Material of Quercus havardii from the Navajo Basin of Utah and adjacent Arizona has been treated as Q . havardii var. tuckeri Welsh. Welsh followed J. M. Tucker (1970) and interpreted these intermediate populations as putative hybrids between Q . havardii and both Q . turbinella and Q . gambelii . Giving varietal rank, instead of nothospecies status, to such populations seems arbitrary, and it certainly is inconsistent with their putative hybrid origins.
Stem density (stems/m²)
Canopy height (cm)
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g. [34,49,95,97]).
Aboveground description: Havard oak is a deciduous, low-growing, thicket-forming, rhizomatous shrub. Shrubs are less than 7 feet (2 m) tall and often less than 3 feet (1 m) tall. Havard oak hybrids measure as tall as 10 feet (4 m) [34,67,70,95,97]. Havard oak is slow growing and long lived [29]. Clones may reach hundreds to thousands of years old [65]. Aboveground Havard oak stems, however, live only 11 to 15 years. Hybrid stems may live longer than 80 years [69,98]. The oldest stems of clones in Ward and Wheeler counties of Texas were 11 years of age [58]. Canopy coverage of Havard oak can be as high as 90% but averages 20% to 30% throughout its range. Biomass production averages 1,000 to 2,200 lbs/acre throughout Havard oak's range [67], although annual production as high as 4,500 lbs (2,000 kg) of air-dry forage/acre has been reported [65].
Leaves, stems, and acorns are highly variable in color, size, shape, and/or texture, and no character is readily associated with another [98]. Stems are rarely larger than 0.8 inch (2 cm) in diameter [58]. Bark becomes rough and scaly with age [49,60]. Leaves are leathery, rippled, entire, alternate, and toothed or lobed. Leaves measure 0.75 to 4 inches (2-10 cm) long and 0.75 to 1.5 inches (2-3.8 cm) wide. Number of lobes or teeth is typically 6 to 10 [34,49,70,95,97]. Male and female catkins occur on the same plant. Male catkins are 0.5 to 1.5 inches (1.5-3.8 cm) long and densely flowered. Pistillate catkins measure 0.1 to 0.3 inch (3-7 mm) long and contain 1 to 5 flowers [60,95,97]. Havard oak acorns mature in 1 year [19,34]. Acorns occur alone or in clusters of 2 or 3. Acorns are 0.5 to 1 inch (12-25 mm) long by 0.55 to 0.71 inch (14-18 mm) wide. The cups enclose 33% to 66% of the acorn [49,70,95,97].
Belowground description: The Havard oak root and rhizome system is extensive. Nellessen [60] reports that rhizomes range from 1 to 6 inches (3-15 cm) in diameter and are concentrated in the top 20 inches (60 cm) of soil, although 30-foot (9 m) penetration depths are possible. Lateral roots and woody rhizomes are widespread near the soil surface. Taproots extend 15 to 20 feet (4.6-6.1 m) deep [20,52,53]. Taproots with a diameter "equal to that of a man's thigh" are not uncommon [52]. Rhizome length is typically from 4 to 40 inches (10-100 cm) [58].
Ninety percent or more of Havard oak's biomass is under ground, and root grafting is common [66]. The mass of Havard oak roots and rhizomes is typically 10 to 16 times greater than that of the aboveground stems [69]. In a Havard oak community in Cochran and Yoakum counties of Texas, an average of 67% of the total Havard oak biomass was below ground. The root:shoot ratio was 11 for Havard oak shrubs measured in July [80].
Drought adaptations: An extensive root and rhizome network as well as physiological and morphological aboveground adaptations make Havard oak highly drought tolerant. Leaves mitigate water loss through a thick, waxy leaf epidermis [65,69]. Leaves may be dropped or leaf out postponed in adverse conditions. The extensive root system is important for water storage and utilization of available water [65].
Havard oak is best represented in southeastern and south-central New Mexico, the panhandle of Texas, and western Oklahoma. Other populations occur in southern Utah, western Colorado, northeastern Arizona, and northwestern New Mexico [59,70,97]. Freeman [31] reports that a native Havard oak population occurs in southwestern Kansas, and a herbarium specimen from Comanche County exists [43]. For additional information on the taxonomy of Havard oak and hybrid populations, see Taxonomy.
Distribution of Havard oak. Distributions of Q. h. var. havardii and Q. h. var. tuckeri, respectively. Maps courtesy of USDA, NRCS. 2017. The PLANTS Database. National Plant Data Team, Greensboro, NC [91].A review reports that Havard oak occupies 5 to 7 million acres (2-3 million ha) in the southern Great Plains [65]. A majority of Havard oak vegetation occurs on private land utilized for agriculture and/or livestock production. It is considered undesirable on grazing lands, and the use of chemical and mechanical control methods has been extensive (see Control), making it unlikely that the Havard oak range is expanding [56].
Fire adaptations: Havard oak sprouts from rhizomes soon after fire [3,11,53]. In Havard oak communities of northwestern Oklahoma, the density of Havard oak stems increased by approximately 15% in the 1st postfire year, and researchers described the communities as "extremely fire hardy" [54].
FIRE REGIMES: The presettlement fire frequency is estimated at less than 35 years for Havard oak communities. Since the late 1800s, however, fire frequency and fire size have decreased as a result of European settler's farming, grazing, and fire exclusion practices.
Presettlement fire frequency: While some have estimated the presettlement fire return interval for Havard oak communities, fire scar records are unavailable, so the fire regime in Havard oak vegetation is largely unknown [11]. Fire frequency estimates of neighboring southern Great Plains grasslands in areas with rolling topography range from 5 to 10 years. In the Rolling Plains and Edward Plateau regions, vegetation is often fragmented by breaks and rivers, and fires likely burned every 20 to 30 years [99].
Researchers report that Native Americans may have burned sand shinnery oak habitats in western Oklahoma as often as every year [65]. Paysen and others [64] suggest that shinnery, Texas savannah, and pinyon-juniper vegetation types experienced a mixture of understory and stand-replacing fires at intervals of less than 35 years.
Changes since settlement: Repeated fires in grasslands were typical in presettlement times; however, since the late 1800s fire occurrence has declined substantially. Together with successful fire exclusion by settlers, heavy livestock grazing in the Southwest removed much of the fire-carrying fuels and reduced the fire incidence in grasslands [30]. In presettlement times, large grassland fires were typical in drought years that followed 2 or 3 years of above average precipitation. However, since most grassland vegetation occurs in patches between agricultural and private lands today, large fires are more rare [101].
"Recurrent fires were a primary influence on stabilizing grassland or savannah vegetation composition" in the Edwards Plateau region of Texas. Most fires burned in the summer. Warm-season grasses of the Edwards Plateau have completed at least 60% of their annual growth by 1 August. Hot, dry conditions in July and August typically coincide with lightning strikes, making summer fires likely. Tall grasses were overgrazed by the late 1800s, reducing fine fuels and fire frequency. Shortly thereafter laws were enacted that successfully excluded fire from the landscape. In the late 1990s, landowners formed the Edwards Plateau Prescribed Burning Association (EPPBA). The EPPBA has restored fire to parts of the Edwards Plateau. The EPPBA provides members with fire safety training, a pool of necessary equipment, and an educated labor force. Prescription fires set by the EPPBA burn mostly in the summer to mimic the area's natural fire regime. Since the founding of the association they have burned approximately 40,000 acres (20,000 ha) [88].
The following table provides fire return intervals for plant communities and ecosystems where Havard oak is important. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or ecosystem Dominant species Fire return interval range (years) bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium 45,64] plains grasslands Bouteloua spp. 64,100] blackbrush Coleogyne ramosissima <35 to <100 juniper-oak savanna Juniperus ashei-Quercus virginiana Ashe juniper Juniperus ashei <35 Rocky Mountain juniper Juniperus scopulorum <35 pinyon-juniper Pinus-Juniperus spp. <35 [64] mesquite Prosopis glandulosa <35 to <100 [55,64] Texas savanna Prosopis glandulosa var. glandulosa <10 oak-juniper woodland (Southwest) Quercus-Juniperus spp. <35 to <200 [64] oak savanna Quercus macrocarpa/Andropogon gerardii-Schizachyrium scoparium 2-14 [64,96] shinnery Quercus mohriana <35 little bluestem-grama prairie Schizachyrium scoparium-Bouteloua spp. <35 [64]Sandy plains, sand dunes, and sand hills of the southern Great Plains are typical Havard oak habitat [19,34,60]. In southern Utah and adjacent Arizona, Havard oak occurs on sandy sites with blackbrush (Coleogyne ramosissima), ephedra (Ephedra spp.), purple sage (Salvia dorrii), and/or pinyon-juniper (Pinus-Juniperus spp.) [97].
Elevation: Havard oak occupies relatively low elevation sites.
State/region Elevation (feet) NM 3,500-4,000 [49] TX, Trans-Pecos 2,300-3,400 [70] UT 3,690-7,000 [97]Soils: Habitats occupied by Havard oak have sandy loam or loamy sand soils. Characteristics of these soils were summarized in a review [65]. Infiltration rates are rapid and pH is neutral or slightly basic. Soils are highly susceptible to erosion and low in organic matter, nitrogen, and phosphorus. A caliche layer more than 3 feet (1 m) below the soil surface is possible. Most sandy soils occupied by sand shinnery oak have a thin clay layer near the surface according to McIlvain [53].
Soils underneath Havard oak shrubs were sampled, analyzed, and compared with soils underneath other vegetation in the Los Medanos area of southeastern New Mexico. Soil samples were taken from 0 to 5.9 inches (0-15 cm) deep. Soil moisture, organic matter, nitrate, and phosphorus were all much lower in the Havard oak-sand sagebrush community type than in the honey mesquite-javelin bush (Condalia ericoides) community type. Average summer soil moisture beneath Havard oak ranged from 2.29% to 2.99% for a 2-year period, and organic matter averaged 0.36%. Below is a summary of the nutrient levels in the soil underneath Havard oak shrubs [81].
Nitrate Ammonium Phosphorus Calcium Magnesiumppm
June July Aug. June July Aug. June July Aug. June July Aug. June July Aug. 0.1 0.1 0.4 0.38 0.36 0.16 0.1 0.1 0.1 18.9 35.1 21.6 1.1 1.4 1.1Researchers found that the abundance of galls on Havard oak was greatest on sites with the highest salt levels. Shrubs were studied at the Waste Isolation Pilot Plant near Carlsbad, New Mexico. Researchers speculated that salt stress may have made Havard oak more susceptible to insect attacks [24].
Climate: Havard oak occurs in semiarid warm temperate and continental climates. Researchers summarized the climate data for 3 areas where Havard oak is common. The table below provides these data [65].
NM, Eddy Co. OK, Roger Mills Co. TX, Yoakum Co. Mean annual precipitation (mm) 316 651 404 Mean date of last frost 31 March 2 April 13 April Mean date of first frost 7 November 28 October 1 November Mean January temperature (? C) 6.7 2.7 5.6 Mean July temperature (? C) 27.5 28.2 26.4Droughts lasting from 2 years to decades are possible in Havard oak habitats. In the northern and southern parts of Havard oak's range, 1 to 2 years and 2 to 3 years, respectively, with precipitation levels of 75% below average are common within a 10-year period [65].
In northwestern Oklahoma, Havard oak rangelands receive an average of 22 inches (560 mm) of precipitation annually, but the range of annual precipitation is 10 to 40 inches (250-1,000 mm). Wind speeds and water evaporation rates are high in this area [54]. In Oklahoma's Black Kettle National Grasslands, where Havard oak dominates some communities, precipitation is variable, with peaks common in May and June, and in August and September [37]. In north-central Yoakum County, Texas, 80% of the annual precipitation is delivered in brief intense storms from May through October [94].
Diverse wildlife species utilize Havard oak habitats for cover and food. Collared peccaries, lesser prairie-chickens, northern bobwhites, and other wildlife species eat Havard oak acorns [19,65,70,95]. Deer, pronghorn, and southern plains woodrats browse Havard oak [60]. In a review, Peterson and Boyd [65] report that black-tailed jackrabbits, desert cottontails, eastern cottontails, wild turkeys, western box turtles, a diversity of arthropods, approximately 25 snake species, and approximately 10 lizard species utilize Havard oak habitats. Scaled quail and northern bobwhites use Havard oak habitats extensively, and Havard oak habitats are considered "the principal home for white-tailed deer in the southern Great Plains" [65].
Livestock: Havard oak is poisonous to horses, domestic sheep, goats, and cattle. Tannic acid is considered the poisoning agent, but other compounds may contribute [1]. Cattle consuming high levels of sand shinnery oak develop rumen ulceration and eventually suffer liver and kidney failure. Domestic goats tolerate a diet with more Havard oak than cows; however, a diet of just Havard oak kills goats [69]. Livestock poisoning is most common in the spring when Havard oak buds and immature leaves are consumed readily and other food is scarce. Without treatment livestock death rates can be as high as 85%. Removing livestock from Havard oak-dominated sites or providing additional feed may reduce the chance of poisoning [1]. For additional information on tips for avoidance and symptoms of poisoning, see James and others [41].
When oak species make up greater than 50% of the forage, livestock poisoning may occur. If diets exceed 75% Havard oak, animals may die [36]. The 50% and 75% consumption estimates, however, are based on consumption of plants with 2% to 6% tannin concentrations. Early in the spring tannin levels can be as high as 18% to 20%, meaning that consumption levels much lower than 50% could cause illness or death [92]. For additional information on stocking rates and rotational grazing patterns that may decrease the chances of poisoning in Havard oak habitats, see Peterson and Boyd [65].
Cattle may be poisoned by consuming Havard oak. Havard oak is most toxic in the spring growing season [56] and if consumed without other forage [57]. Poisoned cattle initially excrete dark, dry feces with mucus and blood and have a decreased appetite. Without supplemental feeding or treatment, cattle have bloody diarrhea, urinate frequently, and drink water excessively. A rough coat, dry muzzle, and reddish urine are typical in the late stages of poisoning. Nursing calves may be the most susceptible. Tannins consumed by the mother concentrate in the milk. Havard oak poisoning is considered a "symptom of overstocking and poor range condition" [92].
Cattle browse high levels of Havard oak when other forage is unavailable, during drought conditions, or in heavily grazed areas. Researchers suggest that animals should be kept off oak-dominated ranges until foliage is at least 30 days old [36]. In sandhills communities in the Texas panhandle, researchers found that Havard oak made up 15.3% of spring, 16.2% of summer, and 23.8% of fall cattle diets. Diet was determined through fecal analysis and observations. Havard oak abundance rather than preference was responsible for cattle consumption levels [77].
Domestic goats: Angora and Spanish goats browse Havard oak extensively without suffering the poisoning described above. Havard oak made up 31.7%, 45.0%, and 55.1% of Angora and Spanish goat diets in June, July, and August, respectively. Goats were in north-central Yoakum County, Texas, where Havard oak made up 80% of the available forage. Tannin content did not change much from June to September and averaged 36.7 mg/g in leaves and 37.6 mg/g in stems [93,94]. A review reports that domestic goats that continually browse Havard oak lose weight [60].
Sheep: In the Palo Duri Canyon of Texas, fecal samples were collected for approximately 2 years. Researchers found that Havard oak made up 31.0% of the relative density of Barbary sheep diets. Diets of mule deer and Barbary sheep in the area had a high degree of overlap [44]. Peterson and Boyd [65] speculate that this study may have mistaken Havard oak for Mohr oak.
Native animals: Native ungulates, small mammals, birds, insects, and lizards utilize Havard oak habitats and/or consume Havard oak.
Pronghorn: Spring, summer, fall, and winter pronghorn diets were 7.8%, 20.8%, 21.4%, and 3.6% Havard oak in sandhill communities of the Texas panhandle. Pronghorn diets had a high degree of overlap with cattle in the same area in the summer and fall months [77].
Deer: Havard oak is important deer browse. In the Palo Duri Canyon of Texas, fecal samples were collected for approximately 2 years. Researchers found that Havard oak made up 37.2% of the relative density of mule deer diets. Armstrong [3] reports that the occurrence of heavily browsed plants in July and August indicates overutilized white-tailed deer range.
Lesser prairie-chickens: Havard oak is important in lesser prairie-chicken habitats and diets. Lesser prairie-chickens are candidates for listing as threatened or endangered throughout their range. For more on the lesser prairie-chicken and its current status, see the U.S. Fish and Wildlife Service website.
Havard oak was a major component of the lesser prairie-chicken's spring and summer diets in the Mescalero Sands of eastern New Mexico. Based on a 2-year study, lesser prairie chicken diets were 15.2% Havard oak acorns, 31.8% catkins, and 2.1% leaves in the spring (March-May) and 21.2% acorns and 0.2% leaves in the summer (Jan-April). In the study area, Havard oak made up 29.1% to 48.8% of the vegetation composition [18]. In Chaves County, New Mexico, Havard oak acorns comprised 17% to 61% of fall diets and 69% of winter diets. Havard oak insect galls made up 5% of winter and 14% of fall diets based on crops collected over a 2-year period. Researchers indicated that Havard oak is the most heavily utilized year-round food source for lesser prairie-chickens [74].
In Cochran County, Texas, 90 lesser prairie-chicken crops were collected and analyzed over a period of 3 years. The frequency of Havard oak leaves, acorns, and galls were 8.0%, 15.9%, and 39.7%, respectively. Percentage of Havard oak leaves, acorns, and galls by volume were 0.3%, 5.0%, and 15.3%, respectively [15]. The percentage of lesser prairie-chicken body fat was significantly (p<0.05) greater on untreated than on herbicide (tebuthiuron)-treated sites in Cochran and Yoakum counties of Texas and Lea and Roosevelt counties of New Mexico. Treated sites had significantly less (p<0.01) Havard oak based on basal composition percentages than untreated sites [61].
Lesser prairie-chicken populations in Havard oak communities in New Mexico and Oklahoma's Sutton Avian Research Centers utilized sites with increased coverage and density of shrubs. Survivorship of the lesser prairie-chicken was greater when sites had over 20% shrub cover than when sites had less shrub cover. Havard oak dominated the shrub layer in New Mexico, but its importance decreased in Oklahoma [63]. In Cochran and Yoakum counties of Texas, fewer lesser prairie-chicken nests were located in herbicide-treated than in untreated Havard oak sites. Of 10 nesting females, 8 nested in untreated sites, and 2 nested in treated sites. Researchers indicated that Havard oak stems and foliage provided important vertical screening cover for nests [38]. However, in high plains bluestem vegetation in southeastern New Mexico nests were more successful when tall grasses provided the principal nest cover. Of the 4 nests with Havard oak as the principal cover, none were successful (hatched at least 1 young) [75].
Other birds: Hawks, mourning doves, and scaled quail occupy Havard oak habitats. Harris' and Swainson's hawks utilize Havard oak shrublands of southeastern New Mexico [4,5]. Mourning doves occupy Havard oak habitats in southeastern New Mexico [7]. Havard oak habitats in northwestern Oklahoma supported 0.035 mourning dove breeding pairs, and ground nesting success was 33% based on data collected for a single nesting season [25]. In southeastern New Mexico, Havard oak was 2.6% of the average volume of foods in 50 scaled quail crops collected in 1971. Crops collected in 1970, 1972, and 1973 did not contain Havard oak. Havard oak was abundant in part of the study area, but abundance values were not reported [17]. The crops of northern bobwhites harvested from Havard oak habitats in northwestern Oklahoma contained 10.9% Havard oak by volume [35].
Small mammals: Havard oak communities provide important habitat and cover and an occasional food source for a variety of small mammals. The following rodents were trapped in Havard oak-dominated sites of Yoakum County, Texas: Ord's kangaroo rat, plains pocket, hispid pocket mouse, deer mouse, western harvest mouse, plains harvest mouse, northern grasshopper mouse, hispid cotton rat, southern plains woodrat, house mouse, and spotted ground squirrel [14]. Based on stomach content analyses, a single Ord's kangaroo rat consumed Havard oak. Havard oak was not recovered from northern grasshopper mouse, southern plains woodrat, or spotted ground squirrel stomachs. All trappings occurred in Havard oak-mesquite grasslands of southeastern New Mexico from March 1978 to December 1979 [8]. Ringtails are also common in Havard oak communities [86].
Herptiles: Snell (personal communication in [79]) indicates that dune lizard populations declined by 70% to 94% in New Mexico when Havard oak was removed.
Insects: Weevils, caterpillars, and grasshoppers utilize Havard oak as a food source or as habitat. Likely the diversity of insects utilizing Havard oak is greater than current research suggests. A puss caterpillar of Oklahoma feeds extensively on Havard oak. Tannins of the leaves are used in their venomous hairs or spines [47]. Havard oak is a primary component of spotted bird grasshopper diets in New Mexico. Levels of grasshopper herbivory were greatest on plots where Havard oak had the fewest galls [24]. Boll weevils overwinter in Havard oak litter, which is considered prime overwintering habitat [85].
Palatability/nutritional value: In western Oklahoma, researchers evaluated the nutritional quality of Havard oak leaf buds and catkins on unburned, ungrazed, grazed, and fall-, winter-, and spring-burned sites. Values were reported for the 2nd postfire year, and ungrazed sites were free of cattle for a single growing season. Nutritional quality of buds and catkins on burned, unburned, grazed, and ungrazed sites were similar. The percentage of phenolics was greater on burned and grazed than unburned and ungrazed sites. Comparisons of catkin nutrition were not made on grazed and ungrazed sites because catkins were too few on grazed sites. The nutritional quality of buds and catkins on burned and unburned sites is summarized below [12].
Crude protein (%) Phenolics (%) Acid-detergent fiber (%) Ash (%) Buds Unburned 22.4 16.1 23.5 4.6 Fall burned 23.2 18.1 20.9 4.3 Winter burned 22.9 15.3 22.5 4.1 Spring burned 24.1 18.6 22.6 4.2 Catkins Unburned 20.7 12.2 18.2 4.4 Fall burned 21.1 15.2 18.9 4.3 Winter burned 21.5 15.0 18.2 4.2 Spring burned 21.7 15.1 21.2 3.9Cover value: The importance of Havard oak as cover and in the habitats of wildlife species has been integrated into the above sections. For additional information on the importance of Havard oak in wildlife habitats, see the species group of interest within Importance to Livestock and Wildlife.
Wildlife-livestock management:
Havard oak habitats are likely candidates for wildlife-livestock management
conflicts. Boll weevil pests overwinter in Havard oak litter [85], and
cattle may be poisoned by Havard oak [92]. Many have attempted to control
Havard oak to lessen the success
of boll weevils, increase available livestock forage, and reduce livestock
poisoning risks [27,39,85]. However, sand shinnery
oak provides important habitat and food for a variety of wildlife and plant species
[65], some of which are endangered as of this writing (2006). The Texas poppymallow (Callirhoe
scabriuscula) and aplomado falcon occur in sand shinnery
oak habitats and are federally endangered. The Texas horned lizard that occurs in
Havard oak habitats is threatened in Texas [21], and the lesser prairie-chicken
that is highly dependent on sand shinnery
oak is a candidate for listing. See U.S. Fish and Wildlife Service, NatureServe,
and/or Texas Parks and Wildlife
databases for more information on the current status of these species.
Competition/allelopathy:
Studies found that extracts of sand shinnery
oak leaves significantly (p<0.05) suppressed the initial root elongation of 'Ermelo'
weeping lovegrass (Eragrostis curvula) seeds. Germination percentage and
shoot elongation of seeds kept moist with Havard oak leaf extracts were
not reduced. Weeping lovegrass seed produced roots that were 1.2 inches (31 mm) in
distilled water, but roots were 0.08 inch (2 mm) long in Havard oak extracts [50].
Rhizome extension is important in the horizontal spread of Havard oak, but sexual regeneration is episodically important [51]. Muller [58] suggests that although abundant seed is produced, seedling establishment is rare and often restricted to open or disturbed sites. In a review, inadequate moisture conditions for seed germination and seedling establishment are proposed as reasons for poor seedling establishment rather than lack of seed production [65].
Work in western Texas and western Oklahoma suggests that sexual regeneration is rare. Researchers conducting studies in the Llano Estacado area of western Texas found no seedlings in a year-long study of Havard oak communities, and later in the same area no viable acorns were found [20]. In observational studies of Havard oak communities in western Oklahoma, Wiedman [98] encountered no seedlings.
Pollination: Oak (Quercus spp.) flowers are wind pollinated. Catkin morphology aids in successful wind dispersal of pollen [82].
Breeding system: Havard oak is a monoecious shrub [34], and the existence of hybrids suggests that outcrossing is common. Self compatibility of flowers was not described in current (up to 2006) literature.
Seed production: On average acorn crops are produced in 3 of 10 years [69]. Nellessen [60] reports that although acorn crops are produced in 3 of every 10 years per clone, acorns are likely found within a community each year. Hanson [35] reported that Havard oak produced large quantities of large-sized acorns during his study of northern bobwhites in 1951 and 1952 in northwestern Oklahoma.
Peterson and Boyd [65] report that "heavy local crops occurred somewhere every year," but within a community, acorn crops were not produced more than 2 years in 5 from 1977 to 1997 in New Mexico. If shrubs experience a freeze after flowering, no acorns are produced [69].
Seed dispersal: Small mammals and birds may aid in the dispersal of oak acorns by abandoning caches. American and European jays often cache acorns a few meters apart in open environments and cover them with debris or soil [26].
Seed banking: Havard oak acorns rarely survive after early January because of heavy predation by insects and other animals [35], suggesting that a seed bank is unlikely. Acorns of the white oak group have little or no dormancy and are capable of germination immediately after falling. It is likely that acorns that survived predation would not persist in the seed bank [9].
Germination: Moisture at the time of seed fall and successful avoidance of predators are necessary for germination of Havard oak acorns [60,69]. Seed viability is lost when acorn moisture levels drop below 30% to 50% [60]. Late July and/or early August moisture is required for successful Havard oak germination [69].
Germinating acorns 1st produce root tissue. The initial root may be 12 inches (30 cm) long before the 1st leaves emerge [69].
Seedling establishment/growth: Havard oak growth rates above and below ground are often greater with increased moisture. Monitoring the root growth of acorns with approximately 0.8 inch (2 cm) radicle emergence using glass-front growth chambers revealed that growth is more dependent on soil moisture than soil temperature. Just 2 of 15 seedlings survived cool-dry treatments, and root extension stopped after 2 weeks in dry-warm treatments. Regardless of temperature, moist and wet treatments produced root extension rates of 0.080 to 0.094 inch (0.20 to 0.24 cm)/day throughout the 52-day study. Researchers suggest that Havard oak regeneration from acorns is likely restricted to long wet periods [84].
Growth rates of Havard oak in Ellis County, Oklahoma were 1.1 mm/year and in hybrid populations in Harmon County were 0.2 inch (5.2 mm)/year [98]. Havard oak ? post oak hybrids in Texas grew at an extremely slow rate. Tree-shrubs over 50 years old were developing a "heart rot like condition," and ring growth was less than 0.2 mm/year. Two growth rings revealed more rapid growth rates that likely coincided with above-average precipitation levels [68].
Asexual regeneration: Havard oak spreads by rhizome growth and sprouts from rhizomes following aboveground stem damage. Lateral woody rhizomes are capable of sprouting along their entire length [52,53]; however, spread by rhizomes is slow. Studies in Oklahoma showed a spread of just 30 feet (9 m) in 50 years [69]. Sprouting after destruction of aboveground stems is quick. New stem growth is typically visible within 1 to 2 months after removal of or damage to aboveground stems [65]. Shoot regeneration rates of 10 to 20 inches (30-60 cm) per year were reported following aboveground kill or damage [60].
Clone size can be very large. In Yoakum County, Texas, 2 acres (1 ha) contained an average of 15 distinct Havard oak clones. Researchers speculated that vegetative reproduction was important in horizontal spread but was not a substitute for sexual reproduction, which may occur only episodically. Clonal shape was variable. Some were densely clumped and circular, others were long and narrow or meandering, and others were convoluted and fragmented by other clones. Clone size ranged from ~100 to 7,000 m? [51]. Clone size in Ward and Wheeler counties of Texas ranged from 10 to 49 feet (3-15 m) in diameter [58]. Havard oak ? post oak hybrid clones in Texas were a product of extensive asexual regeneration. Eighty nine tree-shrubs were determined to be a single clone. Havard oak ? post oak and Havard oak seedlings were rare in the field [68].
In a review, researchers indicate that Havard oak is tolerant of disturbance, occurs as a very old shrub in late seral communities, but colonizes open sites at a very slow rate. Following top-killing disturbances, Havard oak regeneration is typically quick. In some Havard oak communities, root and rhizome systems are estimated to be hundreds to thousands of years old.
Havard oak's response to grazing pressure is mixed. Some have labeled Havard oak as an "increaser" on grazed sites, while others have reported no change or a decrease in Havard oak on grazed sites [65].
Havard oak does not readily colonize open sites. If removed from a site, reinvasion is slow. Old field sites in Oklahoma, abandoned in the early 1900s, were dominated by grasses 50 years later, although Havard oak communities surrounded the sites [53].
The scientific name of Havard oak is Quercus havardii Rydb. (Fagaceae)
[33,34,43,49,97]. Havard oak belongs to the white oak group (Leucobalanus) [19]. Some
[43,91,97] recognize 2 varieties, Q. h. Rydb. var.
havardii and Q. h. Rydb. var. tuckeri Welsh,
but other taxonomic authorities do not [33].
Havard oak populations in the Navajo Basin of Utah and Arizona are
considered pure Havard oak (Q. havardii var. tuckeri)
by some [97], but introgression with
Gambel oak (Q. gambelii) and perhaps shrub live oak (Q. turbinella) make taxonomic
identification of Utah and adjacent Arizona Havard oak populations difficult [90].
Havard oak also hybridizes with Mohr oak (Q. mohriana) and post oak (Q. stellata)
[59,70,95]. Post oak ÃÂ Havard oak hybrids are likely a result of post oak's historic range, which
extended more westerly than it does today. Mohr
oak ÃÂ Havard oak hybrids are restricted to
habitats intermediate to those occupied by the 2 species. Mohr oak
inhabits limestone soils, and Havard oak occurs on deep sand soils [59].
Quercus havardii (common names include shinnery oak, shin oak and Havard oak) is a deciduous, low-growing, thicket-forming shrub that occupies some two million to three million hectares (7,700 to 11,600 square miles) in the southern Great Plains of North America.[2] Clones may reach hundreds to thousands of years old, although aboveground stems typically live only 11 to 15 years. Shinnery oak stems are usually 1 to 2 metres (3 feet 3 inches to 6 feet 7 inches) tall and codominate the plant community with mid- and tall-grasses, which are usually taller than the oaks.
The specific epithet honors U.S. Army surgeon and botanist Valery Havard, who contributed much to the knowledge of southwestern plants. The word "shinnery" seems to be derived from chêne (French for oak), and not from the height of the plant.[3]
Form: A low shrub to 2 metres (6+1⁄2 feet) or occasionally a small tree, Q. havardii forms large clonal thickets by extending rhizomes through the sandy soil where it is usually found.[4] Rhizomes range from 3–15 centimetres (1+1⁄4–6 inches) in diameter and are concentrated in the upper 60 cm (24 in) of soil, although penetration depths of 9 m (30 ft) have been reported in shifting dunes. Lateral roots and woody rhizomes are widespread near the soil surface. At least 90% of shinnery oak's biomass is underground, and fortuitous root grafting is common. These underground stems commonly spread to form plants 3 to 15 m (10 to 49 ft) or more in diameter. Single clones are reported to cover up to 81 hectares (200 acres) and to achieve ages over 13,000 years.[2]
Flowers: Shinnery oak is monoecious with both female and male flowers borne separately on the same plant.[5] Male catkins are densely flowered, 1.5–3.8 cm long, and hang downward. Female catkins are 3–7 millimetres (1⁄8–1⁄4 in) long, contain 1 to 5 flowers, and are usually axillary on young shoots. Shinnery oaks are wind pollinated, and flowering occurs in the spring.
Acorns: Acorns develop in one year, maturing in the autumn. Acorns occur alone or in clusters of 2 or 3, and are 12–25 mm (1⁄2–1 in) long by 14–18 mm wide. A scaly cup covers about 1/3 to 1/2 of the nut. On average, acorn crops are produced in 3 out of 10 years.[4]
Foliage: The leathery, highly variable leaves are grey green to olive green, have a lustrous upper surface, and are whitish and densely hairy below. The leaves are alternate, simple, with variable shape (oblong, ovate, or elliptical), and with wavy or shallowly lobed margins. Their length is 2 to 8 cm (3⁄4 to 3+1⁄8 in) and width 2 to 4 cm.[3]
Twigs: Twigs are brown or grayish, 1–2.5 mm diameter, glabrous or densely covered with short grayish or yellowish hairs, that are lost with age. The buds are dark red-brown, somewhat spherical, about 2 mm long, and sparsely pubescent.
Bark: Bark on the larger stems is light gray and scaly.
Shinnery oak populations in Utah and Arizona are considered a variety of shinnery oak (Quercus havardii var. tuckeri) by some taxonomists. However, introgression of shinnery oak with Gambel oak (Quercus gambelii) and perhaps shrub live oak (Quercus turbinella) make taxonomic identification of these populations difficult.[4]
Shinnery oak hybridizes with Mohr oak (Quercus mohriana) and post oak (Quercus stellata). Post oak × shinnery oak hybrids are likely to be a result of post oak's historic range, which extended more westerly than it does today. Mohr oak × shinnery oak hybrids are restricted to habitats intermediate to those occupied by the two species. Mohr oak inhabits limestone soils, and shinnery oak occurs on deep sand soils.
Shinnery oak is native to western Oklahoma, west Texas, eastern New Mexico, and two isolated populations in southwest Kansas.[6] A variety also occurs in northeastern Arizona and southeastern Utah.[7] Sandy plains, sand dunes, and sand hills of the southern Great Plains are typical habitat.
A majority of shinnery oak occurs on private land used for agriculture and/or livestock production. It is considered undesirable on grazing lands, because it competes with better livestock forage and its buds and leaves are toxic to cattle for several weeks in spring. In addition, where cotton is grown near shinnery oak, boll weevils overwinter in the oak litter and infest nearby cotton fields in spring. Thus, most research and other human effort concerning shinnery oak have been devoted to its eradication.
The use of chemical and mechanical control methods has been extensive, making it likely that shinnery oak's distribution is decreasing. Most authorities do not recommend complete eradication of shinnery oak, indicating that forage production is typically greater if some shinnery oak remains in the community. Stocking rates and rotational grazing patterns exist that reduce the incidence of shinnery oak poisoning of cattle. Excessive control of shinnery oak is controversial because it can open sandy soils to wind erosion and can conflict with wildlife-habitat quality.[2]
Shinnery oak sprouts prolifically from rhizomes soon after fire. Researchers described the communities as "extremely fire hardy".[4]
Because shinnery oak thrives in a harsh environment, it functions as a sand dune stabilizer, protecting sandy soils from wind erosion. It also provides diverse wildlife species cover and food. Collared peccaries, lesser prairie-chickens, northern bobwhites, and many other wildlife species eat shinnery oak acorns. Deer, pronghorn, and southern plains woodrats browse shinnery oak. Shinnery oak habitats are used by black-tailed jackrabbits, desert cottontails, eastern cottontails, wild turkeys, western box turtles, a diversity of arthropods, approximately 25 snake species, and approximately 10 lizard species, including the endangered sand dune lizard (Sceloporus arenicolus).[2]
Many game species also rely on shinnery oak. Scaled quail and northern bobwhites use shinnery oak habitats extensively, and shinnery oak habitats are considered "the principal home for white-tailed deer in the southern Great Plains".[2]
Shinnery oak provides rich habitat for wildlife compared with the surrounding shortgrass plains. Maintaining wildlife habitat and improving vegetation for livestock grazing are often in conflict, and should be carefully considered before undertaking control of shinnery oak.
{{cite journal}}: CS1 maint: multiple names: authors list (link) Quercus havardii (common names include shinnery oak, shin oak and Havard oak) is a deciduous, low-growing, thicket-forming shrub that occupies some two million to three million hectares (7,700 to 11,600 square miles) in the southern Great Plains of North America. Clones may reach hundreds to thousands of years old, although aboveground stems typically live only 11 to 15 years. Shinnery oak stems are usually 1 to 2 metres (3 feet 3 inches to 6 feet 7 inches) tall and codominate the plant community with mid- and tall-grasses, which are usually taller than the oaks.
The specific epithet honors U.S. Army surgeon and botanist Valery Havard, who contributed much to the knowledge of southwestern plants. The word "shinnery" seems to be derived from chêne (French for oak), and not from the height of the plant.
