Tom's Blog

Monday, June 6, 2016

Bur oaks at Black Earth Rettenmund Prairie

I once saw an exhibit at a Nature Center that said: “Where does the prairie end and the savanna begin?” The answer given was: “At the first oak tree.”

There are quite a few small bur oaks at Rettenmund Prairie that have survived various fires over the past 15 or so years. I don’t know where they came from but they are thriving, and some day will cast enough shade to convert their immediate domain into an open savanna.

Saturday after the work party I took some photos of these small trees. They are found in two places: 1) At the top of the North Unit and on the hill facing north to Fesenfeld Road; and 2) on the middle of the Saddle. The photo from Google Earth shows the North Unit. Most of the green in this 2013 photo (cropped) shown here is not bur oaks, but prairie willow and hazel, but several small green “dots” are definitely bur oaks (see the photos of the larger bur oaks farther below).
Georeferenced image cropped from Oct 2013 Google Earth. This is the top of the knoll on the N unit (the footpath shows clearly). The largest bur is at the top of the hill mixed in with the prairie willow (see photo with Kathie below), and the three green “dots” in the upper right hand corner are burs on the hillside (see photo below).

There is a clone of may apple surrounding one of the bur oaks on the side of the hill (see photo below), which represents a “legacy” from when this area was heavily wooded. (See the Converse map from April 1984 below). Although may apples can hang around a long time in sunny habitats, they need fairly substantial shade to get started.

Map drawn for the Nature Conservancy by C.K. Converse in April 1984, before any restoration work had been done at Rettenmund Prairie. Note the woody patch (aspen etc.) on the top of the hill in the North Unit and the large woody patch in the Saddle (labeled by the long leader). Both of these areas have bur oaks today.
Photo taken of the North Unit in 1987 (before any restoration work had begun), 
showing the large woody vegetation in the area where small bur oaks live today. 
Although most of the trees here were aspens, bur oaks might also have been present 
and have been removed as part of the prairie restoration process.

Head-high bur oak on the lower slope of the N unit, surrounded by a clone of may apple. 
Note also the two other bur oaks farther up the slope.

Small bur oak “grub” on the North Unit. 
Presumably arisen from an acorn sometime in the past few years. 
There are no acorn-bearing trees anywhere near Rettenmund Prairie or anywhere within a mile.

Two bur oaks on the Saddle, in an area that once had a large woody patch. The large specimen is about 15 feet tall. The small one consists only of resprouts. The flame height of this year’s fire was probably less than 4 feet high, so that the tall tree, with its corky fire-resistant bark, was unaffected.

The largest bur oak on the top of the knoll in the North Unit. 
Although this area has lots of fine prairie plants, it has also been 
invaded by woody vegetation, including prairie willow, hazel, and gray dogwood.

 Bur oaks and fire: Both units where the bur oaks are present were burned this spring and are burned two out of every three years. Because the grass fuel at Rettenmund is fairly low, flame heights are not very high. The three larger trees shown in photos here would not have been affected, as shown by the fact that their bottom branches have all leafed out. If flames had been higher, the buds on these branches would have been killed. Once a tree gets as high as these three, because of their thick corky bark they can usually survive most fires. Small trees and grubs, however, are almost always killed, even by small fires, but then resprout from the base. Grubs can do that over and over, always extending their root systems, until a year may pass when the fire misses them and they get large enough to turn into “legitimate” trees.

What will happen to Rettenmund as the years go by. Will parts of it turn into a savanna?

Probably only after the three larger bur oaks get old enough to make acorns. How old must a bur oak be before it can flower and make acorns?

According to Silvics of North America, bur oaks must be 35 years old before they can make acorns. However, that age is not based on any real research. In fact, at the UW-Madison Lakeshore Preserve bur oaks that were planted in early 2000s have already made acorns.


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