One of the most
spiriting accounts of oak savanna restoration is that of Steve
Packard, for sites in northeastern Illinois. Packard’s
work has been described briefly in his publications and in
more detail in William K. Stevens book,
Miracle Under the Oaks. These accounts should be read
by anyone interested in oak savanna restoration.
| The
term “tallgrass savanna” should be replaced by the term
“oak savanna”, which places the focus on the critical
component, the open-grown oaks. |
Intact, functioning,
oak savannas are very rare. The main purpose of this web site
is to lay out the principles of oak savanna restoration. The
question first arises: Can an oak savanna be established “from
scratch”? That is, with neither the open-grown oaks nor the
understory vegetation present? From bare ground? The answer
is NO.
Why? Planting oaks is a meritorious undertaking, but one does
not plant oaks for oneself, but for future generations. The
ages of open-grown oaks in functioning savannas will be most
likely over 100 years old, with some trees approaching 200
years in age. Obviously we will be gone before any oak we
plant will become a functioning part of a savanna. This is
not to say that oaks should not be planted, but that planting
oaks is not restoring an oak savanna. If motivated to plant
oaks, try to select sites that are likely to still be here
100 years from now.
Without the presence of open-grown oaks, restoring an oak
savanna is not possible. However, there are numerous sites
across the Midwest with large, older, open-grown oaks, making
them restorable oak savannas. How do we find them?
The
Importance of Air Photography
Air photography is of great value in selecting a site. Here
we are not thinking about recent air photos, but the oldest
images available. Air photography was introduced in the United
States in the mid 1930s by the Soil Conservation Service,
and large parts of the Midwest were “flown” at that time.
States will vary in the extent of availability of air photos,
and within states some counties will have more photos available
than others. An index of available photos should be available
from some agency in the state, perhaps a State Cartographer’s
office, a State Historical Society, a County or Regional Planning
Agency, or a Department of Natural Resources. The U.S. Geological
Survey or a state Geological Survey are other possible sources.
The best older photographs were taken by the Soil Conservation
Service and should be available from the National
Archive and Records Administration, whose headquarters for
this sort of material is currently in College Park, Maryland.
More recent air photos are available from the
National Aerial Photography Program, which operates under
the U.S. Geological Service. Although newer, this photography
is often useful when putting together a time series for a
site. Another useful source of air photos is the current manifestation
of the Soil
Conservation Service called the Natural Resources and Conservation
Service, which has regional offices in each state.
Old photographs are so useful in planning and executing a
savanna restoration that all effort should be expended on
locating them for the area of interest. In virtually all cases,
the images must be purchased, but they are well worth the
cost.
Although many air photos may be available via the Internet,
it will only be in rare instances when older ones are available
this way. Also, the resolution of images downloaded from the
Internet will vary widely, and many will be only of low quality.
Obtain images of the highest resolution possible.
The 1937 year was the first year that SCS did air photographs,
and the quality is fairly good. These old photos are available
as 9 X 9 inch contact prints, and if a hand lens or dissecting
microscope is used, great detail can be seen.
The quality of available photos varies considerably from year
to year, probably because of atmospheric conditions when a
given area was “flown”. Photos from all the early years are
in black and white, and although some later years are in color,
the quality is not necessarily better. Many of the recent
color air photos were taken by the U.S.D.A. Farm Service Agency
primarily to measure crop acreages and are of fairly low resolution.
In some areas, high resolution digital photos are now being
taken.
An example of a typical "time series" for a southern
Wisconsin State Natural Area is shown below. The first photo
below shows clearly the savanna areas of a site as they were
at a time when annual burns were being carried out by the farmer.
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Air
photo from 1937, showing the open savanna area (scattered
trees).
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Air
photo of the same area from 1990, showing how over the
years the area had become heavily wooded. The woody
growth consisted of both "weed" trees and
invasive exotic shrubs. Note also that the planted fields can still be seen. (Original color air photo converted
to grayscale.)
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| Air
photo of the same property in 2007, after extensive savanna
restoration. Many of the large trees seen in the 1937
photo are still present and can be identified. The planted fields are still visible, although now they have all been planted to prairie. (Original
color photo converted to grayscale.) |
Erroneous Definitions of Savanna
If one begins with an incorrect definition of savanna, one
ends up spending lots of time and money on a restoration that
may fail. Early settlers descriptions of oak openings as grassy,
park-like areas have often led later workers to conclude that
“grass” is an essential part of the savanna definition. For
instance: “Savanna is defined as any grassland in which there
are open-grown trees of scattered distribution. A degraded
forest is thus not a savanna, nor is the ecotone between taiga
and tundra, since the ground cover is not primarily grassy.”
WRONG!
Whether or not a savanna is a grassland will depend upon the
spacing of the oaks. If they are very wide apart (such as
the 1 tree per acre sometimes used as the limiting definition
of a savanna), then one would expect this to be a potential
grassland. However, if the oaks are more closely spaced (such
as the 50% canopy often used to define the upper limit of
a savanna), then there will probably not be a continuous grassland,
but “patches” of grass in the more open areas, and a variety
of forbs or cool season (savanna) grasses in the less open
areas.
Another problem is determining the boundaries of the savanna.
As the diagram below suggests, savannas are heterogeneous
and vary widely in percent canopy cover.
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| Example
of the difficulty of defining a savanna based on the canopy
cover. The whole rectangle A represents
a savanna with about 30% canopy cover. However, areas
B and C have canopy covers of less than 10% and area D
has a coverage of greater than 50%. Obviously, the use
of percent canopy as an objective criterion is questionable.
Diagram adapted from Leach and Givnish (1999). |
A savanna is a vegetation complex, with many different light
regimes, some favorable to prairie grasses
and forbs and others favorable to savanna
grasses and forbs. In much of the early literature, although
often not specifically stated, the term “grassland” referred
to prairie grasses, such as big or little bluestem or Indian
grass.
The development of individual plant species is influenced
primarily by available light and soil conditions (especially
whether the soil is sandy or loamy). These two environmental
factors affect available soil moisture, and it is that factor
that most affects the distribution of plant species in an
oak savanna. Extensive research on the interaction between
soil conditions and available light for southern Wisconsin
savannas has been published by Leach and
Givnish (1999) . They state: “Because forb cover and species
richness exceed those of graminoids in most savanna microsites,
it might be more appropriate to describe Midwestern oak savannas
as forblands rather than grasslands.”
If a savanna is not a grassland, how does one select an appropriate
site for restoration? Leach and Givnish
(1998) have analyzed criteria that have been used by various
workers to select sites for restoration work. They state:
“…thousands of acres of highly restorable oak savannas have
been overlooked because of flawed ideas regarding their structure
and composition. Commonly, savannas are defined as having
a specified “percent canopy” and a prairie-like groundlayer.
Percent canopy is a flawed indicator of restorable oak savanna
because it does not account for canopy dynamics nor spatial
heterogeneity and because, by itself, percent canopy is a
poor measure of light penetration to the groundlayer—the home
of most plant species. Likewise, the presence of prairie-like
groundlayers is not a good indicator of species-rich savanna
remnants, especially on more productive soils. We suggest
two elements that are both characteristic and diagnostic of
highly restorable oak savannas:
- the presence
of historic open-grown oaks;
- a groundlayer
vegetation rich in native plant species in both sunny and
shadier locations.”
Leach and Givnish (1988) emphasized
that many sites with well established native groundlayer plants
lacked the presence of warm-season grasses and other characteristic
prairie plants. They suggested that rather than look for prairie
grasses, one should look for sites rich in species that are
characteristic of savannas (savanna specialists). See
link for a discussion of such savanna “indicator” species.
Must a restorable savanna contain presettlement vegetation?
No. The original settlement in the Midwest occurred in the
period from after the Revolutionary War (1776) to the mid
1850s, thus from 150 to 230 years ago. However, many trees
that began growth in the late 19th to early 20th century on
savanna sites should be quite large today. If they exhibit
open-grown characteristics, than they probably grew in savanna-like
conditions, even if none of the original understory vegetation
is present any more.
It is likely that in many areas of the Midwest, especially
in hill country, oak savannas were grazed from the time of
settlement until the 1950s, when farming practices changed
and farmers no longer burned their pastures. Before the 1950s,
grazed savannas were almost certainly burned every spring,
since farmers recognized that burning speeded “green up.”
Between burning and grazing, very little oak reproduction
remained, but the original open-grown oaks remained. In some
areas, grazing of oak savannas still occurs (see photo below).
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| Former
bur oak savanna that is still being pastured for beef
cattle. None of the original savanna groundlayer (understiry) is left,
but the open-grown bur oaks are intact. Fire, use of herbicide,
and planting with savanna understory species would be expected to
eventually restore this site to oak savanna. |
One of the most prominent oak savanna restorationists, Steve
Packard, has frequently used the term “tallgrass savanna.”
From what has been discussed in this section,this term is
misleading, as it focuses on grass. Despite his use of “tallgrass
savanna”, even in Packard’s work, grass is not the major component,
and most of the plant species on his lists are not grasses.
The term “tallgrass savanna” should be replaced by the term
“oak savanna”, which places the focus on the critical component,
the open-grown oaks.
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