Oak savanna

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Oak savanna, California North Slope Santa Ynez Mtns.jpg
Oak savanna, California

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 herb related plants. [1] The terms "oakery" or "woodlands" are also used commonly, though the former is more prevalent when referencing the Mediterranean area. [2] Oak savannas typically exist in areas with low precipitation and 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.

Contents

United States

Map of oak savanna distribution in North America NA-savanna-map.gif
Map of oak savanna distribution in North America

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. [3] [4] 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.)

Wisconsin bur oak savanna in mid-summer Bur-oak-savanna-Wisconsin.jpg
Wisconsin bur oak savanna in mid-summer
Fox River Grove, Illinois oak savanna; winter Oak savanna in Fox River Grove, Illinois.jpg
Fox River Grove, Illinois oak savanna; winter

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 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. [5]

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. [1]

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. [1]

The bur oak (Quercus macrocarpa) is the dominant species in northern oak savannas, although black oak ( Quercus velutina ), white oak ( Quercus alba ), and Hill's oak ( Quercus ellipsoidalis ) are sometimes present. 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 (Baptista lucantha), purple cornflower (Echinacea purpurea) and pasture rose (Rosa carolina). [6] There are also a few species that are unique to oak savannas. [7] [8] [9] 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). [10] 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 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. [11] 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) [12] 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 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. [13]

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.

Fire-tolerant bur oak savanna in Wisconsin hill country Bur-oak-savanna.jpg
Fire-tolerant bur oak savanna in Wisconsin hill country

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. [1]

Prescribed burn; Wisconsin bur oak savanna Fire-sav-S-slope-3720.jpg
Prescribed burn; Wisconsin bur oak savanna

Conservation and restoration

Prescribed burn in oak savanna in Iowa Prescribed Burn in Oak Savannah in Iowa.jpg
Prescribed burn in oak savanna in Iowa

Large areas of the Oak Savanna are affected by woody plant encroachment. [14] With the rising interest in environmental conservation, restoration and preservation of surviving areas of oak savanna began. Low intensity, Spring-time 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. [15] Burned areas are now more savanna-like, having greater grass and forb with lower shrub and tree representation, than unburned areas, but still have higher overstory densities than apparently existed in pre-settlement times. Decreasing canopy cover density is a key part in these restoration efforts. [10]

Two separate studies done by Garrastatxu et al. (2024) [16] and Encinas‐Valero et al. (2024) [17] 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) [18] 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. [19] The Bald Hill Natural Area in Corvallis, Oregon, was established in the 1990s in part to protect oak savanna remnants. [20] [21] 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. [22]

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. [23]

Current distribution

Pre-settlement there were approximately 50,000,000 acres of oak savanna in what is now the Midwestern United States, all of it located 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; they currently exist in many fragmented pockets throughout their native range.

Valadi et al. 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. [24] 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”. [22] 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. [11]

Examples of remnant oak savanna include:

Examples of restored oak savanna sites:

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. [29]

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. [29] 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. [29] 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

Related Research Articles

<span class="mw-page-title-main">Prairie</span> Ecosystems considered part of the temperate grasslands, savannas, and shrublands biome

Prairies are ecosystems considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and a composition of grasses, herbs, and shrubs, rather than trees, as the dominant vegetation type. Temperate grassland regions include the Pampas of Argentina, Brazil and Uruguay, and the steppe of Ukraine, Russia, and Kazakhstan. Lands typically referred to as "prairie" tend to be in North America. The term encompasses the lower and mid-latitude of the area referred to as the Interior Plains of Canada, the United States, and Mexico. It includes all of the Great Plains as well as the wetter, hillier land to the east. From west to east, generally the drier expanse of shortgrass prairie gives way to mixed grass prairie and ultimately the richer soils of the tallgrass prairie.

<span class="mw-page-title-main">Grassland</span> Area with vegetation dominated by grasses

A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur naturally on all continents except Antarctica and are found in most ecoregions of the Earth. Furthermore, grasslands are one of the largest biomes on Earth and dominate the landscape worldwide. There are different types of grasslands: natural grasslands, semi-natural grasslands, and agricultural grasslands. They cover 31–69% of the Earth's land area.

<span class="mw-page-title-main">Savanna</span> Mixed woodland-grassland ecosystem

A savanna or savannah is a mixed woodland-grassland biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses. Four savanna forms exist; savanna woodland where trees and shrubs form a light canopy, tree savanna with scattered trees and shrubs, shrub savanna with distributed shrubs, and grass savanna where trees and shrubs are mostly nonexistent.

<i>Quercus garryana</i> Species of oak tree

Quercus garryana is an oak tree species of the Pacific Northwest, with a range stretching from southern California to southwestern British Columbia. It is commonly known as the Garry Oak, Oregon white oak or Oregon oak. It grows from sea level to an altitude of 690 feet in the northern part of its range, and from 980 to 5,900 ft in the south of the range in California. The eponymous Nicholas Garry was deputy governor of the Hudson's Bay Company.

<span class="mw-page-title-main">Atlantic Forest</span> South American forest

The Atlantic Forest is a South American forest that extends along the Atlantic coast of Brazil from Rio Grande do Norte state in the northeast to Rio Grande do Sul state in the south and inland as far as Paraguay and the Misiones Province of Argentina, where the region is known as Selva Misionera.

<span class="mw-page-title-main">Controlled burn</span> Technique to reduce potential fuel for wildfire through managed burning

A controlled or prescribed (Rx) burn is the practice of intentionally setting a fire to change the assemblage of vegetation and decaying material in a landscape. The purpose could be for forest management, ecological restoration, land clearing or wildfire fuel management. A controlled burn may also refer to the intentional burning of slash and fuels through burn piles. Controlled burns may also be referred to as hazard reduction burning, backfire, swailing or a burn-off. In industrialized countries, controlled burning regulations and permits are usually overseen by fire control authorities.

<span class="mw-page-title-main">Rangeland</span> Biomes which can be grazed by animals or livestock (grasslands, woodlands, prairies, etc)

Rangelands are grasslands, shrublands, woodlands, wetlands, and deserts that are grazed by domestic livestock or wild animals. Types of rangelands include tallgrass and shortgrass prairies, desert grasslands and shrublands, woodlands, savannas, chaparrals, steppes, and tundras. Rangelands do not include forests lacking grazable understory vegetation, barren desert, farmland, or land covered by solid rock, concrete, or glaciers.

<span class="mw-page-title-main">Tallgrass prairie</span> Ecosystem native to central North America

The tallgrass prairie is an ecosystem native to central North America. Historically, natural and anthropogenic fire, as well as grazing by large mammals provided periodic disturbances to these ecosystems, limiting the encroachment of trees, recycling soil nutrients, and facilitating seed dispersal and germination. Prior to widespread use of the steel plow, which enabled large scale conversion to agricultural land use, tallgrass prairies extended throughout the American Midwest and smaller portions of southern central Canada, from the transitional ecotones out of eastern North American forests, west to a climatic threshold based on precipitation and soils, to the southern reaches of the Flint Hills in Oklahoma, to a transition into forest in Manitoba.

<span class="mw-page-title-main">Fire ecology</span> Study of fire in ecosystems

Fire ecology is a scientific discipline concerned with the effects of fire on natural ecosystems. Many ecosystems, particularly prairie, savanna, chaparral and coniferous forests, have evolved with fire as an essential contributor to habitat vitality and renewal. Many plant species in fire-affected environments use fire to germinate, establish, or to reproduce. Wildfire suppression not only endangers these species, but also the animals that depend upon them.

<span class="mw-page-title-main">Riparian zone</span> Interface between land and a river or stream

A riparian zone or riparian area is the interface between land and a river or stream. In some regions, the terms riparian woodland, riparian forest, riparian buffer zone, riparian corridor, and riparian strip are used to characterize a riparian zone. The word riparian is derived from Latin ripa, meaning "river bank".

<span class="mw-page-title-main">California oak woodland</span> Plant community found in the western United States and Mexico

California oak woodland is a plant community found throughout the California chaparral and woodlands ecoregion of California in the United States and northwestern Baja California in Mexico. Oak woodland is widespread at lower elevations in coastal California; in interior valleys of the Coast Ranges, Transverse Ranges and Peninsular Ranges; and in a ring around the California Central Valley grasslands. The dominant trees are oaks, interspersed with other broadleaf and coniferous trees, with an understory of grasses, herbs, geophytes, and California native plants.

<span class="mw-page-title-main">Gallery forest</span> Type of riparian forest in dry regions

A gallery forest is one formed as a corridor along rivers or wetlands, projecting into landscapes that are otherwise only sparsely treed such as savannas, grasslands, or deserts. The gallery forest maintains a more temperate microclimate above the river. Defined as long and narrow forest vegetation associated with rivers, gallery forests are structurally and floristically heterogeneous.

<span class="mw-page-title-main">Eastern woodlands of the United States</span>

The eastern woodlands of the United States covered large portions of the southeast side of the continent until the early 20th century. These were in a fire ecology of open grassland and forests with low ground cover of herbs and grasses.

<span class="mw-page-title-main">Oak forest</span> Forest with tree canopy dominated by oaks

An oak forest is a plant community with a tree canopy dominated by oaks. In terms of canopy closure, oak forests contain the most closed canopy, compared to oak savannas and oak woodlands.

<span class="mw-page-title-main">Upper Midwest forest–savanna transition</span> Temperate broadleaf and mixed forests ecoregion of the United States

The Upper Midwest forest–savanna transition is a terrestrial ecoregion that is defined by the World Wildlife Fund. An oak savanna plant community located in the Upper Midwest region of the United States, it is an ecotone between the tallgrass prairies to the west and the temperate deciduous forests to the east. A part of the Upper Mississippi River basin, it is considered endangered with less than 5% of the original ecosystem remaining intact, due mostly to overgrazing and conversion to agriculture.

<span class="mw-page-title-main">Mediterranean forests, woodlands, and scrub</span> Habitat defined by the World Wide Fund for Nature

Mediterranean forests, woodlands and scrub is a biome defined by the World Wide Fund for Nature. The biome is generally characterized by dry summers and rainy winters, although in some areas rainfall may be uniform. Summers are typically hot in low-lying inland locations but can be cool near colder seas. Winters are typically mild to cool in low-lying locations but can be cold in inland and higher locations. All these ecoregions are highly distinctive, collectively harboring 10% of the Earth's plant species.

<span class="mw-page-title-main">Woody plant encroachment</span> Vegetation cover change

Woody plant encroachment is a natural phenomenon characterised by the area expansion and density increase of woody plants, bushes and shrubs, at the expense of the herbaceous layer, grasses and forbs. It refers to the expansion of native plants and not the spread of alien invasive species. Woody encroachment is observed across different ecosystems and with different characteristics and intensities globally. It predominantly occurs in grasslands, savannas and woodlands and can cause regime shifts from open grasslands and savannas to closed woodlands.

<span class="mw-page-title-main">Wood-pasture hypothesis</span> Ecological theory

The wood-pasture hypothesis is a scientific hypothesis positing that open and semi-open pastures and wood-pastures formed the predominant type of landscape in post-glacial temperate Europe, rather than the common belief of primeval forests. The hypothesis proposes that such a landscape would be formed and maintained by large wild herbivores. Although others, including landscape ecologist Oliver Rackham, had previously expressed similar ideas, it was the Dutch researcher Frans Vera, who, in his 2000 book Grazing Ecology and Forest History, first developed a comprehensive framework for such ideas and formulated them into a theorem. Vera's proposals, although highly controversial, came at a time when the role grazers played in woodlands was increasingly being reconsidered, and are credited for ushering in a period of increased reassessment and interdisciplinary research in European conservation theory and practice. Although Vera largely focused his research on the European situation, his findings could also be applied to other temperate ecological regions worldwide, especially the broadleaved ones.

Oak regeneration failure is a woodland phenomenon whereby insufficient oak (Quercus) seedlings and saplings are recruited into the canopy to replace dead mature oaks. The result is a local decline in oak numbers while other more shade-tolerant trees such as maple, lime, and ash may become more prominent. Oak regeneration failure has been observed across Eastern and Midwestern forests in the United States as well as in Europe.

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