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Quercus taxonomy
Quercus macrocarpa Michx.
EOL Text
Trees , deciduous, to 30(-50) m. Bark dark gray, scaly or flat-ridged. Twigs grayish or reddish, 2-4 mm diam., often forming extensive flat, radiating, corky wings, finely pubescent. Buds 2-5(-6) mm, glabrous. Leaves: petiole (6-)15-25(-30) mm. Leaf blade obovate to narrowly elliptic or narrowly obovate, often fiddle-shaped, (50-)70-150(-310) × (40-)50-130(-160) mm, base rounded to cuneate, margins moderately to deeply lobed, toothed, deepest sinuses near midleaf (at least in proximal 2/3), sinuses reaching nearly to midrib, longer lobes grading into shallow lobes or merely simple teeth distally, shallower, compound lobes proximally, secondary veins arched, divergent, 4-5(-10) on each side, apex broadly rounded or ovate; surfaces abaxially light green or whitish, with minute appressed-stellate hairs forming dense, rarely sparse, tomentum, erect felty hairs absent, adaxially dark green or dull gray, sparsely puberulent to glabrate. Acorns 1-3 on stout peduncle (0-)6-20(-25) mm; cup hemispheric or turbinate, (8-)15-50 mm deep × (10-)20-60 mm wide, enclosing 1/2-7/8 nut or more, scales closely appressed, laterally connate, broadly triangular, keeled, tuberculate, finely grayish tomentose, those near margins often with soft awns to 5-10 mm or more, forming fringe around nut; nut light brown or grayish, ovoid-ellipsoid or oblong, (15-)25-50 × (10-)20-40 mm, finely puberulent or floccose. Cotyledons distinct. 2 n = 24.
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Bur oak is grows natively throughout much of the north-central United States and the eastern Great Plains. It occurs from southern New Brunswick and New England westward to the Dakotas and southeastern Montana, and south to Tennessee, Arkansas, and the central prairies of Texas – with rare outliers in Louisiana, Mississippi, and Alabama. For current distribution, please consult the Plant Profile page for this species on the PLANTS Web site.
More info for the terms: fuel, litter, prescribed fire, presence, surface fire
Several characteristics of oak litter and woody debris make them flammable and important to fire spread. In a controlled experiment, dried bur oak leaves produced temperatures of up to 700 °F (371 °C) [231]. Oak leaves are thick, rigid, and irregularly shaped, which allows for efficient drying and persistence in the litter layer [182]. Leaves curl as they dry and produce a "loose, porous" fuel bed, which can easily carry fire (review by [153]). Oak leaves typically remain curled after snow melt, which allows for drying early in the spring. The high phenolic content of oak leaves means slow decomposition rates, ensuring fuel longevity [182]. Burning oak leaves can also be blown ahead of a fire, potentially igniting spot fires and increasing fire size (review by [153]). When compared, the oak fuel bed is much more "conducive to burning" than that of other hardwoods, which produce thin leaves that stick to the forest floor, trap moisture, provide few air-drying pockets, and decompose rapidly. Woody debris produced by oaks resists decay and provides a long-lasting fuel. Woody debris from other hardwoods decays much more rapidly than that of oak [182].
Prescribed fire in bur oak savanna near Vermont, Wisconsin; primary fuel for the surface fire was bur oak leaves.
Photo © Tom Brock, Universtiy of Wisconsin-Madison
Fuel characteristics in bur oak habitats may be affected by associated vegetation. In riparian areas in central Texas, eastern Oklahoma, southeastern Kansas, southern Missouri, and western Arkansas, bur oak occurred with riverbank grape (Vitis riparia). Often riverbank grape covered the lower branches of bur oak [173]. It is possible that the presence of riverbank grape as a ladder fuel could influence fire behavior and fire effects.
Bur oak is classed as intermediate in tolerance to shade (5). Some consider it more tolerant than northern red and white oaks; but on the prairie margins, bur oak stands are often invaded by black oak, white oak, and bitternut hickory. Bur oak reproduction in old white pine-bur oak stands in Minnesota reaches only sapling size before dying from suppression, and these stands are being replaced by maple-basswood communities.
In the wet bottom lands of northern Ohio, bur oak is a secondary species in the cover type Black Ash-American Elm-Red Maple, together with shellbark hickory, green ash, white ash (Fraxinus americana), pin oak, and swamp white oak. On the better drained bottom lands, bur oak may be successfully replaced by more tolerant species such as sugar maple (Acer saccharum), American basswood, and American beech (Fagus grandifolia).
On the prairie edges, bur oak is a pioneer tree, commonly succeeded by northern pin oak (Quercus ellipsoidalis), black oak, white oak, and bitternut hickory. The climax trees on these sites are sugar maple and basswood or sugar maple and beech. Bur oak may be a climax tree with hickory on extremely dry southern aspects and on thin, stony soils. In general, it is a species well adapted to sites ranging from droughty to moderately wet. But, on any given site, it is largely restricted to plant communities in early successional stages (17).
Please consult the PLANTS Web site and your State Department of Natural Resources for this plant’s current status, such as state noxious status and wetland indicator values.
In the sapling stage, taproot development continues to be rapid, with abundant lateral growth as well. The taproots of 8-year-old saplings in upland clay soils of Missouri were more than 4.3 in (14 ft) long, and primary laterals extended up to 3.4 in (11 ft) (5). In prairie areas, roots of bur oak and hackberry have been found at depths of 3 to 6 in (10 to 20 ft); and a 43-year-old bur oak tree had a lateral spread of 12.5 in (41 ft) although the tree was only 6 in (20 ft) tall. A study of a tree 36 cm (14 in) in d.b.h. revealed that the weight of the roots equaled that of the tops, and root volume was only about 10 percent less than top volume.
More info for the term: fire exclusion
At the edges of its range, bur oak may be a conservation concern [20,167,209].
Bur oak is also a food source of lepidopterans, of which one species is threatened [179,210].
Given bur oak's fragile existence in some areas as well as its importance to the persistence of other threatened species, its response to predicted climate change is considered important.
Conservation concerns: Bur oak is a
conservation concern in Canada [20,167], and several bur oak communities in the Plains region are considered "imperiled" [209]. This topic has also been discussed in Local distribution changes.
The following Great Plains communities are rare and/or vulnerable to extinction [209]:
- bur oak/big bluestem-switchgrass (Panicum virgatum) woodland
- bur oak/big bluestem-porcupine grass (Stipa spartea) woodland
- bur oak-northern pin oak/little bluestem-prairie Junegrass (Schizachyrium
scoparium-Koeleria macrantha) woodland - chinkapin oak-bur oak/big bluestem ravine woodland
- quaking aspen-bur oak-willow (Salix spp.)/big bluestem shrubland
- bur oak-swamp white oak river terrace woodland
- bur oak/chokecherry northern ravine woodland
- bur oak/American hazelnut (Corylus americana) woodland
- bur oak/eastern hophornbeam forest
- bur oak-basswood/eastern hophornbeam/western snowberry forest
- bur oak-swamp white oak-shellbark hickory/cutgrass-woodreed (Carya
laciniosa/Leersia spp.-Cinna spp.) floodplain forest - pin oak-swamp white oak-bur oak-red maple (Acer rubrum) sand flatwoods forest
Bur oak savannas are important for lepidopteran communities. On Iowa's Neal Smith National Wildlife Refuge, smaller forests lacking a prominent bur oak component supported 65 fewer species of moths than larger bur oak-dominated savanna remnants [238]. The barrens dagger moth (Acronicta albarufa)
is declining in the northeastern United States. At the caterpillar stage it feeds on bur oak. Land development and fire exclusion threaten the barrens dagger moth [179]. Within the moth's range in Manitoba, bur oak is the only caterpillar food available [210].
Climate change responses: Bur oak range expansions are predicted in many but not all climate change models. A northern expansion of bur oak's range was predicted from climate change models, assuming that bur oak successfully colonized all habitats made suitable by climate change [166]. Based on model simulations, bur oak was expected to increase in abundance with a 9 °F (5 °C) increase in the annual temperature in northwestern Wisconsin [103]. Conversions from boreal forests to grassland savannas or temperate forests are expected with warmer climates in Minnesota's Boundary Waters Canoe Area. Bur oak is expected to expand its range with increasing temperatures in the area. When warm, dry climates prevailed 8,000 to 5,000 years before present, oak species increased their range to the northeast. Oak species ranges decreased in the last 3,000 years with cool climates [77]. In the eastern United States, bur oak's importance is predicted to decrease with climate change and a doubling of current carbon dioxide levels [123].
Although rarely addressed in climate change analyses, the effects of bur oak pests will likely affect distribution changes associated with climate change. For a discussion of current, common bur oak pests
and diseases, see the following reviews [125,201].
More info for the terms: cover, shrub, tree
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [29,87,90,237]).
Aboveground description: Bur oak typically grows as a large, spreading tree up to 130 feet (40 m) tall; however, growth form and size can vary by site. Branches in the upper portion of the crown are ascending; in the lower crown, branches are larger and horizontal [68,234,237]. The trunks of mature trees have thick, deeply grooved bark [121] and may measure 8.5 feet (2.6 m) in diameter [63]. In the western part of its range on exposed, harsh sites, bur oak grows as a small tree or shrub [72,90,118] and may produce crooked, gnarled branches [68]. Bur oak growth forms may relate to moisture availability. In the Niobrara Valley Preserve, bur oak may only reach 15 feet (4.6 m) tall on moisture-limited sites but may reach 50 feet (15 m) tall on floodplains [99]. Growth may also be affected by browsing pressure. Bur oak was dwarfed in heavily browsed areas of Manitoba and Saskatchewan [23].
Bur oak is a long-lived tree. It is common to find remnant trees that are 300 to 400 years old [55,93], and in a savanna in Kentucky, a bur oak tree was an estimated 440 years old [38].
The sizes and shapes of bur oak leaves are variable, but generally leaves are deeply lobed and large, up to 12 inches (30 cm) long and about half as wide [90,121,211]. Shallowly lobed leaves may occur on bur oak sprouts or deeply shaded branches [90], and small leaves are common in the Northern Great Plains [234]. Leaves are deep green shiny above and coated with white hairs below [118]. Bur oak produces male flowers in 3- to 4-inch (7-10 cm) long catkins, and female flowers are solitary or in clusters of up to 4 [90,234].
Bur oak acorns are generally 1-seeded with a cup that covers at least 33% of the nut and may, though rarely, cover the entire nut [63,87,90,211,237]. Acorn size and cup coverage can vary by site. In the Northern Great Plains, bur oak produces small fruits and cups with low coverage, which may be the result of past hybridization with Gambel oak [234]. In general, acorn size decreases with increasing latitude [72]; bur oak acorns from a site in Texas averaged 7.5 g, while in Minnesota they averaged 0.9 g [140]. Diameter of acorns in Texas can be 2 inches (5 cm) [60,222]. Bur oak acorn size differences can even occur over small changes in latitude. Acorns produced in Wisconsin were much smaller than those produced in southern Illinois and Missouri [55]. Acorn size can also vary with shadiness of habitat. In east-central Nebraska, bur oak acorns from shady habitats weighed more than those from open habitats. Acorns collected from a closed-canopy floodplain forest weighed 1.3 to 6 g and those from open savannas weighed 0.5 to 2.5 g [144].
Belowground description: Typically bur oak produces extensive root systems with wide-spreading laterals and a deep taproot [8,68,118]. Several studies describe the root system of bur oak from seedling stage to maturity, although mature trees in these studies were not very old (43-80 years old).
Seedlings and saplings: Bur oak rapidly develops deep and wide-spreading roots. At the end of the 1st growing season, bur oak roots may reach 4.6 feet (1.4 m) deep and spread 2.54 feet (0.76 m) (review by [125]). Root systems of bur oak saplings are described in the table below.
| Characteristics of root systems of bur oak saplings at different ages and locations | ||||
| Tree age (yrs) | Tree height | Taproots | Lateral roots | Site |
| 3 | 3.5 ft | 5 ft deep | 2.5-ft spread | silt loam soil in Lancaster County, Nebraska [225] |
| 8 | ----* | 14.6 ft deep; 1.3-in. diameter at 4 ft deep |
11-ft spread; 18-24 laterals from top 14 in. of taproot |
upland clay soil near Fayette, Missouri [24] |
| 12 | 14 ft | 13 ft deep | 11.5-ft spread | silt loam soil in Richardson County, Nebraska [225] |
| *No information. | ||||
Mature trees: In the few excavation studies involving mature bur oak trees, root spread and penetration increased with tree age in clay, loam, and loess soils. A 43-year-old, 20-foot (6 m) tall bur oak tree growing in clay soil in North Dakota produced a taproot that was a little over 8 feet (2.4 m) long. The longest lateral root was 41 feet (12.5 m) [274]. A review reports that a 43-year-old bur oak growing in a prairie had nearly equal weights of above- and belowground biomass [125]. In eastern Nebraska, researchers excavated and described the entire root system of a 65-year-old bur oak tree growing in a deep, fertile, fine-textured loam with a high water-holding capacity. The tree was 37.5 feet (11.4 m) tall with a basal diameter of 14 inches (36 cm). There were 64 main roots with diameters greater than 2 inches (5 cm) that were 3.5 to 15 feet (1.1-4.6 m) long. The taproot was 14 feet (4.3 m) deep. Within a 12-foot (3.6 m) radius of the tree trunk, there were 64 main taproot branches, 82 secondary roots, and an abundance of rootlets. Bur oak aboveground biomass was 1,285 lbs, and the root system weight was nearly the same [262]. In Nance County, Nebraska, an 80-year-old, 20-foot (6.1 m) tall bur oak growing in loess soil produced a root system that reached 16 feet (4.9 m) deep and 72 feet (22 m) wide [225].
More info for the terms: avoidance, basal area, density, fire frequency, fire-return interval, frequency, fuel, fuel moisture, litter, natural, prescribed fire, severity, top-kill, tree
Mature bur oak trees are rarely killed or even top-killed by one or more fires [5,30]. Mortality and top-kill by fire typically decrease with increasing tree age or size [122,190,192]. However, postfire sprouting typically decreases with tree age [219,221]. Bur oak seedling establishment on burned sites is variable and limited on repeatedly burned sites [3,5,30,139].
Fire case studies: Various aspects of bur oak survival and recruitment have been studied and reported in areas managed with prescribed fire. These studies provide additional, site-specific details on the effects of fire on bur oak trees, saplings, and seedlings.
In Meade County, South Dakota, fire effects were evaluated on 24 prairie and woodland plots burned by prescribed fires in April. Bur oak was the dominant tree in the plots; there were 1,097 bur oak trees/ha, and the basal area of bur oak was 39 m²/ha. In the understory, bur oak was rare. Fine fuel loads averaged 590 kg/ha; fine fuel moisture averaged 14.6%; woody fuel loads averaged 11 mt/ha; woody fuel moisture averaged 11%; and soil moisture averaged 38%. Fires spread at an average rate of 0.13 foot (0.04 m)/s. Fire spread was "poor" and several ignitions were often necessary [221]. Mortality of bur oak was rare in burned plots, but 2 fire-scarred bur oak trees with heart rot burned for up to 2 weeks. Just 1 large diameter bur oak tree was consumed by fire, and it produced 1 sprout. Bur oak sprouts were more abundant on burned than unburned plots. The number of bur oak sprouts/tree increased with increasing scorch heights but decreased with tree age. Sprouts were more abundant on bottomland sites than on floodplain or slope sites. Bur oak germination was not increased on burned sites, and seedling survival was similar on burned and unburned plots. Seedling survival between the 1st and 2nd postfire growing seasons was 58.5% on burned and 71.8% on unburned plots. The abundance and survival of bur oak sprouts and seedlings on burned and unburned sites are summarized in the table below [219,221].
| Bur oak sprouts and seedlings on burned and unburned plots 1 and 2 growing seasons after a spring prescribed fire in Meade County, South Dakota [219,221] | ||||||
| Time since fire | 1st postfire growing season | 2nd postfire growing season | ||||
| Burned | 2.2 sprouts/tree | 0.4 shrub-sized stems/m² | 706 seedlings/ha | 2.1 sprouts/tree | 0.5 shrub-sized stems/m² | 429 seedlings/ha |
| Unburned | 0.5 sprouts/tree | 0.4 shrub-sized stems/m² | 1,071 seedlings/ha | 0.5 sprouts/tree | 0.4 shrub-sized stems/m² | 692 seedlings/ha |
Effects of spring prescribed fires were studied in oak savannas in east-central Minnesota's Cedar Creek Natural History Area [190,192]. Prescribed fires occurred in April or May, typically under the following weather conditions: air temperatures between 59 °F (15 °C) and 77 °F (25 °C), relative humidity from 25% to 45%, and wind speeds less than 12 miles (20 km)/hour. Prescribed fire frequencies for individual plots ranged from 0 to 26 fires in 32 years.
A study of 1st postfire growing season effects after a single, mid-May prescribed fire revealed that in general, the greater the sapling height at the time of the fire, the greater the number and height of postfire sprouts produced. This "low intensity" prescribed fire burned when the air temperature was 59 °F (15° C) and winds were less than 6.2 miles (10 km)/hour. Dry northern pin oak leaves were the primary surface fuel; flame lengths were 4 to 12 inches (10-30 cm). Before the fire, the site supported 800 bur oak saplings 10 to 26 feet (3-8 m) tall, with basal diameters of 1 to 3.5 inches (3-9 cm). The fire top-killed all but 2 saplings. For saplings greater than 3 feet (1 m) tall, the postfire sprouting frequency was 95% [192].
Bur oak seedlings and sprout densities were similar among plots with varied fire frequency in the Cedar Creek Natural History Area, but seedlings and sprouts were "suppressed" in frequently burned plots (11-26 fires in 32 years). Most sprouts grew from grubs. Generally, bur oak stems reached sapling height (5 feet (1.5 m)) in 3 years in the absence of fire. Dense thickets of bur oak saplings occurred in plots burned at low frequency (4 fires in 32 years). Mature bur oaks, even those in smaller size classes, were rarely killed by fire. When mortality occurred on burned sites it was typically from damage caused by the fall of another tree of a different species that was killed by fire. Mortality of bur oak trees on unburned sites was often the result of shading [190]. In the most frequently burned plots (26 fires in 32 years), the only tree species present were bur oak and northern pin oak [191]. The density and fate of bur oak saplings and trees on burned and unburned plots are summarized below.
| Abundance of bur oak saplings and abundance and fate of bur oak trees on unburned and repeatedly burned plots in the Cedar Creek Natural History Area in east-central Minnesota [190] | |||
| Unburned | Low-frequency fire (4 fires in 32 yrs) |
High-frequency fire (11 fires in 32 yrs) |
|
| Sapling* density (stems/ha) | 89 | 215 | 2 |
| Tree** basal area (m²/ha) | 0.91 | 0.07 | 0.11 |
| Tree density (stems/ha) | 36 | 7 | 3 |
| Tree mortality | 17.5% | 8.3% (all burned plots) | |
| *Saplings: ≥1.5 m tall and <5 cm DBH. **Trees: ≥5 cm DBH. |
|||
Earlier studies of the unburned and burned plots in the Cedar Creek Natural History Area described effects of prescribed fires that ranged from 11 to 17 fires in 17 years. Density of bur oak stems increased on unburned sites. Bur oak mortality averaged 29% across all plots, but recruitment exceeded mortality. Mortality of the oaks (bur oak and northern pin oak) averaged 75% for trees with diameters less than 4 inches (10 cm) and 30% for trees with diameters between 4 and 6.7 inches (10-17 cm). Oak mortality was least on plots burned 2 times in 17 years. Oak mortality was higher on plots burned 6 to 9 times in 17 years than on plots burned 11 times in 17 years. Researchers suggested that fires may have been more severe on the less frequently burned plots due to increases in woody stem densities with longer fire-free periods [122].
Fires every 3 to 5 years limited bur oak recruitment at Allison Savanna in east-central Minnesota, where prescribed fire is used to manage bur oak-northern pin oak barrens. The density of bur oak was 107 stems/ha on unburned plots and 53 stems/ha on high-frequency burned plots (25-year fire-return interval of 1.6-1.9 years). In unburned plots and low-frequency burned plots (25-year fire-return interval of 3.1-5 years), bur oak was abundant in the 4- to 9.8-inch (10-25 cm) DBH size classes. In unburned plots, bur oak trees ranged from 20 to more than 200 years old. In high-frequency burned plots, all bur oak stems were over 70 years old [67].
In the Namekagon River Barrens in northwestern Wisconsin, frequency of bur oak averaged 28% on burned and 16% on unburned sites. Burned sites experienced 1 or 2 spring fires, and postfire sampling occurred in the 1st or 2nd postfire growing season [258]. Bur oak density and basal area were greater on burned than unburned woodland plots in the Marengo Ridge Conservation Area of Illinois. Burned plots experienced 2 fall prescribed fires that were 4 years apart. Fires were low to moderate severity and burned when air temperatures were 60.1 °F (15.6 °C) and 47 °F (8.3 °C), relative humidities averaged 80% and 63%, and winds were 5 miles (8 km)/hour and 15 miles (24 km)/hour, respectively. Two years after the last fire, the density of bur oak (≥2-inch (5 cm) DBH) was 11 stems/ha on burned plots and 3 stems/ha on unburned plots. Basal area was 7.1 ft² (0.66 m²)/ha on burned and 2.9 ft² (0.27 m²)/ha on unburned plots [227].
Postfire seedling establishment: Fires may affect bur oak seedling establishment directly by removing litter and exposing mineral soil and indirectly by influencing the behavior of seed predators. However, the importance or inhibitory effects of litter and moisture on bur oak seedling establishment are unclear (see Seedling establishment). One researcher suggests that litter benefits establishment and survival of oak seedlings [153], while another researcher found that removal of litter improved oak seedling establishment in the field [141]. Because seed predators reduce the number of acorns available for establishment (see Seed predation), Lorimer [153] suggests that acorns on burned, open sites, which are not attractive feeding sites for many small mammals, may avoid predation better than those in unburned areas. However, such avoidance of predation could be counterproductive: The field study conducted by Krajicek [141] found that burial in mineral soil was most important to successful oak seedling establishment, and small mammal caches may be important for burial [74].
Because the combination of factors most conducive to bur oak seedling establishment is unclear, it is not surprising that fire studies fail to report clear patterns of postfire seedling establishment. It does appear however, that annual fires limit bur oak seedling establishment. In Madison County, large bur oak trees were frequent but there were no bur oak seedlings in a prairie remnant burned annually for at least the last 8 years [139]. In the Morton Arboretum in DuPage, Illinois, bur oak trees survived 17 years of annual, dormant-season, low-severity fires, but there was "little evidence for regeneration of oak species" [30]. See the Research Paper by Bowles and others 2007 for further information on prescribed fire and postfire responses of several plant species, including bur oak. On the Konza Prairie, bur oak seedlings were not present before burning in oak gallery forests, but in the 1st growing season after a late-April prescribed fire, the density of bur oak seedlings was 50/ha. The same bur oak seedling density was reported after another prescribed fire in early April of the following year. Prescribed fires moved slowly, 3 to 6.6 feet (1-2 m)/min, produced low flame heights (<1.6 feet (0.5 m)), and did not burn into tree crowns [3]. However, in a later study on the Konza Prairie, bur oak seedlings present before fires were absent 2 years after fire [5].