Geography of Oak Savannas

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 and Oregon in the west; 2) Southwestern United States and Mexico; and 3) the prairie/forest border of the Midwest. Because most of the restoration work has been done in the Midwest, this web site will focus on this area. Included are savannas in the states of Ohio, Indiana, Michigan, Illinois, Wisconsin, Minnesota, Missouri, and Iowa. Although much of these savannas areas have been destroyed by grazing, row agriculture, and urbanization, there remain significant areas currently under restoration, or areas that could be restored.

A useful book summarizing all savanna types in North America is that edited by Anderson, Fralish, and Baskin.

A survey of oak savannas in the Midwest was carried out by Nuzzo in the early 1980s. This work was important, because it emphasized the rarity of the oak savanna ecosystem and encouraged workers in the various Midwestern states to promote savanna restoration.

An updated map, which also shows the distribution of other North American savannas, based on McPherson, is shown here.



North American Oak Savannas

Although the present web site deals primarily with Midwestern oak savannas, for comparison the west coast (California/Oregon) and Southwestern U.S. oak savannas are discussed briefly here.

Southwestern U.S. oak savannas

Oak savannas are found in vast areas of southeastern Arizona, southwestern New Mexico, and adjacent land in Mexico. These are predominantly grasslands with scattered oaks, mainly at altitudes between 4000 and 6000 feet.

Open-grown Emory oaks (Que-rcus emoryi) on a hillside in southeastern Arizona. Vast acreages of oak savannas can be found at the appropriate altitudes in this part of Arizona.
Typical open-grown Emory oak in southeastern Arizona. This species is evergreen, but generally loses some or all leaves during the early summer drought. These habitats are primarily prairie grasslands with scattered oaks. Fire is a constant threat, either human- or lightning-induced, and under appropriate conditions great acreages burn. The large oaks are fire-resistant.


A survey of some of these savannas was made in December 2009 and January 2010 and is summarized in a report (PDF), which can be downloaded here.



California oak savannas

A good overview of California oak savannas can be found at this link.

There are vast oak savanna areas in interior California, primarily at altitudes of between 200 and 2000 feet in the Coast Ranges and in a ring around the Central Valley in the foothills of the Sierra Nevada and Coast Ranges. (See map above) They are situated between annual grasslands at lower altitudes and mixed conifer or ponderosa pine forests at higher elevations.

The dominant species of oaks in California oak savannas are blue oak (Quercus douglassii), valley oak (Q. wislizenii), coast live oak (Q. agrifolia), and Engelmann oak (Q. engelmannii). Much of the California oak savannas are in private hands and are used primarily for cattle grazing.

Open-grown oaks in a savanna on Mount Diablo State Park, California. Photographed in October, just at the end of the dry season. The grasslands provide extensive fuel for either wildfires or prescribed burns.
Extensive oak savanna area in the California foothills.
Open-grown oaks in a grazed savanna area in the foothills, southern California.

Topography of Oak Savannas

The rest of this web site deals only with Midwest oak savannas.

Oak savannas can be found on a wide variety of landscapes, from perfectly flat, to rolling, hilly, and very steep terrain. Today, savanas are found mainly on dry or dry-mesic sites. Sites with excessive moisture, such as wetlands, bottomland forests, gullies and ravines, and wet-mesic sites are not conducive to savanna development. Since they are fire-dependent, savannas are found where fires can start and spread, areas where soil moisture levels are lower.

Oak savannas may be found more commonly in hill country because these areas burn better. Fires generally move uphill more rapidly and with higher flame heights and more intense heat. Also, hill country is less suitable for agriculture and if it is far from a highway it often remains undeveloped. Fire-sensitive trees such as maples, walnuts, and basswood are killed whereas oaks, with their thick, fire-resistant bark, survive. In level sites, savannas may be more common in sandy than loamy soil, again because they burn hotter. In addition to trees, shrubs (brush) is often a major component of a degraded oak savanna. All shrubs are fire sensitive and are top-killed by fire but are not eradicated.

Presettlement Oak Savannas

Before first settlement by humans, Midwest oak savannas were maintained by lightning-caused fires. The frequency of lightning-caused fires was almost certainly low, perhaps once every ten to fifty years. Once humans arrived everything changed.

North America has been occupied by humans for about 12,000 years. At the time humans arrived, the last continental glacier (the Wisconsin ice age) still existed, so that much of the Midwest was covered with ice. As the glacier receded, more and more of the land was occupied. By the beginning of the Christian era, there were large numbers of Native peoples living in North America, and they had created large and complex societies. It is estimated that at by 1600 A.D., over 20,000 people lived in Wisconsin alone, and all the other Midwestern states were similarly occupied.

The Native peoples vastly altered the landscape of the Midwest. One of the most important causes of alteration was the use of fire. It is now well accepted that Native peoples burned the land yearly, primarily in the late fall of the year. Fires were created for a variety of reasons, but an important consequence was that the oak savanna landscape flourished.

Public Land Surveys

Several types of information are available for assessing the presettlement vegetation of the United States: travelers accounts; original surveyors records; fragments of original (old-growth) forests.
The most useful records are the original surveyors records, created by the Public Land Surveys (PLS). These records were developed when the country was systematically settled in the early part of the 19th century. Most of United States was surveyed by the Public Land Survey, which was adopted in 1785. Only the original 13 colonies, plus Maine, Vermont, and Texas were not surveyed by this system.

The PLS system, which is still the system upon which all real estate transactions are based, uses a grid with north-south strips six miles wide, determined from a series of principal meridians (north/south lines). The 1st principal meridian is in eastern Indiana, and subsequent principal meridians were established as the survey moved westward. Each six mile area (called a township) is divided into 36 one square mile units called sections, which are numbered in sequence beginning at the upper (northeast) corner of the township. Each one square mile section is 640 acres. Each section is divided into quarter sections by dividing the grid lines into two. Thus, each quarter section consists of 160 acres. For real estate purposes, quarter sections were often divided into four parts, each 40 acres in extent, and land transactions still often involve sales of "40's".

The original survey notes were recorded by the surveyor in a special log book, and these log books still exist today. (For some states, these log books have been digitized and all the information is available on line. Thus, Wisconsin’s surveyor records can be seen at This web site also provides detailed information about how the land surveys were done, limitations on the data, and other details.)

Page from the surveyor's notebook, showing the records of the "witness" trees used to mark the corner posts. All the trees marked are bur oaks. This area today remains a bur oak savanna. Image downloaded from the UW-Madison Digital Collections.  

The Public Land Survey (PLS) data provide detailed insight into the original vegetation of a state. This is possible because at each section corner a monument was created, using stakes, stones, or earthen mounds. This monument was marked by several witness trees. Each witness tree was noted by its diameter, species, and directional coordinates and distance from the monument. In addition to the section corners, witness trees were used at every half section mark.

In addition to the above information, the surveyor also made notes of the vegetation along the route, and any particular noteworthy items. For each township, a hand-drawn map was made.
The PLS data have been used for many years for preparing presettlement (close to original) vegetation maps of counties or even complete states. Maps have been published for each of the Midwestern states and can be purchased from an appropriate state agency in that state, such as a Natural History Survey, Natural Resources department, or state university. In recent years, computer technology and GIS software has made it possible to develop even more accurate maps of the vegetation of complete states.

The most recent work of Rhemtulla and others on the extent of oak savannas in the Upper Midwest analyzing the surveyors’ records with digital and GIS technology, has shown that the oak savanna was the most common vegetation type (69%), with prairies (6%) embedded in the savanna matrix. This work considerably expands on the Nuzzo study, and provides even stronger support for the importance of oak savannas.

Importance of Fire

The oak savanna would not exist without fire. Although fire resistant, oaks are shade intolerant and are unable to maintain populations underneath the shade of maples and other shade-tolerant trees. Thus, in the absence of fire maples and other shade-tolerant trees are eventually able to take over the forest.

Why are oaks fire resistant?

  • The thick bark of oaks is very fire resistant and has low thermal conductivity so that heat does not penetrate well to the living tissues underneath.
  • Oaks form dormant buds at the base of the trunk. These buds are protected from fire by the root collars. These dormant buds are below the soil surface where they are protected from the lethal effects of fire. If the stem is killed by fire, one or more of these dormant buds grow.
  • Oak saplings become resistant to fire when they are smaller than those of other tree species.
  • Oak saplings subject to periodic (annual) fires produce extensive root systems even if top-killed. These large root systems permit better growth of shoots in fire-free years.
  • Tree competitors are destroyed by fire, thus permitting selective oak reproduction.
  • The seeds of oaks (acorns) serve as food for rodents. Rodents bury acorns under ground, thus bringing them into an environment where they are protected from fire.

The table below compares the shade tolerance and fire sensitivity of various hardwood trees to oaks. As seen, the oaks are the most fire resistant of the hardwoods.

Shade tolerance and fire sensitivity of deciduous treespecies in upland Midwest forests.

Tree species Common name Shade tolerance Fire sensitivity
Acer negundo Box elder Somewhat tolerant Sensitive
Acer rubrum Red maple Tolerant Sensitive
Acer saccharum Sugar maple Very tolerant Sensitive
Betula papyrifera White birch Intolerant Sensitive
Carya ovata Shagbark hickory Intermediate tolerance Intermediate
Fagus grandifolia Beech Very tolerant Sensitive
Juglans nigra Black walnut Intolerant Sensitive
Populus tremuloides Quaking aspen Intolerant Sensitive
Prunus serotina Black cherry Intolerant Sensitive
Quercus alba White oak Intermediate tolerance Resistant
Quercus macrocarpa Bur oak Intermediate tolerance Very resistant
Quercus rubra Red oak Intermediate tolerance Resistant
Quercus velutina Black oak Intermediate tolerance Resistant
Tilia americana Basswood Tolerant Sensitive
Ulmus rubra Slippery elm Tolerant Sensitive

Trees which are intolerant or very intolerant to shade, such as aspen and birch, are pioneer colonizers of open land, but are unable to reproduce in their own shade. Also, tree species which are very tolerant to shade, such as maple and beech, are able to form dense forests with essentially no tree reproduction except for their own species. However, all of these species are very sensitive to fire.

The oaks are intolerant to shade and hence are unable to reproduce under their own canopy. Oaks, however, are resistant to fire (in the case of bur oak, very resistant) and hence are able to reproduce in a fire-prone ecosystem. Although the table is an oversimplification, it can be used to predict the type of forest that will eventually evolve under different sorts of regimes. This table clearly explains why the oaks are the savanna trees par excellence.

The most fire-resistant of the oaks is the bur oak, Quercus macrocarpa, which is a preeminent tree of the oak savanna. The bark of the bur oak is very thick and unusually fire resistant.

Measuring the oak canopy with fish-eye lens photography

A fish-eye lens takes a photograph that encompasses a complete 180 degrees, from the horizon to the azimuth. Such photographs can be used to measure the oak canopy at different locations on a site. Simple measurements on these photos with a grid system can be used to calculate the percent canopy, or more complex measurements with computer software can be used to obtain more precise readings. For oak savanna restoration, the complex measurements are not needed.

Typical fish-eye lens photograph of a bur oak savanna, with a canopy cover of about 50%.

Although measurements of canopy cover with fish-eye lens photography are not necessary to assess the restoration value of a site, they do permit an objective assessment of the canopy.

Impact of Forestry and Agriculture

The early surveyors called oak savannas “second” or “third rate timber”, indicating that there was little wood suitable for commercial cutting. The oaks did provide wood for many uses by early settlers, however, such as material for log homes and barns, furniture, fence posts, and fire wood. However, without power equipment, cutting trees is a laborious occupation, so that much of the oaks, especially the larger trees, were probably left alone.
The focus of early settlers was on producing food for their own needs, so that the land that could be cultivated without clearing was favored. Oak savannas, with their open-grown character, were, however, suitable for pasturage, so that almost all of the savannas in settled areas were grazed. The widely spaced oaks provided no difficulty for grazing animals, which generally benefitted from the shade they provided.

Savanna areas for grazing were burned every spring, in order to “green up” the pasturage. In many parts of the Midwest, burning of savannas by farmers continued well into the 20th century. The open character of these savannas is strikingly revealed by the air photography that was carried out by the U.S. Soil Conservation Service in the late 1930s. Open-grown trees (almost certainly oaks) stand out on all these air photos. Many of the oaks that can be seen on these old air photos still exist on the landscape today, especially in hill country where row-crop agriculture was not possible.

Oak savannas and forestry

Professional foresters, interested primarily in woodlands for commercial production, often do not look favorably on oak savannas, because silvicultural characters such as “stocking rate” and “reproduction rate” are low. From the forester’s point of view, the ideal oak forest is one sufficiently “stocked” so that it is able to “produce” at maximum rate. The term “basal area per acre” is often used to express the “quality” of a forest.

Landowners with potentially high quality oak savannas may be discouraged by professional foresters from managing their woodlots as savannas. It must be emphasized that oak savanna management and commercial logging are incompatible activities. This is unfortunate for the future of oak savanna restoration. Since there is no reasonable way that an oak savanna can be managed for a profit, many landowners may be discouraged from savanna restoration.

However, the aesthetic value of an oak savanna is vastly superior to that of a logged forest.


Nuzzo, V.A. 1986. Extent and status of Midwest oak savanna: presettlement and 1985. Natural Areas Journal 6: 6-36.

McPherson, Guy R. 1997. Ecology and Management of North American Savannas. University of Arizona Press, Tucson.

Jeanine M. Rhemtulla, David J. Mladenoff and Murray K. Clayton (2007). Regional land-cover conversion in the U.S. upper Midwest: magnitude of change and limited recovery (1850–1935–1993). Landscape Ecology Volume 22:

Anderson, Roger A., Fralish, James S. and Baskin, Jerry M. editors.1999. Savannas, Barrens, and Rock Outcrop Plant Communities of North America. Cambridge University Press.

McClaran, Mitchel P. and McPherson, Guy R. Oak savanna in the American Southwest. pp. 275-287 in Anderson et al. 1999.

Allen-Diaz, Barbara, Bartolome, James W., and McClaran, Mitchel P. California oak savannas. pp. 322-339 in Anderson et al. 1999..


Return to TopReturn to Top