Archive for December 18th, 2009

The Bur Oak (Quercus macrocarpa) that stands just south of the house is attractive in all seasons, but it is in the winter that its hidden character is revealed. When all its leaves have fallen, its branches stand out starkly, revealing the bur oak’s typical ridged, corky bark and gnarled profile.

The range of the bur oak includes the prairies south from Manitoba and west to Texas, where it is a savannah tree. A savannah is a grassland with widely-spaced trees. The bur oak’s thick bark enables the tree to resist grassland fires, keeping trees as young as 15 years old safe from flames. As a prairie tree, in addition to producing acorns that are a food source, the bur oak is important as cover and nesting habitat for wildlife.

Oaks are divided into two general categories: white oaks and red oaks. Red oaks have leaves with pointed tips and their acorns take two years to mature. The leaves of white oaks have rounded lobes and their acorns mature in one growing season. You can see from its rounded leaves that the bur oak is placed with the white oaks.

The bur oak derives its name from its acorns, which have a deep cap with a fringed, or burred edge. The acorns, like those of other oaks, are prized by a range of wildlife, including black bears, deer, raccoons, squirrels and a variety of birds including blue jays, wood ducks, nuthatches, woodpeckers and grouse. The large acorns contain about half the tannin of those of the red oaks, and are thus enjoyed by a wider range of wildlife. An abundant crop of acorns, called the mast, is produced every two or three years.

The oak is utilized by hundreds of different species of insects. There are about 50 species of leaf miners that feed on oak leaves. In early summer, round holes in oak leaves may be a sign that Junebugs (Phyllophaga spp) have been feeding on the leaves at night. Walkingsticks (Diapheromera femorata) eat entire leaves except for the main veins, working their way in from the edge of the leaf. Many types and shapes of galls are found on oaks. Of about 800 gall-makers on oaks, most are from the Cynipidae family of wasps. For more about oak bullet galls, see the October 22 post.

With its long tap root and deep lateral roots, the bur oak is securely anchored and is rarely blown over. It is a long lived tree, with a lifespan of 200 to 300 years.

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