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Kuchler Type: Oak-hickory forest
PHYSIOGRAPHY : Oak-hickory forests are found on all topographic positions including dry rocky ridges, deep coves, and well-drained valley floors [81]. West of the Appalachian Mountains, deciduous forests occupy areas geologically classified as plateaus, although erosion has produced rolling to mountainous topography [44]. CLIMATE : The climate of the north-central forest region (Minnesota, Wisconsin, Michigan, Iowa, Illinois, Indiana, Ohio, Missouri, and Kentucky) is continental and ranges from humid to subhumid. Winters are cold and summers hot. Temperatures differ greatly from north to south through the region, both in summer and winter. Temperature, growing season, and precipitation increase from north to south; at a given latitude, precipitation increases from west to east [44]. In the southeastern United States along the eastern edge of the oak-hickory forest, mean annual precipitation is approximately 45 inches (1,140 mm); at the western edge, annual precipitation averages 39 inches (980 mm). Lower precipitation in the western parts of this area is exacerbated by an episodic pattern of rainfall and more frequent drought. In southeastern oak-hickory forest, average annual precipitation increases from 42 inches (1,060 mm) in Indiana to 55 inches (1,400 mm) in Alabama [15]. At the southwestern limit of oak-hickory forest, central Texas has an average annual precipitation of 22 inches (560 mm) and mean annual temperatures around 65 degrees Fahrenheit (18.3 deg C). In eastern Texas mean annual temperatures are 62.6 to 66.2 degrees Fahrenheit (17-19 deg C), and the growing season is approximately 225 days [15]. SOILS : Oak-hickory forests occur on soils ranging from cool-moist Boralf and Orthod Spodosols to warm-dry Millisols and Alfisols. Soils range from clay to loam and are derived from glacial material, residual sandstones, shales, limestones, gneisses, schists, and granites [81]. In southeastern oak-hickory forests, soils encompass a wide range of physical and chemical features, ranging from shallow to deep, infertile to rich, and high clay fraction to very little clay. Soils are formed in residuum, primarily from limestone parent materials, but also from sandstones and shales. Soils in oak-hickory forests are fertile compared to other southeastern soils and represent some of the most productive agricultural land on these uplands [15]. Parent materials on the Ozark plateau are predominantly limestones, cherts, and dolomites. In Arkansas, soil parent materials in the eastern Ouachita province are mostly shales, sandstones, quartzites, and cherts, and in the western Ouachita province are mostly sandstones and shales [12]. VEGETATION : Classification: There is much debate over the most useful classification for the eastern deciduous forests including oak-hickory forest. Oak-hickory forest is not uniform across its range; dominants vary with climatic and edaphic conditions. The many differing interpretations of oak-hickory forest are probably a function of its vegetational complexity [15]. The most extensive area of unequivocal oak-hickory forest is found in the Ozark Mountains, where it covers most upland sites [15]. Hicks and Chabot [44] described oak-hickory forest that is largely equivalent to Kuchler's oak-hickory type, but Braun [12] characterized the central and eastern portions of K100 as western mesophytic forest. She defined the oak-hickory forest as the most westerly part of the deciduous forest region. Some authors use a definition of oak-hickory forest that is broader than either Kuchler's or Braun's, and include all forests dominated by oaks and having at least some hickory component as oak-hickory. These definitions usually include Kuchler's oak-hickory forest (K100), oak-hickory-pine (Pinus spp.) (K111), Appalachian oak forest (K104), and Braun's mixed mesophytic forest [37,81]. Monk and others [65] divided the eastern deciduous forest into three regions: the northern region, occupied by eastern hemlock (Tsuga canadensis), eastern white pine (Pinus strobus), and northern hardwood forests; the central region which included oak-hickory; and the southern region, occupied by pine and oak-pine forests. They defined oak-hickory as inclusive of Braun's oak-hickory, oak-chestnut (Castanea spp.), and about half of her western mesophytic forest; this definition is inclusive of Kuchler's oak-hickory forest but somewhat broader [65]. The Forest and Range Ecosystems of Garrison and others [37] include FRES 15, the oak-hickory ecosystem. This ecosystem includes Kuchler's oak-hickory but also the mosaic of oak-hickory and bluestem prairie (K082), Cross Timbers (K084), Appalachian oak (K104), oak-pine-hickory (K111), and oak savanna (K081) types [37]. This write-up includes mostly information about communities within Kuchler's [51] oak-hickory forest region as it was mapped, with the major focus on oak-hickory communities in the Ozarks. Communities further north (in Wisconsin and Michigan) are included where information is deemed relevant. An oak savanna type was mapped by Kuchler as a potential type for Wisconsin and Michigan (K081), but forests of similar structure and containing mostly the same species occurred widely in presettlement times throughout the prairie-forest interface. Savannas in the central states are therefore included because of their relevance to the fire ecology of oak-hickory forests. Other classifications describing oak-hickory forest include the following: The central hardwood forest [20] Plant geography--with special reference to North America [28] Ordination and classification of western oak forests in Oklahoma [31] A classification of the deciduous forest of eastern North America [65] The study of plant communities [71] Classification and evaluation of forest sites in the Cumberland Mountains [86] The natural vegetation of North America [97] Flora: Oak-hickory forests as described by Kuchler [51] are dominated by white oak, black oak, northern red oak, bitternut hickory (Carya cordiformis), and shagbark hickory (C. ovata). Other trees in alphabetical order of scientific name include pignut hickory (C. glabra), white ash (Fraxinus americana), black walnut (Juglans nigra), black cherry (Prunus serotina), chinquapin oak (Q. muehlenbergii), and basswood (Tilia americana). In the northern parts of the oak-hickory forest, other components are northern pin oak (Q. ellipsoidalis), and shingle oak (Q. imbricaria). In the southern parts of the oak-hickory type, forest tree species include black hickory (C. texana), mockernut hickory (C. tomentosa), southen red oak (Q. falcata), overcup oak (Q. lyrata), blackjack oak (Q. marilandica), Shumard oak (Q. shumardii), and post oak (Q. stellata) [51]. Occasional eastern white pine and eastern hemlock also occur [44]. Burkman and others [17] described oak-hickory forests as the largest and most diverse forest type in the eastern United States. Cover types include post oak-blackjack oak, bur oak (Q. macrocarpa), bear oak (Q. ilicifolia), northern pin oak, chestnut oak (Q. prinus), white oak-black oak-northern red oak, white oak, black oak, and northern red oak [17]. Oak-hickory forest sensu Kuchler is approximately equivalent to the Society of American Foresters white oak (Q. alba)-black oak (Q. velutina)-northern red oak (Q. rubra), white oak, and northern red oak forest cover types [39]. Braun's description of the oak-hickory forest flora resembles Kuchler's. In northern stands, bur oak has great prominence. Northern pin oak is confined to the northern division. Flowering dogwood is a common understory tree. Blueberries and huckleberries (Vaccinium spp.) are abundant shrub layer components, as is ceanothus (Ceanothus spp.). The herb layer often contains poverty oatgrass (Danthonia spicata), bird's-foot viola (Viola pedata), Appalachian mountainmint (Pycnanthemum flexuosum), stone-mint (Cunila origanoides), clasping aster (Aster patens), flax-leaved aster (A. linariifolius), and legumes including downy trailing lespedeza (Lespedeza procumbens), round-leaved tick-trefoil (Desmodium rotundifolium), wild sensitive plant (Chamaecrista nictitans), goat's rue (Tephrosia virginiana), pencil-flower (Stylosanthes biflora), and Sampson's snakeroot (Orbexilum pedunculatum) [12]. The Texas Plant and Wildlife Department published a list containing current information on the stability and distribution of plant communities occurring in Texas. They listed a post oak-black hickory series and post oak-blackjack oak series which occur within Kuchler's oak-hickory forest. In each of these community types, woody species diversity is highest to the east, where closed canopies may form. Post oak-black hickory forms open woodlands over deep sands in the western portion of its Texas range; species composition varies with soil texture and depth and herbs have higher species diversity than stands farther east. Components of this type include white oak, southern red oak, blackjack oak, pines, other hickories, flowering dogwood, yaupon (Ilex vomitorium), and beautyberry (Callicarpa americana). Common components of the post oak-blackjack oak series include cedar elm (Ulmus crassifolia), southern red oak, yaupon, beautyberry, water oak (Q. nigra), black hickory, eastern redbud, and deciduous holly (I. decidua), but composition is variable [89]. Savanna: Some authors define oak savanna as open-grown oaks with 10 to3 80 percent crown cover, with or without a shrub layer, and with a ground cover of grasses and forbs [23]. The understory vegetation of savanna is a mixture of both prairie and forest species, with prairie forbs and grasses more abundant in areas of high light, and forest forbs and woody species more abundant in areas of low light [13,69]. Early descriptions of Missouri savannas are of very open woodlands with ground covers of grasses and forbs and no underbrush or small timber [22]. The herb layer in oak savannas consists mostly of bluestem prairie species, namely big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), Indian grass (Sorghastrum nutans), switchgrass (Panicum virgatum), and many prairie forbs [97]. In Indiana typical savanna herbs include little bluestem, butterfly weed (Asclepias tuberosa), flax-leaved aster, and yellow sedge (Carex pensylvanica) [7]. Savanna trees are oak-hickory dominants; bur oak is common in northern savannas and post oak and blackjack oak are dominant in southern savannas. In Texas junipers (Juniperus spp.) and mesquites (Prosopis spp.) are common associates [97]. Indiana black oak sand savannas are dominated by black oak and white oak. Understory trees include black cherry, sassafras (Sassafras albidum), shining sumac (Rhus copallina), and smooth sumac (R. glabra) [7]. No species is known to be endemic to oak savannas, and relatively few species are modal (i.e., have their highest occurrence in this habitat) [69]. In Wisconsin only six species are considered modal in oak savanna (K081): sunflower-everlasting (Heliopsis helianthoides), kitten-tails (Besseya bullii), cancer-root (Orobanche uniflora), smooth phlox (Phlox glabberima), thick-root buttercup (Ranunculus fascicularis), and mountain deathcamas (Zigadenus elegans) [25]. Geographic Trends: Major vegetational trends from east to west include an increasing importance of oaks, particularly post oak, and a reduction in canopy tree species diversity. Mesic sites and vegetation are more restricted in the western part of the range, particularly with decreased precipitation and increased incidence of drought and fire. Pines increase in importance from north to south [15]. Oak-hickory merges with oak-hickory-pine (K111) in eastern Texas, grades into post oak-blackjack oak savanna (Cross Timbers [K084]) farther south and west in Texas, and alternates with tallgrass prairie on the western edges of its distribution (within the region mapped as mosaic of bluestem prairie and oak-hickory forest) [12]. WILDLIFE : Birds: Few bird species can be considered distinctive of oak-hickory forests. Species frequently encountered in oak-hickory forests include red-bellied woodpecker (Melanerpes carolinus) and cerulean warbler (Dendroica cerulea) [61]. Avian species richness is well documented within the southeastern portion of the oak-hickory forest. Breeding bird communities within southeastern oak-hickory forests range from about 120 species in western Tennessee to about 93 species in the southern extreme [38]. Dominant members of the avifauna of southeastern oak-hickory forests include downy woodpecker (Picoides pubescens), great-crested flycatcher (Myiarchus crinitus), red-bellied woodpecker, eastern wood-pewee (Contopus virens), tufted titmouse (Parus bicolor), Kentucky warbler (Oporornis formosus), red-eyed vireo (Vireo olivaceus), and summer tanager (Piranga rubra) in the Bluegrass region of Kentucky. On the Highland Rim and Knobs of Kentucky and Tennessee, dominant avifauna also include ovenbird (Seiurus aurocapillus), black-throated green warbler (Dendroica virens), black-and-white warbler (Mniotilta varia), blue jay (Cyanocitta cristata), red-eyed vireo, wood thrush (Hylcichla mustelina), Carolina chickadee (Parus carolinensis), hairy woodpecker (Picoides villosus), and eastern wood-pewee. In the Central Basin, Tennessee, dominant avifauna include most of the abovementioned species and yellow-billed cuckoo (Coccyzus americanus). In the Ozark Plateau blue-gray gnatcatcher (Polioptila caerulea) and northern cardinal (Cardinalis cardinalis) occur in addition to previously mentioned species; on the Coastal Plain additional species include Carolina wren (Thryothorus ludovicianus), northern parula (Parula americana), and hooded warbler (Wilsonia citrina). In lowland communities the avifauna is dominated by prothonotary warbler (Protonotaria citrea), American redstart (Setophaga ruticilla), wood duck (Aix sponsa), red-eyed vireo, American woodcock (Scolopax minor), yellow-throated vireo (Dendroica dominica), northern parula, hooded warbler, Acadian flycatcher (Empidonax virescens), pileated woodpecker (Dryocopus pileatus), white-breasted nuthatch (Sitta carolinensis), Carolina chickadee, and tufted titmouse [15]. Probst [75] summarized oak forest bird communities for the eastern United States, including both oak-hickory forests and oak-pine forests. The blue jay and the brown-headed cowbird (Molothrus ater) are species of generalized habitat requirements that are found in almost every stand, regardless of age or structure. However, most birds require particular habitat features. Five categories of birds that are present in oak-hickory forest, based on response to habitat feature include: 1) bark foragers such as northern flicker (Colaptes auratus), white-breasted nuthatch, downy woodpecker, and hairy woodpecker, 2) active gleaners feeding in the tree canopy such as chickadees (Parus spp.), tufted titmouse, and red-eyed vireo, 3) pursuers feeding in tree canopy with a sit-and-wait hunting strategy such as flycatchers (Empidonax spp.) and tanagers (Piranga spp.), 4) ground species associated with shrub and sapling layers such as thrushes (family Muscicapidae), rufous-sided towhee, and ovenbird, and 5) species associated with dense growth of saplings and small trees, mostly warblers (Emberizidae) [75]. Mammals: Southeastern oak-hickory forests generally are low in mammalian species richness with the exception of bats (Chiroptera). The limestone-based geology of the region has fostered a rich cave fauna including little brown myotis (Myotis lucifugus), big brown bat (Eptesicus fuscus), and pipistrelles (Pipistrellus spp.). The white-footed mouse (Peromyscus leucopus) is very common in southeastern oak-hickory forest; other common species include short-tailed shrews (Blarina brevicauda and B. carolinensis), eastern chipmunk (Tamias striatus), eastern gray squirrel (Sciurus carolinensis), eastern fox squirrel (S. niger), gray fox (Urocyon cinereoargenteus), raccoon (Procyon lotor), opossum (Didelphis virginiana), striped skunk (Mephitis mephitis), and white-tailed deer (Odocoileus virginianus) [15]. Reptiles: The garter snake (Thamnophis sirtalis) is very common in oak-hickory forests of the Southeast; other common reptiles include black racer (Coluber constrictor), black rat snake (Elaphe obsoleta), five-lined skink (Eumeces fasciatus), fence lizard (Sceloporus undulatus), rough green snake (Opheodrys aestivus), ring-necked snake (Diadophis punctatus), eastern hognose snake (Heterodon platyrhinos), and eastern box turtle (Terrapene carolina). Reptile species richness generally increases to the south and east of the southeastern oak-hickory region. Amphibians: Moist sites within oak-hickory forest support amphibians, most commonly slimy salamander (Plethodon glutinosus), dusky salamander (Desmognathus fuscus), American toad (Bufo americanus), Fowler's toad (B. woodhousei), spring peeper (Hyla crucifer), and gray treefrogs (H. versicolor and H. chrysoscelis) [15]. ECOLOGICAL RELATIONSHIPS : Species Distribution: Oak-hickory associates tend to clump together on ridges and upper slopes. In grassy openeings and on steep slopes and other dry exposures oak-hickory associates cooccur with plant species such as eastern redcedar (Juniperus virginiana) that are tolerant of xeric conditions [15]. On the Ozark Plateau dry ridges and south-facing slopes are usually occupied by open communities of post oak and blackjack oak. Locally, shortleaf pine (Pinus echinata) may form mixed stands with the oaks. White oak is usually more abundant on north-facing slopes, entering some ridge forests. In the western border area, deep ravines and gorges are occupied by mesophytic species; sugar maple (Acer saccharum), white oak, and northern red oak are abundant. Open forests of low-statured trees with sparse herbs occur on xeric plateaus [12]. On the Ozark Plateau in southeastern Missouri, ordination of forest stands showed a clear separation of upland forests into two groups. Acid upland stands were dominated by black oak, black hickory, and white oak. Other stands (intermediate in elevation and soil acidity) tended to be dominated by chinkapin oak, sugar maple, eastern redcedar, and basswood. White ash and slippery elm (Ulmus rubra) were sometimes present [101]. The eastern deciduous forest reaches its western limit in the central portion of Oklahoma as oak forest and oak savanna. Farther west, occasional stands occur in sandstone canyons in west-central Oklahoma and in the Wichita Mountains of southwestern Oklahoma [31]. These forests are attributed to climatic fluctuations which favored the westward migration of eastern species [12]. Prior to European settlement oak savanna was common in western Oklahoma; these stands now have a closed canopy developed with the advent of grazing and fire suppression. Ordination of forests in this area demonstrated that the vegetation pattern corresponded to a complex moisture gradient. Post oak and blackjack oak codominated forests occuping the xeric end of the gradient, post oak dominated stands located in the middle (slightly more mesic), and forests at the mesic end of the gradient included sugar maple, elm, and Shumard oak-dominated stands [31]. Post oak and black hickory appear to require relatively high levels of nutrients and moisture, while blackjack oak is tolerant of drier, less fertile sites [47]. Phenology: The understory herbs begin growth earliest in spring; the herb stratum is dominated by species that overwinter as rosettes or underground perennating organs [44]. Disturbance and Succession: Oak-hickory forests have a long history of disturbance; the presettlement pattern was a mosaic of seral stands, many due to Native American activities. Continuing and overlapping disturbances in the range of oak-hickory forest arise from the high value of the land for agriculture [15]. In some regions in the eastern United States, current oak dominance is similar to and is related to the importance of oaks in presettlement forests. In other regions, particularly in the Midwest, the current distribution of oaks exceeds that of presettlement vegetation [1]. Much development of oak forests has occurred through a variety of ecological pathways and disturbance patterns [2]. In northern Michigan sites on upland sandy soils that were dominated by eastern white pine, red pine, or mixtures were converted to northern red oak stands after disturbance by logging and fire [46]. In Missouri stands that were oak-pine or pine at the turn of the century are now dominated by scarlet oak (Quercus coccinea) and black oak following heavy cutting of the pine and post-harvest fires [24]. Most oaks are considered early to mid-successional species; there is recent evidence of a widespread potential for oak replacement by more shade-tolerant tree species in mature forests [53,68]. This phenomenon seems to be more prevalent on mesic than on xeric sites [1,2]. There is a strong successional trend to shade-tolerant species such as sugar maple and American beech (Fagus grandifolia) on moist sites [15]. The persistence of relatively shade-intolerant oaks in mesic forests may be related to cycles of synchronous tree death similar to the fir (Abies spp.) waves seen in New England, or shifting mosaic conditions caused by climate changes and/or large-scale disturbance [102]. Changes in fire frequency have had an impact on the species composition and structure of many forests [15]. For further discussion, see FIRE ECOLOGY AND MANAGEMENT. Old-field Succession: Species composition and the successional role of species may vary geographically. Generally, the first 3 years following the last crop are dominated by herbaceous members of the Asteraceae such as fleabanes (Erigeron spp.), asters (Aster spp.), ragweeds (Ambrosia spp.), and goldenrods (Solidago spp.), and grasses such as crabgrasses (Digitaria spp.), bristlegrasses (Setaria spp.), threeawns (Aristida spp.), and panic grasses (Panicum spp.). In many instances, woody species such as persimmon (Diospyros virginiana), elms (Ulmus spp.), hackberries (Celtis spp.) and junipers seed in but are not dominant for a number of years. Broomsedge (Andropogon virginicus) is a common dominant from the fourth year on, and may be a component for over 20 years. On limestone soils, eastern redcedar is a major old-field inhabitant. Oak-hickory and dry open forests of post oak, junipers, blackjack oak, and black hickory eventually develop on at least 50 percent of the relatively dry uplands. Other forest types develop in mesic coves and in bottomlands [15]. Old-field succession occurred on abandoned fields in western Illinois following a period in which the area was heavily cultivated and suffered severe erosion, then converted to pasturage and hay fields. Early succession was dominated by cool-season grasses including Kentucky bluegrass (Poa pratense). Early establishing woody species included boxelder (Acer negundo) and elms. Trees with heavy seeds only later invaded the edges of fields. The most rapid tree development took place in gullies [16]. Savanna Vegetation: Depending on local conditions including fire regime, climate, and settlement patterns, very different descriptions of the same area were sometimes recorded within a few years [69]. The structure of most savannas is highly dependent on fire frequency; savannas are converted to more closed forest in less than 50 years without fire. Further discussion of the relationship of savanna and fire is in FIRE ECOLOGY AND MANAGEMENT [15]. Nuzzo [69] distinguished between open savanna and scrub savanna. Open savanna is a parklike community with widely spaced trees, virtually no shrub layer, and an herbaceous ground layer. Scrub savanna is made up of moderate to dense thickets of oak sprouts within a prairie matrix, with a few fairly dwarfed open-grown trees. Open savannas usually occur on flatter, usually mesic areas, and scrub savannas are generally located on the dry to dry-mesic areas of steeper topography, particularly hillsides, dunes, and ridges. Both kinds of savanna vary in structure through time and space, depending on fire occurrence [69]. The persistence of some high-quality savannas may be related to the droughty soils on which they occur. Some savannas, in particular those located on thin soil or rocky substrates, have survived moderate grazing, exclusion of fire, and competition with alien and woody species. Light grazing may have helped maintain the savanna. Some stands have survived because they have been burned relatively frequently [69]. The roots of savanna trees may show morphological responses to growing in dry soils; bur oak taproots are at least 13.2 feet (4 m) deep in dry soils but are not as deep in moist soils [103].

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