Recognition of Restorable Oak Savannas

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.

Air photo from 1937, showing the open savanna area (scattered trees).


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.)

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.

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:

  1. the presence of historic open-grown oaks;
  2. 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).

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.


Summary of Steve Packard's Work
Here is a brief summary of Packard’s work that I prepared for a group of interns: 1) Although the oak savanna habitat was one of the most widespread in the Midwestern United States, there are no remaining examples in pristine condition. 2) However, there are areas with original or close to original oak trees, that contain partial or degraded remnant herbaceous populations. 3) If such degraded savannas are subject to controlled burns, suppressed species in the understory are often released. 4) After burning, seeding savannas with a wide variety of species collected from nearby local areas should be done. Those that thrive are probably typical savanna species. In a sense, the restoration process itself is being used as a research endeavor, and the results help in finding out what sorts of herbaceous plants are savanna species.

What are typical savanna species? One of Packard's contributions was to discern that there were plant species that were actually unique to savannas and were absent or unimportant in prairies or oak woods. These unique savanna species may be adapted to light intensities lower than full sunlight, but considerably more than the light intensities found in oak woods. Some of these typical savanna species can be identified because they thrive under restoration conditions. Examples Packard gives of such savanna species are purple milkweed, various grasses of the genera Elymus and Bromus, upland boneset, cream gentian, broad-leaved panic grass, elm-leaved goldenrod, and Tinker's weed. Such species have been called "indicator" species, and a more detailed list was published in the 1995 Midwest Oak Savanna Conference.

Follow Link to Continuation of this Topic

Stevens, William K. 1995. Miracle Under the Oaks: the revival of nature in America. New York. Pocket Books

Leach, Mark K. and Givnish, Thomas J. 1999. Gradients in the composition, structure, and diversity of remnant oak savannas in southern Wisconsin. Ecological Mongraphs 69: 353-374..

Leach, Mark K. and Givnish, Thomas J. 1988. Identifying highly restorable savanna remnants. Transactions Wisconsin Academy of Sciences, Arts, and Letters Vol. 86, pp. 119-128.