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Bur Oak - Quercus macrocarpa
State Rank Reason (see State Rank above)
DRAFT: Requesting feedback on the 2026 revised rank, factors, and State Rank Reason outlined below and in the Conservation Status Rank Report.
Quercus macrocarpa is rare in Montana, being restricted to bentonite soils in extreme southeast Montana, where it reaches its western range extent. Occurrences are on private and state-managed lands. No populations are currently in protected areas or on conservation-managed lands. Some occurrences are threatened by and may have been impacted by bentonite mining in the area. Non-native grasses and forbs are common in some stands and may be negatively impacting the regeneration of the oak. Populations appear to consist of at least several thousand trees based primarily on surveys in 1992. Actual trends are unknown though populations have likely experienced at least minor declines in extent, numbers and condition.
Climate change may negatively affect this species over the next several decades. However, the timing and magnitude of potential impacts are largely unknown. Although climate change is listed as a potential threat to the species, it does not affect the overall threat rank or scoring, or the final subnational rank.
Survey and monitoring data are greatly needed to be able to more precisely determine population levels, assess condition and for assessing population trends.
General Description
Monoecious trees to 10 m with gray, scaly bark. Twigs pubescent, often with corky wings. Leaves alternate, simple, petiolate; the blades obovate, 6–15 cm long, pinnately lobed, the lobes rounded with wavy margins. Flowers unisexual with a 6-lobed perianth. Male flowers borne in a drooping, interrupted, spike-like inflorescence; perianth 6-lobed with 6 to 10 stamens. Female flowers in clusters of 1 to 3; the perianth adnate to the ovary, surrounded by a scaly, cup-like involucre; pistil 1 with 3 styles. Fruit a hardened, ovoid nut (acorn) 15–50 mm long, surrounded at the base by the hardened, cup-like, scaly and hairy involucre (
Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).
Phenology
Flowering takes place in spring, and acorns mature in one year.
Diagnostic Characteristics
Bur oak is Montana's only native oak.
Species Range
Montana Range
Range Descriptions
Native
Range Comments
In MT bur oak is restricted to the extreme southeastern corner of the state in Powder River and Carter counties(Vanderhorst et al. 1998; Lesica 2012); eastern temperate N. Am., in Canada from SK to NB, central U.S. south to TX, east to ME and AL (Nixon and Muller 1997 in Fl. N. Amer. Vol. 3).
Observations in Montana Natural Heritage Program Database
Number of Observations: 24
(Click on the following maps and charts to see full sized version)
Relative Density
Recency
(Observations spanning multiple months or years are excluded from time charts)
Habitat
Most locations are on bentonitic shale ridges trending WNW to ESE and extending into Wyoming (Vanderhorst et al. 1998). Bur oak forms solid or mixed stands on slopes, and is widely scattered in a savanna-like association on the ridgetops. It co-dominates in various proportions with Rocky Mountain juniper (Juniperus scopulorum) and ponderosa pine (Pinus ponderosa). The understory is relatively depauperate, even under full canopy cover, and there is a high diversity of annuals, including Nuttallanthus texanus, and a high proportion of exotic grasses and forbs. Openings are often dominated by the rhizomatous sedge, Carex inops (C. heliophila), with shrubs confined to the more wooded stands.
A secondary habitat is the alluvial terraces along the Thompson Creek tributary of the Little Missouri River, where bur oak is the dominant tree or is occasional in stands of green ash (Fraxinus pennsylvanica). The understory of these habitats is dominated by introduced pasture grasses, and in many places, a near monoculture of Bromus inermis.
National Vegetation Classification System Groups Associated with this Species
Forest and Woodland
Deciduous Forest and Woodland
Low Elevation - Xeric Forest and Woodland
Ecology
Bur oak is a member of the white oak group. It is a slow-growing, long-lived tree capable of living 200-300 years and it can reproduce both vegetatively and sexually. Wnd is the primary means of pollination.
Acorns mature in one year and are typically produced in abundance every two to three years. However, consumption of acorns by wildlife can significantly reduce numbers available for germination. Leaves of bur oak may be used as forage by cattle and mule deer and are most vulnerable to herbivory after they emerge, when the concentration of tannins and phenols is lowest. Galls are common and leaf-mining occasional; heart rot was noted in one tree cored along the Little Missouri River and suspected in trees on the ridge slope.
Fire historically played an important role in maintaining stands of Quercus macrocarpa throughout its geographic range (Abrams 1992). Bur oak will resprout from a damaged trunk. Prescribed fire was found to increase the number of bur oak sprouts in stands in the Black Hills but did not increase the density of seedlings (Sieg and Wright 1996). Minimum seed-bearing age for bur oak has been reported to be 35 years.
Management
The "Alzada Oaks" constitute Montana's only known stand of bur oak, and the stand lies at the westernmost edge of the species' natural range. They have been affected by mining, associated road construction and maintenance, and grazing (Vanderhorst et al. 1998). The long history of grazing by sheep and horses and the harsh environment at the site favor high numbers of introduced species and a relatively impoverished native flora (Heidel 1993). Competition from weeds may limit the establishment and growth of bur oak. Bur oak provides forage for cattle and mule deer though effects of grazing on the the Montana population are not known. Fire may have played a role in maintaining Q. macrocarpa by reducing competition from non-stump sprouters such as ponderosa pine and juniper, and suppression of wildfire may be having an adverse effect on Q. macrocarpa recruitment.
Stewardship Responsibility
Threats or Limiting Factors
STATE THREAT SCORE REASON
Reported threats to Montana's populations of Bur Oak are due to ongoing bentonite mining in the vicinity of most populations. Bur Oak populations at the extreme edge of its range are clustered where demand for bentonite is also concentrated. Thus, negative impacts to this very limited habitat are potentially severe (MTNHP Threat Assessment 2021).
References
- Literature Cited AboveLegend:
View Online Publication- Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
MTNHP Threat Assessment. 2021. State Threat Score Assignment and Assessment of Reported Threats from 2006 to 2021 for State-listed Vascular Plants. Botany Program, Montana Natural Heritage Program, Helena, Montana.
- Additional ReferencesLegend: View Online Publication
Do you know of a citation we're missing?- Abrams, M. D. 1992. Fire and the development of oak forests. BioScience 42(5):346-353.
- Girard, M. M., H. Goetz, and A. J. Bjugstad. 1989. Native woodland habitat types of southwestern North Dakota. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station. Research Paper RM-281. Fort Collins, CO. 36 pp.
- Heidel, B.L. 1993. Survey for Quercus macrocarpa in the Powder River Resource Area, Miles City District, Bureau of Land Management, Montana. Unpublished report to the Bureau of Land Management, Miles City District. Montana Natural Heritage Program, Helena, MT. 13 pp. plus appendices.
- Lesica, P., M.T. Lavin, and P.F. Stickney. 2022. Manual of Montana Vascular Plants, Second Edition. Fort Worth, TX: BRIT Press. viii + 779 p.
- MacCracken, J.G. and D.W. Uresk. 1984. Big game habitat use in southeastern Montana. The Prairie Naturalist 16(3):135-139.
- Sieg, C. H. 1991. Ecology of bur oak woodlands in the foothills of the Black Hills, South Dakota. Ph.D. dissertation, Department of Range and Wildlife Management, Texas Tech University.
- Sieg, C. H. and H. A. Wright. 1996. The role of prescribed burning in regenerating Quercus macrocarpa and associated woody plants in stringer woodlands in the Black Hills, South Dakota. International Journal of Wildlands Fire 6: 21-29.
- Vanderhorst, J.P., S.V. Cooper, and B.L. Heidel. 1998. Botanical and vegetation survey of Carter County, Montana. Unpublished report prepared for the Bureau of Land Management. Montana Natural Heritage Program, Helena. 116 pp. + app.
- Web Search Engines for Articles on "Bur Oak"
