Spotlight on Bur Oak by Sam Cox
Bur Oak – Quercus macrocarpa
by Sam Cox
In centuries past, long before a plow was seen upon the Great Plains, vast blankets of tall grass on the plains were broken up by pockets and strings of a particularly tough tree that specialized in invading grassland prairies. Few of the massive eastern tree species ventured west of the Missouri River out into the plains, where fire, drought, hail and wind provided little opportunity for trees to take hold. Among the eastern forest monarchs, bur oak is not the tallest or the most colorful, it doesn’t have the widest canopy or the thickest-trunk, it doesn’t produce commercially-desirable nuts, nor is it a major timber species; but more than any other tree from the vast eastern forests that stretched from New England into the Great Lakes region, it is perhaps the toughest.
The toughness of bur oak manifests physically in thick, leathery leaves that resist desiccating prairie winds; in thick, corky bark that shrugs off pelting hailstones as well as simultaneously protecting the tree from fire and frost-cracks; in cold-tolerance that exceeds any other North American oak; and in a root system deep and fibrous enough to outcompete native grasses for water though long, dry summers. No other American oak can match the drought tolerance of bur oak, thanks in part to a deep taproot whose first-year growth has been measured at 4.5 feet. All of these traits allow bur oak to thrive not only in Ohio, Kentucky, Indiana and Illinois, but beyond the Mississippi River into Missouri, Iowa and Minnesota and further still beyond the Missouri River, across the grasslands of the Dakotas, Nebraska and Kansas, even into the Black Hills of South Dakota and the Bear Lodge Mountains of Wyoming, down through the baked plains of central Oklahoma and the Hill Country of Texas to the Gulf of Mexico. The bur oak’s natural range spans 25 degrees of latitude and 7 USDA growing zones, more than any other American oak.
Bur oak is an instantly-recognizable tree. It’s grand, rounded form strikes a spectacular silhouette on the landscape. Like all oaks, it produces acorns. Bur oak derives its common name from the thick growth of burs along the acorn cap, and it derives its scientific name, Quercus macrocarpa (literally: Big-seeded oak), from the massive 2-inch diameter acorns that utterly dwarf acorns from any other oak. Bur oak acorns are on the menu for all types of forest wildlife, including bears, squirrels, turkeys, ducks, rabbits and rodents. Native Americans consumed the acorns, but only after roasting or boiling them to remove the bitter taste. Bur oak bark is grey, rough and corky from a very early age, and this gnarled bark gives bur oak a lot of winter interest, in addition to excellent protection from the elements. The leaves are 8-10” long with deep sinuses, extending almost to the leaf midvein, separating rounded lobes. Leaves are deep green and thick, glossy above and rough underneath. While forest cohorts burst forth in reds, purples and orange in fall, bur oak enters autumn quietly and humbly, with leaves turning pale yellow to brown before dropping off. Bur oak trees can grow to heights of 100 feet in excellent forest growing conditions, but more typically reach 60-80 feet tall through most of their natural range. In western parts of the range, it is rare for a bur oak to grow taller than 40 feet. In fact, many attributes of bur oak vary dramatically with geography.
Many tree species vary in physical characteristics somewhat with geographic location, but Bur Oak takes this to an extreme. Bur oak acorns from Texas are commonly 2.5 inches in diameter, whereas those from northern Wyoming rarely exceed ½ inch. Leaves grow twice as large on trees located further south and east, and the trees themselves grow almost twice as tall. In fact, geographic differences in bur oaks have led some authorities to classify the western bur oak as its own species: Quercus mandanensis, a hybrid of bur oak and gambel oak that occurred in the distant past when these two species overlapped in range. Other authorities, including the US Forest Service, strike a middle ground of recognizing two distinct varieties of bur oak: var. macrocarpa, present throughout most of the range and characterized by large acorns, large leaves and large tree height, and var. depressa, present in the western plains and mountains, characterized by shrubby growth, smaller leaves, and small acorns with less fringe. The classification of bur oak is even further muddled by its propensity to freely hybridize with 7 other species of white oaks, leading to individual trees that exhibit physical characteristics of bur oak and white oak, or bur oak and English oak, among others. Taxonomists have a tough time reconciling bur oak with the classical species definition, but then that is true with all oaks, as well as many other tree species.
In eastern parts of its range, bur oaks achieve massive proportions. The McBaine bur oak near Columbia, Missouri is that state’s largest specimen. At 400 years old, it is 90 feet tall, with a 130-foot canopy spread and a 24-foot trunk circumference. The Kentucky state champion bur oak is 104 feet tall, with a 93-foot canopy spread and a 24-foot trunk circumference. But the largest bur oak anywhere is a 500-year old giant in Posey, Indiana, at 99 feet tall, with a 128-foot canopy spread and a 25-foot trunk circumference.
Closer to home, Beverly and Don Kurtz own the house in Cody where Don’s grandfather planted a bur oak seedling that came from Illinois in a bucket around 1904. Today that bur oak is 65 feet tall, with a 66-foot canopy and a trunk circumference of 7.5 feet. This tree held the Wyoming state champion bur oak title until just last year, when an even larger specimen from Crook County topped the list at 71 ft tall, 52-foot canopy spread and a 12-foot trunk circumference. Though very large, the Wyoming champion is by most metrics only half as large as its eastern counterpart. The largest bur oaks in Cheyenne sit just north of the Wyoming State Museum at Central Ave and 24th St. Each of them are well-shaped, beautiful specimens. The largest of the three is 45 feet tall, with a 51-foot canopy spread and a 7.5-foot trunk circumference.
Among Cheyenne’s urban forest, there are only a few native hardwoods grown in any abundance: cottonwood, aspen and bur oak. Aspen sprint to full size in a decade, and usually die within 25 years. Cottonwood grow fast and provide magnificent shade for a perhaps a century, but then come tumbling down as they rot from within. Every fall, early snowstorms snap cottonwood branches all over town. But bur oaks will neither shed their limbs in foul weather, nor wither and die within a human lifetime. Their strong wood withstands the snow and wind, their thick corky bark scoffs at hail, their deep roots find water even during the hottest August days, and their propensity to stay dormant through late spring, even while other trees ostentatiously bloom and leaf out, serves them well when the inevitable May snowstorm comes along and smashes trees that leafed out too soon. Additionally, bur oak grows well in the alkaline, clay soil that underlies Cheyenne and defeats so many eastern trees. In cold tolerance, it is almost unmatched, with a capability to withstand -50°F in midwinter. All of these traits keep bur oaks growing steadily and reliably, if not quickly – usually about 1-foot/year until maturity – for hundreds of years. There are few trees better suited to Cheyenne’s growing conditions, yet gratification requires patience. Someone who plants a bur oak today and laments that it may take a lifetime to reach mature size may take comfort in knowing children 20 generations from now may someday sit and enjoy a break from a summer day’s heat under its sprawling, shady canopy.
Acorns from bur oak vary dramatically in size depending on geography. The acorn on the left came from a bur oak in North Texas, whereas the one on the right came from a bur oak in northeast Wyoming. The two are so strikingly different that some have suggested the two belong to different species altogether.