- Scientific name: Quercus macrocarpa Michx.
- Species of Greatest Conservation Need (MA State Wildlife Action Plan)
- Special Concern (MA Endangered Species Act)
Description
Bur oak, a member of the beech family (Fagaceae) is a deciduous tree with alternately arranged leaves, twigs, and branches. It grows to a maximum height of about 30 m (100 feet) and has gray, scaly or flat-ridged bark. The twigs are finely pubescent and often corky-winged. The leaves are usually fiddle-shaped in outline, dark green above and pale pubescent below, with 4 to7 rounded lobes on each side of the midrib, and the tips do not have bristles. The center or near-center pair of lobes is cut nearly to the midrib. Staminate flowers are born on small, pendant catkins. The pistillate flowers are solitary to few and are in the leaf axils. Acorns are produced singly or in clusters of up to three. The acorns may lack an attaching stalk or may be short-peduncled (on a stalk less than 25 mm or about 1 inch long). The cup holding the acorn encloses one half to nearly the entire nut, and bears on its margins an array of soft awns 5-10 mm (up to about four-tenths of an inch) long, which form a prominent fringe along the bottom of the cup. The acorns can be large, up to 5.1 cm in length, though the fruits are generally much smaller in Massachusetts. They ripen in one growing season and seldom persist on the twig past November.
The genus Quercus in Massachusetts includes a large number of species. Among the most common native species are chestnut, white, red, black, scarlet, and swamp white oaks. Naturalists familiar with those and perhaps other less common oaks can distinguish bur oak from them by its combination of: 1) occasionally corky twigs; 2) fiddle-shaped leaves with their high “waist;” and 3) a fringed acorn cup.
White oak (Quercus alba) leaves are highly variable and may occasionally take on a fiddle shape. White oak acorns, however, are not fringed. Swamp white oak (Quercus bicolor) leaves are less deeply lobed than bur oak leaves, the branches have peeling bark, the acorns are borne on long stalks, and the acorn caps are fringeless. Hybrids between bur oak and swamp white oak are known in Massachusetts.
Life cycle and behavior
Bur oak flowers in late May shortly after the leaves appear. Acorns mature in one growing season and drop from the tree from August through November. Like other members of the white oak group, acorns germinate shortly after dropping and require no cold stratification. Seed leaves or cotyledons remain below ground upon germination.
Population status
Bur oak is listed under the Massachusetts Endangered Species Act as a species of special concern. All listed species are legally protected from killing, collection, possession, or sale, and from activities that would destroy habitat and thus directly or indirectly cause mortality or disrupt critical behaviors. Nineteen populations of various degrees of vigor and both with and without reproduction have been recorded in Massachusetts since 1999. An additional 7 populations are considered historical as they haven't been observed in the past 25 years.
Distribution and abundance
Bur oak is known over most of eastern North America, from New Brunswick and Quebec to Alberta and south to North Carolina and Texas. It is considered uncommon in many states at the edge of its range, but secure in the middle. In New England, it is critically imperiled in New Hampshire, vulnerable in Connecticut and Massachusetts, and apparently secure in Vermont. Maine and Rhode Island have not assessed it.
Habitat
Bur oak is widespread and common in some parts of North America, where it is found in bottomlands, on well-watered sites, and on less wet upland sites. In Massachusetts, it is most often found in wet locations, and often in calcareous or limestone-influenced soils. Such locations include fen-like seepage swamps; forested swamps, floodplains, and seepage slopes; and wet meadows and moist uplands.
Healthy habitats are vital for supporting native wildlife and plants. Explore habitats and learn about conservation and restoration in Massachusetts.
Threats
Invasion by exotic species (both plant and animal), diseases, and logging are current or potential threats to all native tree species and bur oak populations in Massachusetts are no exception. Since many Massachusetts bur oak populations are wetlands, beaver and the hydrologic changes they bring may be considered a potential threat. More positively, some studies indicate that reproductive isolation (such as might occur in the fragmented populations of bur oak in Massachusetts) and a resulting loss of genetic diversity may not be a major concern for wind-pollinated trees. High deer densities can reduce the number of seedlings dramatically, thereby limiting recruitment to the sapling and canopy stages. Competition by other species, such as red maple, can also reduce long-term recruitment, particularly on wet and mesic sites.
Conservation
Bur oak may be attacked by several insects such as spongy moth (Lymantria dispar) and redhumped oakworm (Symmerista canicosta) in the northeast, and other insects in other parts of the range. Oak wilt (Ceratocystis fagacearum) has damaged bur oaks in the Midwest, though bur oaks are more tolerant of attack than red oaks. Bur oak is moderately shade tolerant and may be replaced by more shade-tolerant species (e.g., red maple) on mesic sites. They are tolerant of a wide range of soil conditions and are relatively easy to propagate. As with all oaks, the key is to have sufficient numbers of seedlings and saplings to replace canopy oaks over time. This requires some reduction in competitors and opening of the canopy using mechanical means or periodic fire. High deer densities reduce seedling numbers in oaks. There is some question whether the bur oaks in Massachusetts are hybridizing with other white oaks. Research into the genetic make-up of these trees is ongoing.
References
Johnson, P.S. 1990. Bur Oak. In Silvics of North America. U.S. Department of Agriculture, U.S. Forest Service Agricultural Handbook 654. Online: http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_ of_contents.htm. Accessed January 17, 2010.
NatureServe. 2025. NatureServe Network Biodiversity Location Data accessed through NatureServe Explorer [web application]. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org/. Accessed: 5/27/2025.
POWO (2025). Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Published on the Internet; https://powo.science.kew.org/ Accessed: 5/27/2025.
Tirmenstein, D. 1988. Quercus macrocarpa. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Online: http://www.fs.fed.us/database/feis/ [2010, January 11].
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| Date published: | May 21, 2025 |
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