wisconsin, chinquapin oak. Red oak, black oak, and shagbark hickory may also be present and can sometimes account for 50% relative cover or more of the tree layer. Canopy closure can vary widely. The best quality sites have a canopy 41-65% closure but can vary from 30-80% closure. "Canopy closure" is the proportion of shrub and ground layer that is covered in shade or shade flecks at noon on a sunny day.
Oak woodlands historically experienced near-annual surface fires. As a result, the subcanopy is very sparse in good quality sites, though fire-suppressed sites often contain mesophytic species such sugar maple, red maple, ironwood, hackberry, American elm, and bitternut hickory. The shrub layer is also typically sparse but may include low growing species like low bush honeysuckle (Diervilla lonicera), snowberry (Symphoricarpos albus or S. occidentalis), blueberry (Vaccinium spp.), huckleberry (Gaylussacia baccata), New Jersey tea (Ceanothus americanus), and lead plant (Amorpha canescens).
The groundlayer in oak woodlands is often dominated by a matrix of sedges like Pennsylvania sedge and savanna running sedge (Carex siccata), sometimes also with a variety of other graminoids that thrive under dappled light conditions such as bottlebrush grass (Elymus hystrix), silky wild-rye (Elymus villosus), and wide-leaved panic grass (Dichanthelium latifolium). Forbs overlap significantly with oak openings and oak forest, but some of the best indicators include upland boneset (Eupatorium sessilifolium), prairie alumroot (Heuchera richardsonii), two-flowered Cynthia (Krigia biflora), veiny pea (Lathyrus venosus), pale vetchling (Lathyrus ochroleucus), blunt-leaved sandwort (Moehringia lateriflora), wood betony (Pedicularis canadensis), eastern shooting-star (Primula meadia), yellow-pimpernel (Taenidia integerrima), Culver's-root (Veronicastrum virginicum), Carolina vetch (Vicia caroliniana), and Short's aster (Symphyotrichum shortii). Additional oak woodland indicator species can be found in the Coarse-level Monitoring Protocol for Oak Woodlands (Carter et al, 2023).
Oak woodlands can occur in a variety of landscape settings, usually in sites that allowed them to persist with frequent fire but not so severe as to convert them to prairie or oak opening. Examples include cooler slope aspects adjacent to prairies and oak openings, knolls and ridgetops surrounded by mesic to dry-mesic forest, and upland islands surrounded by wetlands.
