An oak savanna is a type of savanna (or lightly forested grassland), where oaks (Quercus spp.) are the dominant trees. It is also generally characterized by an understory that is lush with grass and herbaceous plants. The terms "oakery" or "woodlands" are also used commonly, though the former is more prevalent when referencing the Mediterranean area. Oak savannas typically exist in areas with low precipitation or high fire frequency and may feature poor soils. Predominant land uses include rangeland agriculture. Naturally, these savannas depend on natural wildfires (e.g. caused by lightning) to maintain their open vegetation.
United States Although there are pockets of oak savanna almost anywhere in North America where oaks are present, there are three major oak savanna areas: 1) California, British Columbia, Washington and Oregon in the west; 2) Southwestern United States and northern Mexico; and 3) the prairie/forest border zone of the Midwestern United States. There are also small areas of oak savannas in other parts of the world. (See also Eastern savannas of the United States for information on pine savannas of the U.S. South.)
California A mix of grassland and savannas make up about 15% of California. Oak subcategories within the state are separated by region, with the northern type including valley oak (Quercus lobata), which is the largest species of oak in the US, garry oak (Quercus garryana) and blue oak (Quercus douglasii), the southern type including Englemann oak (Quercus englemannii) and coast live oak (Quercus agrifolia), and the foothill woodlands including interior live oak (Quercus wislizenii) and blue oak as well.
Midwestern oak savannas The oak savannas of the Midwestern United States form a transition zone between the moist broadleaf and mixed forests to the east and the arid Great Plains to the west. They are found in a wide belt from northern Minnesota and southern Wisconsin, down through Iowa, Illinois, northern and central Missouri, eastern Kansas, and central Oklahoma to north-central Texas, with isolated pockets further east around the Great Lakes, including Ontario. According to the findings of Victoria A. Nuzzo, since there is much variation per state, other names used for the Midwestern oak savanna are oak opening, scrub prairie, oak barrens, and brush savanna. The bur oak (Quercus macrocarpa) is the dominant species in northern oak savannas on rich soils, although black oak (Quercus velutina), white oak (Quercus alba), and Hill's oak (Quercus ellipsoidalis) are sometimes present. On sandy soils, black oak is the dominant tree. The dominant tree in the south is usually black oak, although the chinquapin oak (Quercus muhlenbergii), post oak (Quercus stellata), and black-jack oak (Quercus marilandica) are also common. The flora of the herbaceous layer generally consists of species associated with tallgrass prairies, both grasses and flowering plants, although some woodland species may be present. Some of these plants can include white wild indigo plant (Baptisia leucantha), purple coneflower (Echinacea purpurea) and pasture rose (Rosa carolina). There are also a few species that are unique to oak savannas. Some important species in the midwest oak savanna include a variety of bats, like the evening bat (Nycticeius humeralis) found in Missouri, and many types of birds, including the blue winged warbler (Vermivora cyanoptera), field sparrow (Spizella pusilla), and summer tanager (Piranga rubra). Oak savannas, because of their mixture of grassland, woodland, and unique savanna species, typically have a higher plant diversity than grasslands and woodlands combined.
History Before European settlement, the oak savanna, a characteristic fire ecology, was extensive and was often a dominant part of the ecosystem. Fires, set by human activity or by lightning, ensured that the savanna areas did not turn into forests. Savannas normally were found next to large prairies near rolling hills and this combination of habitat was perfect for deer, bison, elk, and other grazing animals. Only trees with a high tolerance for fire, principally certain oak species, were able to survive. On sandy soils, black oak (Quercus velutina) predominated. On rich soils bur oak (Quercus macrocarpa) was the major tree in Midwestern North America. These savanna areas provided habitat for many animals, including American bison, elk, and white-tailed deer. The understory flourishes from exposure, as the oaks tend to be scattered, making for a less dense canopy. Fires contain the oak population, as too much shade would eliminate the grasses underneath. The most fire-tolerant of the oak species is the bur oak (Quercus macrocarpa), which is especially common in hill-country savannas in the Midwest.
European settlers cleared much of the savanna for agricultural use. In addition, they suppressed the fire cycle. Thus surviving pockets of savanna typically became less like savannas and more like forests or thickets. Many oak savanna plant and animal species became extinct or rare. However, a lack of generalized terminology or documentation in historical records has made it difficult to know about the past state of oak savannas.
Conservation and restoration
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