MONTANA TREE ADVENTURE

MONTANA TREE ADVENTURE "ALZADA OAKS"

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.

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.

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.

Citation for data on this website:

Bur Oak — Quercus macrocarpa. Montana Field Guide. Montana Natural Heritage Program. Retrieved on February 26, 2023, from https://FieldGuide.mt.gov/speciesDetail.aspx?elcode=PDFAG05190