Bur Oak

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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Oak Bullet Galls

There is a young Bur Oak (Quercus macrocarpa) growing outside the front door of Willow House. I was examining it recently to see how the acorn crop was shaping up. I did find acorns, such as that pictured above, but I also found clumps of small, hard round growths along the tree stems. Being familiar with the various and sometimes puzzling forms galls can take, I suspected I was looking at a new gall that I was unfamiliar with. Sure enough, a quick bit of research proved this suspicion to be correct.

I was surprised to learn that oaks host more types of gall-makers than any other plant. Of the nearly 1500 North American galls identified to date, more than half of them are associated with oaks. There are galls found on all parts of the oak tree: leaves, twigs, roots, flowers and even fruit. The largest number of varieties form on the leaves. Twigs come in second.

Most oak galls are formed by the Cynipid family of wasps, sometimes called gall-wasps. These cool wasps are known to have alternating generations, one generation without males, the next with both sexes. Dr. Alfred Kinsey, the sex researcher, began his career studying cynipids.

Similar galls can be formed by a variety of species. The hard, round galls that I noticed along a twig are called bullet galls. Over fifty types of bullet galls are known. Among the most common species forming bullet galls are Oak Rough Bulletgall wasps (Disholcaspis quercusmamma). The bullet galls are pictured below.

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You’re invited…

Bur Oak (Quercus macrocarpa), March 1, 2009

…to drop over to the Festival of the Trees 33. If you are a tree lover, tree hugger, or would just like to add a bit of green to your day, you’ll enjoy the festival. All kinds of trees, all kinds of places. Look for M.A. Sheehan’s sad story of the rise and fall of his urban forest, “What will the Neighbours Think?”

If you’re in the market for a good book to read, you’ll find lots of suggestions at the March update of The 2nd Canadian Book Challenge. See what others are reading and check out reviews of the books that interest you. As they say at Chapters, the world needs more Canada!

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