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).

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.