This is the tree that is most associated with the savannas of our area. The thick, corky bark of mature trees insulated the living tissues within from the grass fires that frequently swept the savanna. For further protection from fire the bur oak leafs out fairly late--usually after the spring fire season--compared to other trees, thus protecting vulnerable leaves from heat and smoke damage. Other tree species, being less well adapted to fire, are often excluded from fire dominated ecosystems such as grasslands.
These “open-grown” bur oaks may grow 50-80 feet tall and have thick trunks with widely-spreading, often crooked branches. The acorn is large and has a “burry” cap, from which this oak gets its name. The leaf has rounded tips on its lobes, but with a deeply incised “waist,” a characteristic it doesn’t share with other oaks in our area. The bark is gray and deeply furrowed in older trees. Twigs are both thick and blunt compared to many other trees.
Savanna is among our rarest plant communities, yet they are among one of our most popular places to visit. City parks, essentially grasslands with widely spaced trees, are modeleds after savannas.
In much of the Midwest, savannas are also called “oak openings,” as there are many small, grassy openings among the groves of oak trees.
As the Mississippi River corridor was settled in our area, grass fires became less frequent and fire intolerant trees grew up around the bur oaks. These lesser trees block light to the lower branches of the bur oaks. When branches no longer have access to light, the tree often “self-prunes” letting those useless branches die so valuable resources can be used on those branches that still have access to sunlight. Look for open grown bur oaks with dead lower branches that were once located in what was savanna, but is now forest.
Did You Know?
At the headwaters of the Mississippi, the average surface speed of the water is 1.2 miles per hour. People typically walk 3 miles per hour.