Oak savanna

Midwestern oak savannas

The oak savannas of the Midwestern United States form a transition zone between the arid

Before European settlement, the oak savanna, a characteristic fire ecology, was extensive and was often a dominant part of the ecosystem. Fires set 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.^[8] 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. Research by Granado-Díaz, Villanueva, & Colombo (2024)^[9] on land manager preferences for environmental services in oak savannas offers historical insights into the evolution of conservation strategies. This study illuminates the critical role of managing understory vegetation for ecological health, integrating seamlessly with the natural fire regimes that shaped these ecosystems. These findings underscore the importance of preserving the intricate balance between fire, vegetation, and wildlife that defines the oak savanna's unique ecological identity.

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.

Restoration

With the rise in interest in environmental conservation, restoration and preservation of surviving areas of oak savanna began. Low intensity, spring prescribed burns have been used since 1964 at the Cedar Creek Ecosystem Science Reserve in Minnesota in an attempt to restore the area to an oak savanna.^[10] Burned areas are now more savanna like (having greater grass and forb and lower shrub and lower tree representation) than unburned areas but still have higher overstory densities than apparently existed in presettlement times. Two separate studies done by Garrastatxu et al. (2024)^[11] and Encinas‐Valero et al. (2024)^[12] have identified the crucial role of fungal symbionts and the tree-soil microbial system in the health of oak savannas, underlining the importance of these factors in restoration practices. This insight is pivotal, as it highlights the interconnectedness of all elements within the oak savanna ecosystem, emphasizing that successful restoration efforts must account for the complexity of biological relationships. Similarly, the study by Hsiao et al. (2024)^[13] on the impact of vegetation and geology on soil nutrient storage in juniper–oak savannas emphasizes the need to consider soil health in restoration efforts. Restoration work in the US began in the 1970s in Illinois, followed by work in Wisconsin, Indiana, Iowa, Missouri, and Minnesota.^[14] The Bald Hill Natural Area in Corvallis, Oregon was established in the 1990s in part to protect oak savanna remnants.^[15][16] At one point in time the oak savanna was the most common type of vegetation found in the midwest, but it is now endangered with many ecologists every year working on replacing the oak savannas that have been destroyed in the past.^[17]

For centuries, the issue of oak regeneration failure has been acknowledged. Historical records, such as French ordinances dating back to the 13th century, illustrate early recognition of obstacles to oak regeneration. These ordinances required the planting of oak seedlings to prevent overharvesting, given oak's significant timber worth and adaptability.^[18]

Current distribution

Presettlement there was approximately 50,000,000 acres of oak savanna in Midwestern United States, all of it being exclusively in a wide strip stretching from southwestern Michigan to eastern Nebraska and from southern Manitoba to central Texas. After Europeans arrived, fire suppression and settlement diminished the oak savannas to a fraction of their former expanse, which currently exist in many fragmented pockets throughout its native range. Valadi et al. (2023)^[19] have explored the conflicting edge influences on herbaceous species in oak savannas, shedding light on the complex interactions that impact the distribution and conservation status of these ecosystems today. This research underscores the need for nuanced conservation strategies that address both the macro and microecological pressures impacting oak savanna preservation. Many sites are protected and maintained by government bodies or non-profit organizations such as The Nature Conservancy, the Wisconsin and Minnesota Departments of Natural Resources, and the Iowa Natural Heritage Foundation. In the midwest, about only 30,000 acres total of oak savanna still exist. The savannas that remain are fairly small at about 100 acres and this rarity has caused them to be categorized as “globally imperilled”.^[17] It is estimated that about only less than 0.02% of the original savannas remain due to human interaction and many organizations and conservations are prioritizing restoring and recreating these ecosystems in the midwest.^[8]

Examples of remnant oak savanna include:

• Neal Smith Wildlife Refuge near Prairie City, Iowa
  .
• Indiana Dunes National Park in Indiana.
• Hobart Nature District in Hobart, Indiana.
• Numerous State Natural Areas in Wisconsin.
• Lake County Forest Preserve District
  in Illinois.
• Waterfall Glen Forest Preserve near Darien, Illinois.
• Cook County Forest Preserve District and Iroquois County State Wildlife Area
  in Illinois.
• Oak Openings Preserve Metropark in Ohio.
• Pinery Provincial Park on Lake Huron in Ontario, Canada.

Examples of restored oak savanna sites:

• Indiana's Conrad Savanna Nature Preserve in north Newton County.^[20]
• Oak savanna restoration project site on Winchell Trail in Minneapolis, Minnesota^[21]
• Wisconsin's Pleasant Valley Conservancy State Natural Area.^[22]
• Minnesota's Belwin Conservancy.^[23]

In the Western Palearctic

In Europe, wood pastures, semi-open oak woodlands used for grazing, were widespread and common landscape features across the continent during the Middle Ages, but have since declined. Traditionally, they have been considered purely anthropogenic landscapes, the result of human clearing of virgin forest since the beginning of the European Neolithic. However, according to proponents of the controversial wood-pasture hypothesis, they can be viewed as functional analogs to the oak savannas that may have naturally covered large parts of Europe during the early Holocene. After the Industrial Revolution and the accompanying intensification of land use, they became rarer and were displaced by forests, open croplands, pastures, and meadows. Today, they are largely confined to small patches in western and central Europe, while intact forest pastures are still found in eastern Europe, especially Romania. In the Mediterranean region, oak savannas are still widespread, especially on the Iberian Peninsula.^[24]

In the Mediterranean

In the Western Mediterranean, oak savannas, known as dehesas in Spanish and montados in Portuguese, are at present widespread, concentrated in Southern Portugal, the Extremadura and Andalusia. They are actively managed and used for the production of cork and as pastures for Spanish fighting bulls and the black Iberian pig for the production of Jamón ibérico, among other uses. The main tree components are ballota oak (Quercus rotundifolia) and cork oak (Quercus suber), but Portuguese oak (Quercus faginea) and Pyrenean oak (Quercus pyrenaica) may also be common.^[24] Other oak species found in the area include Mirbeck's oak (Quercus canariensis), downy oak (Quercus pubescens), pedunculate oak (Quercus robur), sessile oak (Quercus petraea), Quercus estremadurensis, Quercus × cerrioides, kermes oak (Quercus coccifera) and Lusitanian oak (Quercus lusitanica).

In the Eastern Mediterranean, oak rangelands are still common landscape elements in Greece, Anatolia and the Levant.^[24] They are formed by a number of oak species, including pedunculate oak (Quercus robur), sessile oak (Quercus petraea subsp. polycarpa), downy oak (Quercus pubescens), Hungarian oak (Quercus frainetto), Quercus dalechampii, Strandzha oak (Quercus hartwissiana), Austrian oak (Quercus cerris), Macedonian oak (Quercus trojana), holly oak (Quercus ilex), Kasnak oak (Quercus vulcanica), Aleppo oak (Quercus infectoria), Lebanon oak (Quercus libani), Tabor oak (Quercus ithaburensis), Valonia oak (Quercus ithaburensis subsp. macrolepis), Brant's oak (Quercus brantii), Quercus aucheri, Quercus look and kermes oak (Quercus coccifera).

See also

• California oak woodland
• Dehesa
• Wood-pasture-hypothesis

References