Purpose of this web site
This web site has been created to provide land managers, ecologists, foresters, land owners, and interested laypeople with the tools needed to restore oak savannas in Midwestern North America.
The oak savanna was once one of the most common vegetation types in the Midwest but is today highly endangered. Intact oak savannas are now one of the rarest plant communities on earth. However, many degraded oak savannas still remain and can be restored. The detailed information in this web site shows the way.
Savanna Oak Foundation, Inc.
This web site has been created and is maintained by the Savanna Oak Foundation, Inc., a nonprofit foundation dedicated to the oak savanna community. The Foundation was established by Tom and Kathie Brock of Madison, Wisconsin, who manage Pleasant Valley Conservancy, a Wisconsin State Natural Area, which has extensive restored oak savannas.
Another web site maintained by the Savanna Oak Foundation is Pleasant Valley Conservancy, which describes in detail the restoration work on oak savannas and other fire-dependent ecosystems being carried out at this outstanding State Natural Area in southern Wisconsin.
What is an oak savanna?
A savanna is
generally defined as a plant community where trees are a component
but where their density is "...so low that it allows
grasses and other herbaceous vegetation to become the actual
dominants of the community." (Curtis,
The Vegetation of Wisconsin). Savannas are found throughout the world, but the dominant trees differ. In North America, a major type of savanna has oaks as the principal trees. Three major areas of oak savanna in North America are found: 1) California and Oregon on the west coast; 2) Arizona, New Mexico, and Mexico in the southwest; and 3) the Midwestern United States. This web site deals exclusively with Midwestern oak savannas, of which many restorable sites exist. Oak savannas in the Midwest are most commonly
found in a climatic zone intermediate between woodland and
prairie, which is often called the prairie/forest border.
Savannas are often
defined in terms of the openness of the tree canopy. Thus,
the upper limit between savanna and forest
is generally considered to be a tree canopy with 50% coverage. Therefore, if more
than one-half of the ground area is in the sun at noon
in midsummer, the vegetation is classed as a savanna. It the canopy has greater than 50% tree canopy coverage, the vegetation is called a woodland or forest. The lower canopy coverage,
between savanna and prairie, is generally considered to be 10% tree coverage, although these upper and lower limits are only approximate.
Another term sometimes used as equivalent to oak savanna is
“oak opening”, which refers to the open-grown characteristic
of the trees. Early travelers in the Midwest remarked
at the “park-like” character of the vegetation. Some alternate
terms occasionally used to describe a savanna-like setting
are barren, brush prairie, glade, and open woodland.
The oak savanna landscape
is a mosaic of vegetation types, with sizes varying with the
topography and other characteristics of the land. The term
“oak savanna landscape” refers to a natural area or a complex
of natural areas with a wide diversity of species. The dominant
trees of the oak savanna are several major species of
oaks. Within and among this oak tree canopy are numerous
smaller trees, shrubs, grasses, and forbs. Thus, although
the oaks are the most obvious plants, we must keep in mind
the high diversity that is present. This diversity is important
in the changes that take place in the landscape with time.
Gradual climate change, year-to-year variation in weather,
and external disturbance (logging, disease, wind, etc.) may
lead to the elimination of individual trees or even wide swaths
of the forest, but in a diverse natural area, other individuals
usually move in and fill the gaps.
How does a savanna differ from a woodland?
Two major types of oak ecosystems in the Midwest are savanna
and woodland. The essential character of an oak savanna is
the presence of open-grown oaks. When a single tree grows
isolated from all other trees, it does not face competition
and its lower limbs are able to spread out and become large
and substantial. A savanna oak generally develops in an open
area where competition is reduced, which in the Midwest are
usually prairies. Thus, oak savannas and prairies are closely
linked ecologically as well as topographically.
The openness of the oak savanna is usually maintained by fire,
and of the major tree species in the Midwest the oaks are uniquely fire
resistant. Over time, these scattered oaks develop into large
trees and each open-grown tree receives maximum sunlight and
there is little competition between individuals. Oak savannas
generally develop in drier areas, on south- or southwest-facing
slopes or other areas where many other tree species are unable
In contrast, in an oak woodland the trees grow in a crowded situation,
with each tree shading its neighbors. The lower limbs of these
trees receive inadequate sunlight and eventually die. Woodlands
develop in moister areas, on north- or northeast-facing slopes
or in lowlands where tree growth is favored. In a woodland,
the faster growing trees overtop slower growing ones and become
dominant. The slower growing, shaded, trees will eventually
die. The characteristic structure of a healthy woodland consists
of tall straight trees without significant lower branches
and these trees have top canopies that spread out, making
it possible for them to capture maximum sunlight. The trees of a woodland can also be oaks, although maple, basswood, beech, and other species may dominate, depending on the environmental conditions.
Because of the
open nature of the savanna, there will be many areas of scattered
light and shade, where herbaceous plants can thrive even during
the summer. An oak woodland, in contrast, is so shady during
summer that significant herbaceous plants are only found before
the tree canopy has leafed out (these plants are therefore
called “spring ephemerals”).
The open nature
of the oak savanna results in the establishment of numerous
kinds of prairie plants, both grasses and forbs. If the tree
canopy is very sparse, the vegetation will be more prairie-like
than woodland-like. On the other hand, when the tree canopy
approaches 50%, prairie plants will not grow as well but many
woodland plants will thrive. Because of the scattered nature
of the oaks, some parts of an individual savanna will be very
open and other parts more closed.
In addition to
the prairie-like and woodland-like herbaceous plants, there
is a third category, the savanna specialists, that grow best
at intermediate light intensities. Thus, the diversity of
plants in an oak savanna is higher than either a prairie or
woodland, because it has species representing all three categories
of plants: prairie plants, savanna plants, and woodland plants.
An open-grown tree
The first step in recognizing an oak savanna is to look for
open-grown trees. The best example of an open-grown tree (whether
oak or other species) is a “street tree.” Cities plant trees
widely spaced along streets so that they do not crowd each
other. Such trees are generally planted as saplings and are
cared for until they reach maturity. A large street tree is
a glorious object, and because it takes many years to mature
it is irreplaceable. Yard trees, planted by homeowners, are
also open-grown trees, and provide shade and comfort to the
typical open-grown oak that has developed in an urban
settling. Because there was no competition, the branches
remain alive and intact close to the ground.
are also found in the wild, as part of oak savannas. In a
well-managed savanna open-grown oaks are readily recognized.
However, most savannas that exist today are highly degraded
and although open-grown oaks may be present, they are generally
crowded from overgrowth of weedy trees or woody shrubs. Although
large lower limbs may be present, they are often dead. In
some case, the only evidence that a tree was once open-grown
is the presence of large knots where branches once stood.
A major task of oak savanna restoration is the liberation
of open-grown oaks from competition, a process called
"daylighting the oaks."
Note that the open character of the forest alone does not
signify savanna. If an oak forest has been logged, it may
be open, but the remaining trees were not open-grown. Such
trees can be distinguished from open-grown ones by the
lack of lower branches, or the by the absence of the large
knots that show where lower branches were once present. In
degraded savannas, the large lower limbs that have died (due
to shading) may remain on the tree for many years and can
still be seen. Also, lower branches that are in a moribund
state due to shading in a degraded savanna may actually revive
once the tree is opened up.