Rare Species Guide

  Basis for Listing

Quercus bicolor (swamp white oak) is now rare in Minnesota and is apparently restricted to lowland forests on the floodplain of the Mississippi River. Historically, it occurred as far upstream as the Twin Cities area, but it has not been found above Frontenac (Goodhue County) for many years. Most of its habitat was lost early in the twentieth century, when a series of 8 locks and dams were completed on the River. They turned the Mississippi from a free-flowing river into a series of interconnected navigation pools. The only original forests that survived intact were immediately downstream from the dams, where the water levels remained more or less natural. This is predominately where Q. bicolor occurs today (Blufflands Subsection). Given the very small number and size of remaining populations in Minnesota, the limited amount of potential habitat, and the concerns over natural regeneration, Q. bicolor was listed as a species of special concern in 2013.

There have been recurring reports of Q. bicolor in the Minnesota River Valley as far upstream as Granite Falls (Yellow Medicine County) and from scattered locations elsewhere in central Minnesota (Little 1971; Ownbey and Morley 1991). Most of these reports have been traced to misidentified herbarium specimens of Q. macrocarpa (bur oak). Although plausible, there is no convincing evidence that Q. bicolor has ever occurred naturally in the Minnesota River Valley (Smith 2008).

  Description

Quercus bicolor is a large tree, up to 24 m (80 ft.) tall and 70 cm (28 in.) dbh (diameter at breast height). The bark is gray or brownish gray, with thick flat-topped or scaly ridges separated by deep furrows. The leaves are 10-19 cm (4.0-7.5 in.) long and 7-11 cm (3.0-4.3 in.) wide, with 3-9 small to midsize rounded or blunt lobes; the deepest sinuses usually extending 15-50% of the distance to the midvein. The flowers are unisexual, with male flowers and female flowers borne separately on the same branch. The male flowers are in slender pendulous catkins, 2-8 cm (0.8-3.0 in.) long, and the female flowers are on a stiff peduncle. The fruit (acorn) is a broadly ovoid or ellipsoid nut, 1.7-2.4 cm (0.7-0.9 in.) long, and 1.3-1.7 cm (0.5-0.7 in.) wide, with a scaly dome-shaped cup at the base (Smith 2008).

Quercus bicolor is most likely to be confused with Q. macrocarpa, but the leaves of Q. bicolor have smaller lobes, and the lobes are more uniform in size and shape. Also, the acorns of Q. bicolor are on a long slender stalk (peduncle). The two species sometimes grow side by side, and they hybridize freely (Bray 1960).

  Habitat

Despite its common name (swamp white oak), Q. bicolor does not actually occur in swamps, at least not in Minnesota. Instead, it occurs in floodplain forests on the floodplain of the Mississippi River. Quercus bicolor typically occurs mixed with Populus deltoides subsp. monilifera (plains cottonwood), Acer saccharinum (silver maple), Betula nigra (river birch), or sometimes Q. macrocarpa.

  Biology / Life History

Quercus bicolor is a slow-growing and relatively long-lived (300+ years) canopy tree. It is only known to occur in a handful of widely separated sites along the Mississippi River, and there is a concern over whether these populations are regenerating. These habitats are typically flooded in the early spring, when snowmelt and rainfall cause the river level to rise. Flooding typically lasts only a few weeks and usually subsides before the growing season begins. The shallow root system of Q. bicolor is particularly well adapted to survive this type of flooding and accompanying sedimentation.

  Conservation / Management

Since loss and degradation of habitat is the main cause of Q. bicolor decline, recovery of the species may depend on improving and increasing appropriate habitat as well as preserving existing habitat. Preserving existing habitat may be relatively simple, at least from an ecological perspective; however, recreating appropriate habitat or augmenting existing habitat presents unique challenges. Certainly there is land on the floodplain of the Mississippi River in Houston, Winona, and Wabasha counties that has been cleared in the past and used for agricultural purposes (usually unsuccessfully), which could be restored to forested native plants. Some land restored in this manner could, eventually, be able to support reproducing populations of Q. bicolor. However, it is notoriously difficult to reestablish forest trees on agricultural land. Agricultural practices alter the soil environment to such an extent that tree growth is greatly impaired. Competition from invasive plant species, especially Phalaris arundinacea (reed canary grass), will also be a challenge. So any efforts at habitat restoration would require decades of active management and a significant commitment of resources.

  Best Time to Search

Without acorns, it can be very difficult for a non-specialist to distinguish Q. bicolor from the more common Q. macrocarpa. Mature acorns can be found on the tree from about the middle of August to late September. Shed acorns (or the acorn cups) can usually be found on the ground beneath the tree for up to about a year after they were shed. Be aware that oak trees will not produce acorns every year (Smith 2008).

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Bray, J. R. 1960. A note on hybridization between Quercus macrocarpa Michx. and Quercus bicolor Willd. in Wisconsin. Canadian Journal of Botany 38(5):701-704.

Little, E.L., Jr. 1971. Atlas of the United States trees: conifers and important hardwoods. Volume 1. Miscellaneous Publication No. 1146. U.S. Forest Service, Washington, D.C. 200 pp.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.

Nixon, K. C. 1993. Quercus. Pages 445-506 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 3. Oxford University Press, New York, New York.

Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis. 320 pp.

Smith, W. R. 2008. Trees and shrubs of Minnesota. University of Minnesota Press, Minneapolis. 703 pp.