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
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
An updated map,
which also shows the distribution of other North American
savannas, based on McPherson, is shown
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
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
http://digicoll.library.wisc.edu/SurveyNotes/. 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
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.
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.
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.
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
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.
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.
tolerance and fire sensitivity of deciduous treespecies
in upland Midwest forests.
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.
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.
the oak canopy with fish-eye lens photography
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.
fish-eye lens photograph of a bur oak savanna,
with a canopy cover of about 50%.
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.
of Forestry and Agriculture
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
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.
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.
savannas and forestry
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.
the aesthetic value of an oak savanna is vastly superior
to that of a logged forest.
V.A. 1986. Extent and status of Midwest oak savanna:
presettlement and 1985. Natural Areas Journal 6:
Guy R. 1997. Ecology and Management of North American
Savannas. University of Arizona Press, Tucson.
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..