Midwestern Oak Savanna

This system is found primarily in the northern glaciated regions of the Midwest with the largest concentration in the prairie-forest border ecoregion. It is typically found on rolling outwash plains, hills and ridges. Soils are typically moderately well- to well-drained deep loams. This system is typified by scattered trees over a continual understory of prairie and woodland grasses and forbs. Quercus macrocarpa is the most common tree species and can range from 10-60% cover. The understory is dominated by tallgrass prairie species such as Andropogon gerardii and Schizachyrium scoparium associated with several forb species. Historically, frequent fires maintained this savanna system within its range and would have restricted tree canopies to 10-30%. Fire suppression in the region has allowed trees to establish more dense canopies. Periodic, strong wind disturbances and browsing also impact this system. Much of this system has also been converted to urban use or agriculture, and thus its range has decreased considerably.

Quercus macrocarpa is the most common tree species and can range from 10-60% cover. The understory is dominated by tallgrass prairie species such as Andropogon gerardii, Calamagrostis canadensis, and Schizachyrium scoparium associated with several forb species.

This system is typically found on rolling tillplains, hills, and ridges in the glaciated Midwest. Soils are typically moderately well- to well-drained deep loams and fertile. Because fire is critical to maintaining this system, it is not found in fire-protected portions of the landscape.

Historically, frequent fires maintained this savanna system within its range and would have restricted tree canopies to 10-30% cover with some portions having up to 60% tree canopy. On average, surface fires were very frequent (1- to 5-year return intervals) and maintained the open, herbaceous understory. Canopy trees were replaced when periodic longer fire-return intervals, due to chance, multi-year wet climatic cycles, or lack of burning by Native Americans, allowed oak seedlings to grow large enough to survive surface fires when they returned. If fire is absent for more than about 20-40 years, a site will transition to oak woodland/forest (Cottam 1949, Curtis 1959, Grimm 1981). Fire suppression in the region has allowed trees to establish more consistent dense canopies. Periodic, strong wind disturbances and browsing/grazing also impact this system through modification to the herbaceous layer and tree seedlings.

Before European settlement, oak savanna, essentially this system and North-Central Oak Barrens (CES202.727), was estimated to have covered 11-13 million ha in the Midwest. The extent in 1985 was estimated at just over 2600 ha and all but 40 ha was on dry or rocky sites (Nuzzo 1986) and likely North-Central Oak Barrens (CES202.727). Fire suppression and conversion to agriculture or urban development have nearly eliminated this system from the landscape. Fire suppression for more than a few years allows woody species to proliferate. This system was most common on flat to rolling fertile sites in the "corn belt" of the United States, so many sites were used for agricultural and the landscape they were in was fragmented. Agricultural grazing can degrade sites, especially when combined with fire suppression, and other agricultural uses can outright destroy them. Conversion to urban uses was also common in the industrial and agricultural Midwest. A lack of fire allows native woody species to expand but also allows exotic species into the shrub and herbaceous strata. Rhamnus cathartica and Lonicera spp. are particularly common invaders. In Michigan, invasive species that threaten the diversity and structure of this system include Berteroa incana, Celastrus orbiculata, Centaurea biebersteinii (= Centaurea maculosa), Cynanchum louiseae (= Vincetoxicum nigrum), Cynanchum rossicum (= Vincetoxicum rossicum), Elaeagnus umbellata, Hieracium spp., Hypericum perforatum, Leucanthemum vulgare (= Chrysanthemum leucanthemum), Lonicera japonica, Lonicera maackii, Lonicera morrowii, Lonicera sempervirens, Lonicera tatarica, Lonicera x bella, Lonicera xylosteum, Poa compressa, Poa pratensis, Rhamnus cathartica, Rosa multiflora, Rumex acetosella, and Saponaria officinalis. Due to their high edge-to-area ratio, savannas are susceptible to exotic species invasion by such aggressive shrubs as buckthorns and honeysuckles (Apfelbaum and Haney 1991), which create dense shade that depresses or eliminates graminoid species that provide fine fuels for surface fires (Anderson and Bowles 1999). Ground layer vegetation of savanna remnants has been inhibited by low levels of light filtering through the dense overstories and impenetrable understories (often dominated by exotic shrubs) and by the thick litter layers that have accumulated from over a century of fire suppression (Bowles and McBride 1994, Abella et al. 2001) (from Cohen 2004).

This system is found throughout the northern glaciated regions of the Midwest. Its main concentration, where it was likely the matrix type, is within the Prairie Forest Border of Minnesota, Wisconsin, Iowa, and Illinois. Conversion to urban uses and agriculture and fire suppression have significantly impacted the range of this system.