According to numerous reports (
reviewed in this publication), bur
oak (
Quercus macrocarpa) populations have decreased dramatically since European
settlement, and conservation and restoration of this species has become a
management priority in many areas. According to the U.S. Forest Service, in the
early 1900s, bur oak savannas occupied 32,000,000 acres (13 million hectares)
in the Midwest U.S., of which only about 6,400 acres (2,600 ha) remain. Loss is
due primarily due to agricultural development, but fire exclusion is another
important factor.
The seriousness of the loss of this important species has
motivated some parks and natural areas to carry out programs with school
children and others to collect acorns and raise oaks for planting. This is a
meritorious endeavor although not likely to produce ancient oaks in the foreseeable future.
However, it seems to me that there is another approach that
holds promise.
There are extensive areas in the Midwest where venerable bur
oaks already exist. Observations of wooded hillsides in some areas often reveal
the presence of large bur oaks that are hidden among extensive invasive brush
and trees. These trees,
often called “wolf trees”, can often be seen through
the dense shrubbery, especially in the winter when leaves are absent and snow
provides a contrasting background. Although these open-grown bur oaks are probably
in weakened condition due to shading, it is possible that many might still be
saved.
Pleasant Valley Conservancy (PVC) can provide an object
lesson of what can be done.
When restoration began at PVC about 20 years ago, we knew
that hidden among all the invasive brush and trees were many large bur oaks,
especially on the upper slopes and ridge tops. Many of these were large open-grown
trees and subsequent age determination showed that many were over 200 years
old.
In the 1937 air photo shown here most of the large scattered trees are bur oaks (see later photo below).
 |
| Early air photo of the area that after restoration became Pleasant Valley Conservancy |
As the years went by in the absence of fire, this landscape filled in with invasive trees and brush, as the air photo from 1990 shows. However, the large bur oaks are still present, but hidden from view.
 |
| Air photo from 1990 showing the same area, before restoration work had begun. |
After some years of restoration work, the habitat was opened up, permitting the existing bur oaks to flourish. The 2013 air photo shows the result.
 |
| Air photo from 2013, after restoration was complete. |
When restoration work began, the goal was to remove all
woody plants, both shrubs and trees, that were crowding open-grown oaks. Many
of the open-grown oaks, especially on the ridge top and south-facing slope,
were either bur or white oaks. Over a six-year period, about 60 acres of woody
areas were restored. All invasive woody plants and trees were removed. Trees
removed included all fire-sensitive species such as black walnut, slippery elm,
black cherry, aspen, box elder, red maple, and basswood. Even red, black, and
Hill’s oaks were removed if they were crowding open-grown bur or white oaks.
Once the restoration work was completed, a census of all
trees over 10” in diameter was made. Those trees greater than 20 inches in diameter would be trees that were present at the time of the 1937 air photo. There were 251 bur oaks and 111 white oaks that were greater than 20 inches in diameter.
The table below provides the bur oak data.
Size classes; inches diameter
|
Number of bur oaks
|
10-14.9
|
183
|
15-19.9
|
223
|
20-24.9
|
146
|
25-29.9
|
76
|
30-34.9
|
22
|
35-39.9
|
5
|
>40
|
2
|
Using GIS, a map was prepared that overlays the >20 inch
oaks onto a georeferenced version of the 1937 air photo.
 |
| GIS map showing the 2013 distribution of large bur and white oaks overlaying the 1937 air photo |
Note that the large bur oaks overlay quite well the 1937 trees (within the accuracy of the GPS device used). It is evident that
most of the large oaks are superimposed on top of the black tree patches in the
1937 air photo.
It can thus be concluded that most of the large oaks that were
present in 1937 are still present today. The ages of some of these venerable oaks were determined by the U.W. Platteville TREES laboratory by dendrochronology. The oldest living tree has a start date of 1736 (280 years old in 2016).
The map shows that bur oaks are mainly in the prairie border and savanna areas of the Conservancy (see yellow dots on the map). The largest area, which includes the major ridge-top savanna and upper part of the south-facing slope, was 18 acres. The smaller area, which includes the basin savanna, was 4 acres. (There are also bur oaks scattered within the North Woods and the smaller woodlands on the far east side of the Conservancy.)
It is likely that PVC is not an isolated example, and that
there are numerous other sites in in the Midwest where ancient bur oaks still
survive. They may be heavily overgrown and struggling, but the bur oak is a
remarkably resilient tree, and with care may still be saved.
Using automobile surveys, venerable bur oaks may well be
located today by casual observation in heavily wooded areas. Once such areas
are found, the historic air photo of that area can be used to confirm the site,
and then a detailed “on-the-ground” visit made.
It is recommended that using historic air photos (in the
public domain and available on-line in Wisconsin or from the U.S. National
Archives), potential areas for bur oak restoration should be located and
visited today. Look for the presence of open-grown oaks on south-facing slopes,
most of which will probably be bur oaks. Visit the same locations now. Because
an identified area now is most likely unlogged and hence thickly wooded, the
open-grown oaks may still be visible within the thicket. The best time to do an
on-the-ground survey is in the winter with snow on the ground.
Note that recent
air photos will not reveal the presence of these ancient bur oaks (unless the area
has been restored) because of the heavy tree growth. One must use the oldest
available air photos. Fortunately, the air photos taken in 1937-1941 by the
Soil Conservation Service are now available free of charge (in Wisconsin from
the State Cartographer’s Office. Access the Wisconsin Historic Aerial ImageryFinder.
