Big Thicket

The Big Thicket is the name given to a somewhat imprecise region of a heavily forested area of Southeast Texas in the United States. This area represents a portion of the mixed pine-hardwood forests or " Piney Woods" of the Southeast US. The National Park Service established the Big Thicket National Preserve (BTNP) within the region in 1974 and it is recognized as a biosphere reserve by UNESCO. Although the diversity of animals in the area is high for a temperate zone with over 500 vertebrates, it is the complex mosaic of ecosystems and plant diversity that is particularly remarkable. Biologists have identified at least eight, and up to eleven, ecosystems in the Big Thicket area. More than 160 species of trees and shrubs, 800 herbs and vines, and 340 types of grasses are known to occur in the Big Thicket, and estimates as high as over 1000 flowering plant species and 200 trees and shrubs have been made, plus ferns, carnivorous plants, and more. The Big Thicket has historically been the most dense forest region in Texas.

Existing literature states that Native Americans were known to have lived and hunted in the area nomadically, but did not establish permanent settlements there before the Alabama–Coushatta settled in the northeast about 1780. However, there is insufficient archaeological evidence to support this claim. What records that do exist could suggest human occupation dating back to the Clovis culture 13,400–12,700 years ago, with numerous era diagnostic points being found in all but one of the counties commonly considered to be in the Big Thicket. Spanish explorers and missionaries had a sporadic presence in the region, however colonization and settlement was not their aim, preferring to establish forts outside of the Region where the French were encroaching from the east (namely around Natchitoches, Nacogdoches, and the lower Trinity river valley). Logging in the late 19th and 20th centuries dramatically reduced the forest concentration. Efforts to save the Big Thicket from the devastation of oil and lumber industries started as early as the 1920s with the founding of the East Texas Big Thicket Association by Richard Elmer Jackson.

Conservatively the area occupies all of Hardin County, most of Polk, and Tyler Counties, and parts of Jasper, Liberty and San Jacinto Counties, including areas between the Neches River on the east, the Trinity River on the west, Pine Island Bayou on the south, to the higher elevations and older Eocene geological formations to the north. Broader interpretations have included the area between the Sabine River on the east and the San Jacinto River on the west including much of Montgomery, Newton, Trinity, and Walker Counties, as well. Several attempts to define the boundaries of the Big Thicket have been made, including a biological survey in 1936 which included over covering 14 counties. A later botanical based study in 1972 included a region of over . This same habitat extends into Louisiana and eastward.

Geography

Physical geography

Geology: The Big Thicket is notable for the unusually high diversity of plants concentrated in a relatively small area. A major factor enabling this diversity is an unusually high diversity of soils. The exposed surface soils are relatively recent, late Cenozoic Era, predominantly Miocene, Pliocene, and Holocene Epoch formations. Other than some slight and uniform tilting, the strata are not greatly deformed. The strata were laid down in several cycles of glaciations and warmer interglacial interludes. Although glaciers never extended as far south as Texas, it was during these cold periods that enough water was frozen in polar regions to lower sea levels and the Big Thicket area was dry land well above sea level. Conversely, during the warmer interludes, enough ice melted to increase sea levels and submerge Southeast Texas in the sea. It was during those warmer periods of submersion that alluvial and delta deposits from the streams and rivers of the continent laid down the succession of strata, each new layer burying and compacting the previous. At some point there was an approximately 1% tilting of the layers. Some say it was caused by the rising of the Rocky Mountains,Fritz, Edward C. (1993) ''Realms of Beauty: A Guide to the Wilderness Areas of East Texas'', revised edition. University of Texas Press, Austin. 120 pp. while others say it was the weight of the successive deposits that eventually caused the Gulf of Mexico to subside. The older Pliocene, Miocene, and Oligocene formations are exposed in the north (Polk, San Jacinto, and Tyler counties), while the more recent Quaternary Period formations are exposed in the south (Hardin and Liberty counties).Geology of Texas, Bureau of Economic Geology, The University of Texas, Austin (Accessed 7 December 2019)

Soils: The soils of the various formations at the surface have shifted and intermingled through erosion and other factors over time and can be locally quite complex. The Big Thicket contains a greater variety of soils than any area of comparable size in the United States. Sources vary, but from 50 to 100 soil types are said to occur in Hardin County alone. The surface soils tend to be slightly to strongly acidic, and only occasionally neutral in much of the Big Thicket Basin. However, calcareous or alkaline soils do occur in the region, particularly along the Trinity River, in the Beaumont Formation to the south, the Fleming Formation to the north, and northern areas of the Willis Formation where the layer is thin and erosion and road cuts have exposed the Fleming Formation. Most plants are sensitive to soil pH levels (acidic, neutral, or alkaline) and many species will only grow in areas with a specific soil type. Claude A. McLeod's study stated that the Big Thicket was "an edaphicmesophytic climax forest" – '' edaphic'', meaning the soil is a greater influence on the plants than the climate, and ''mesophytic'' meaning a medium moisture level.

Topography: The Big Thicket Basin has been described as shallow bowl or dish, tilted to one side, with the high rim on the interior north and low rim on the coastal south. Eastern and western rims consist of the low ridges that divide the Neches River and Sabine River drainages on the east and the Neches River and Trinity River drainages on the west. Elevations range from about in the north to in the south, dropping as low as at confluence of the Neches River and Pine Island Bayou. Northern regions are characterized by low gently rolling hills and well-formed drainage systems of streams and creeks in the landscape. The southern regions are comparatively flat, with poor drainage and support moderately extensive wetlands.Cozine (2004) p. x.

Hydrology: Hardin, Tyler, western Jasper and southeast Polk counties are drained by the Neches River and its tributaries: Village Creek and its smaller tributaries (Beach, Theuvenins, Turkey, Hickory, Kimball, Big Sandy, and Cypress creeks) in the north; Pine Island Bayou and its smaller tributaries (Little Pine Island Bayou and Mayhaw Bayou) in the south. Eastern regions, including eastern Jasper and Newton counties, are drained by several relatively short creeks running into the Sabine River. Western regions including central Liberty, northeastern San Jacinto, northern Walker counties and southwest Polk County (via Kickapoo, Long King, and Menard creeks) are drained by the Trinity River. Areas farther west are drained by the San Jacinto River, with east and west forks. Additional wetlands are extensive, including innumerable nameless swamps, cypress sloughs, oxbow lakes, wetland savannas, and baygall bogs. Floodplains and bottomlands in the south, can retain surface water for days and weeks after rain. The Sabine and Neches rivers flow into Sabine Lake, a natural occurring brackish water estuary, just southeast of the Big Thicket. The other large lakes in the region were constructed in the decades following the Second World War, including B.A. Steinhagen Lake (1947–53) and Sam Rayburn Reservoir (1956–65) on the Neches River drainage, Lake Livingston (1966–69) on the Trinity River, Lake Houston (1953) and Lake Conroe (1970–73) on the San Jacinto River.

Climate: The proximity to the Gulf of Mexico gives the region a climate of mild winters, hot summers, and high humidity most of the year. Average precipitation is a year, with northern areas receiving about , and southern areas receiving about . However, tropical storms and hurricanes can increase annual rainfall to over . Humidity stays well above 60% most of the time and exceeds 90% often. Winters are mild, with nighttime lows averaging and daytime highs . Winter cold fronts can drop temperatures to freezing, but freezes rarely last for more than a few days, sometimes only hours, averaging about 20 mostly nonconsecutive days each year. Snow is rare, occurring only once every ten years or so. Summer temperatures are hot, with nighttime lows averaging and daytime highs averaging . Temperatures exceed about 110 days each year and days with temperatures above and even above are not uncommon. The high summer temperatures with high humidity levels can produce very high heat index numbers.

Human geography

Population: In 2010, the United States Census Bureau reported a population of 54,635 for Hardin County, with an average of in a county of . The larger towns (and populations from 2010 census) in Hardin County are Kountze, the county seat (2,123), Lumberton (12,448), Silsbee (6,611), and Sour Lake (1,813). Smaller unincorporated communities in the Hardin County include Batson, Honey Island, Saratoga, Thicket, Village Mills, and Votaw.United States Census Bureau
(Accessed December 2019)

Tyler County had a total population of 21,766, with an average population of in . The larger towns in Tyler County are Chester (312), Colmesneil (596), Warren (757), and Woodville, the county seat (2,586) with smaller unincorporated communities including Doucette, Fred, Rockland, and Spurger.

Polk County had a total population of 45,413, with an average of in an area of in 2010 including Livingston (5,335), the county seat and the largest town in the county.

Larger towns and cities in the greater region include Beaumont, Jefferson County, located at the southeast corner of the Big Thicket area with a population of 118,296 and Liberty, Liberty County (8,397), on the southern edge of the Big Thicket area. Houston, Harris County, with a population of 2,320,268 (greater metropolitan area over 7,000,000) is one of the largest metropolitan areas in the US with its central, downtown area located by air southwest of the town of Kountze, Hardin County.

Economy: Timber, oil and gas, and agriculture continue to be significant to the economy of the area. Many people commute to work in oil refineries and related industries in the Beaumont area. Other sources list industries providing significant employment in Hardin County as: educational, health and social services, construction, recreation, accommodation and food services, finance, insurance, real estate, and rental and leasing.

Biology

Ecosystems

On a larger scale of ecoregions of the United States, the Big Thicket is part of the Piney Woods. The Big Thicket is located in the southwest of the Piney Woods and transitions into the Western Gulf coastal grasslands immediately to the south. To the west are the Texas Blackland Prairies and East Central Texas forests (a.k.a. post-oak savanna). Within the Big Thicket, a major component in the high diversity of plants and animals is the variety of ecosystems present in a relatively small area. The National Park Service identifies eight plant communities in Big Thicket National Preserve.National Park Service, Big Thicket National Preserve
Plant Communities
Some biologist and plant ecologists identify as many as nine, ten, or eleven ecosystems in the Big Thicket. In nature these ecosystems are not always found in pure, large tracts of land, and often occur in intertwined and checkerboard mixes and ecotones, transitioning into one another. Ecosystems do not have universal standardized names. Many ecologist, botanist, zoologist and others have studied and classified ecosystems in the Big Thicket region (and elsewhere), defining and naming these communities as relevant to their own disciplines and studies.P. A. Harcombe, P. A., J. S. Glitzenstein, R. G. Knox, S. L. Orzell, & E. L. Bridges. (1993)
Vegetation of the Longleaf Pine Region of the West Gulf Coastal Plain. 83-104 pp.
In Hermann, Sharon M. (ed.). Proceedings of the Tall Timbers Fire Ecology Conference J. No. 18, The Longleaf Pine Ecosystem: ecology, restoration and management. Although the exact names, definitions, and boundaries might vary (subdivided, compounded, or overlapping), below are summaries of ten major ecosystems recognized by most who have studied and written about the Big Thicket, starting with higher elevations in the north, and in a generalized way, moving to lower elevations in the south.

Uplands (aka: longleaf pine-bluestem uplands; dry upland forests;Bridges, E. L. and S. L. Orzell. 1989. Longleaf pine communities of the West Gulf Coastal Plain. Natural Areas Journal 9:246-263. longleaf pine uplands) In its natural state, this ecosystem is often described as having a park-like appearance, with widely spaced pine and oak trees, and grasses growing in the partially shaded open areas. Longleaf pine-bluestem grass uplands are most extensive in northern areas, on higher elevation plateaus between streams and dry upper slopes, where the soils of the Catahoula, Fleming, Willis, and Bentley formations often have several feet of moderately coarse sand over an iron oxide clay or plinthite hardpan stratum. The permeable sands and topography provide good drainage in the uplands which are relatively dry. Smaller fragments of this type are also found throughout the south where higher ridges, hills, and knolls occur. Longleaf pine (''Pinus palustris'') is a common species in this ecosystem and one of the dominant trees along with shortleaf pine (''Pinus echinata'') and a variety of oaks (''Quercus''). Understory shrubs include flowering dogwood (''Cornus florida'') and American holly (''Ilex opaca''). Plant ecologist Geraldine Watson stated the U.S. Forest Service grass herbarium in Louisiana has over 200 species of grasses from this range and various bluestem grasses (''Andropogon'') are the dominant ground cover. Wildflowers like purple pleat-leaf ('' Alophia drummondii'') and bird-foot violet ('' Viola pedata'') grow in the lighter, semi-shaded areas, while Texas Dutchman's-pipe ('' Aristolochia reticulata'') and false foxglove ('' Aureolaria grandiflora'') grow in the deeper shade. Streams flowing through these areas can support large stands of southern bracken fern (''Pteridium aquilinum''). Longleaf pine, bluestem grass, and many other plants in these areas are pyrophytic (adapted and resistant to wildfire), and wildfires are a critical factor in maintaining this ecosystem. Historically, naturally occurring fires would periodically eliminate non-pyrophytic plant species encroaching on this plant community. However, decades of fire suppression has allowed a wide mixture of species to invade and in time has turned many of the upland localities into beech-magnolia-loblolly slope forest or oak-hickory forest. Additionally, many areas clearcut for lumber have been replanted with rows of the faster growing, non-native slash pine (''Pinus elliottii'') eliminating much of the longleaf pine-bluestem grass uplands from Texas.

Sandylands (aka: arid oak-farkleberry sandylands; sand ridge savanna; sandhill pine forests; arid sandylands): Sandylands resemble and have much in common with upland forest, with open grassy areas and widely spaced trees. However, the fundamental difference lies in the soils. These areas reside on deep sand dunes deposited by ancient seas and rivers. In contrast to the uplands where a few feet of coarse sand sits on a hardpan stratum, sandylands sit on layers of fine sand with thin layers of siliceous silt that can be over a hundred feet deep, with little clay and low fertility. Although these areas receive the same rainfall as the rest of the region, water drains quickly in the deep sand which dries in the sun leaving an arid surface. The ground cover can be meager in places with sparse grasses, lichen, or even exposed sand in some spots. Plants adapted to arid environments like Louisiana yucca ('' Yucca louisianensis'') and prairie prickly pear (''Opuntia macrorhiza'') are conspicuous indicators of sandylands in the humid Big Thicket climate. Although found throughout the region, some of the largest and best examples are in the vicinity of the Neches River and Village Creek. In the more arid sites bluejack oak (''Quercus incana''), sand post oak ('' Quercus margaretta''), north blackjack oak (''Quercus marilandica''), post oak (''Quercus stellata''), black hickory ('' Carya texana''), and mockernut hickory (''Carya tomentosa'') are dominant hardwoods. With increasing moisture levels longleaf pine (''Pinus palustris'') and loblolly pine (''Pinus taeda'') emerge over the oaks. Longleaf pine is characteristic of sandylands, but not always dominant. Farkleberry ('' Vaccinium arboreum'') is the dominant shrub in the understory. Several rare wildflowers with limited distributions in Texas can be found in this environment; examples include Oklahoma prairie clover ('' Petalostemum griseum''), Carolina vervain (''Stylodon carneus'' = '' Verbena carnea''), Hooker palafoxia ('' Palafoxia hookeriana''), green-thread ('' Thelesperma flavodiscum''), and trailing phlox ('' Phlox nivalis texensis'') which is also an endangered species. Like the uplands, periodic wildfires are essential to keeping the understory open and preserving this ecosystem. These areas are said to have been preferred habitation sites for the nomadic Atakapa-Ishak people in Pre-Columbian times.

Savannas (aka: longleaf - black gum savannahs; wetland longleaf pine savannas; wetland pine savannas; pine savannah wetlands) If the sandylands described above resemble a well drained and arid version of the uplands, the savanna wetlands might be described as a poorly drained, wet version. This ecosystem is characterized by the same open spaces with widely spaced pine trees as uplands and sandylands, but differs in being very flat, with a relatively thin layer of low nutrient, fine, packed sand, sitting on hardpan which is very near the surface. The nearly impervious hardpan both inhibits drainage in wet periods as well as prevents moisture from rising in dry periods. The severity of the fluctuating water table, along with occasional low-intensity ground fires that are necessary to sustain this ecosystem, limit the plants that can grow there. Savanna wetlands are most often found to the south of the uplands, on the Montgomery and recent Bentley formations, although they are not entirely restricted to this zone. Longleaf pine (''Pinus palustris'') along with black gum (''Nyssa sylvatica''), often stunted, make up much of the overstory, being among the few trees that grow in the severe conditions. Ground cover is dominated by rushes, sedges ('' Rhynchospora'' and '' Scleria''), and grasses including ''Andropogon'', ''Aristida'', ''Muhlenbergia'', and little bluestem ('' Schizachyrium scoparium''). Within the flat savannas, the slightest change in elevation can produce significantly different vegetation. Occasional mounds with two or three feet of sand, support upland habitat, while depressions of just a few inches can hold acid bogs, with low pH levels and low levels of nutrients such as nitrogen, potassium, phosphorus, and calcium. Even the peripheral areas around acid bogs, distinguished by subtle sloping elevations of only an inch or two, sustain a distinct zone of vegetation. ''Sphagnum'' mosses and carnivorous plants are adapted to these environments, including the pitcher plant ('' Sarracenia alata''), annual sundew ('' Drosera brevifolia''), rush bladderwort ('' Utricularia juncea''), and small butterwort ('' Pinguicula pumila''). A number of orchids are found in savannas including the grass pink orchid ('' Calopogon tuberosus''), yellow fringed orchid (''Platanthera ciliaris''), snowy orchid ('' Platanthera nivea''), and snake mouth orchid ('' Pogonia ophioglossoides'').

Slope forests (aka: beech-magnolia-loblolly slopes; mesic upland forests: lower slope hardwood pine forest; beech-magnolia-loblolly pine association): Slope forest have a wide mix of conifers, deciduous, and evergreen broad-leaved trees occupying the gentle slopes between the wet bottomlands and the dryer uplands. Small spring-fed streams drain the slopes before reaching the larger creeks. The closed canopy filters the light reaching the forest floor where an ample layer of leaf litter holds water, creating a mesic understory. The soils are relatively fertile with fine, loamy, sands and are well to moderately drained. The purest examples of slope forest are found on the soils of the Willis Formation with red clay in Jasper, Newton, Polk, and Tyler counties, but they extend well into the lower, southern regions where soils and topography are suitable, particularly along the slopes of the waterways. A variety of trees co-dominate slope forest and these can vary with elevation and moisture. In higher and less humid areas, forest shortleaf pine (''Pinus echinata''), loblolly pine (''Pinus taeda''), white oak (''Quercus alba''), and southern red oak ('' Quercus falcata'') co-dominate, but sweetgum (''Liquidambar styraciflua'') and blackgum (''Nyssa sylvatica'') are also common. More humid areas tend to shift to American beech (''Fagus grandifolia''), southern magnolia (''Magnolia grandiflora''), and American holly (''Ilex opaca'') co-dominating with loblolly pine (''Pinus taeda''), but include a wide variety of other hardwoods such as white oak (''Quercus alba'') and sugar maple (''Acer saccharum''). In the lowest, flatter areas of the south, beech trees tend to drop out and are replaced by chestnut oak or basket oak ('' Quercus michauxii'') and laurel oak ('' Quercus laurifolia''). Understory trees and shrubs include Hercules-club or toothache tree ('' Zanthoxylum clava-herculis''), hoary azalea ('' Rhododendron canescens''), redbud (''Cercis canadensis''), and silver bell ('' Halesia diptera''). A rich diversity of wildflowers are found in slope forest such as three jack-in-the pulpits ('' Arisaema dracontium'', '' A. quinatum'', '' A. triphyllum''), Indian pipe (''Monotropa uniflora''), and wake-robin ('' Trillium gracile''). Orchids include the yellow lady's-slipper ('' Cypripedium calceolus''), spring coral-root ('' Corallorhiza wisteriana''), whorled pogonia ('' Isotria verticillata''), southern twayblade ('' Listera australis''), crippled crane fly ('' Tipularia discolor''), and three birds (''Triphora trianthophora''). Wildfires are much less important to the ecology of the humid slope forest than in the uplands and sandylands, both of which will slowly revert into slope forest when fire is suppressed and slope vegetation that is not tolerant of fire takes hold and fills in the open areas.

Floodplains: (aka: sweet gum - oak floodplains; wet forests: floodplain hardwood forest; oak-gum floodplain; stream floodplains): Often referred to as bottomlands, floodplains are low laying areas adjacent to the larger creeks, bayous, and rivers which overflow and flood with some regularity. The floodwaters deposit rich alluvial soils, moderately permeable silty, sandy loams that support a wide diversity of vegetation. The major floodplains in the region were formed during the Wisconsin glaciation, when sea levels were low and great rivers cut deep and wide channels in the landscape. These channels are now largely filled with sediment and deposits through which the present day rives, much smaller than in the past, wind and meander. Floodplains often occur in terraces with various swales, ridges, and levees stepping down in tiers to the water level. Some ecologists subdivide these terraces into separate ecosystems, including but not limited to, baygalls on the outer margins of higher terraces, and cypress sloughs in the lowest areas. Pine trees are uncommon or absent in the floodplains, which are dominated by deciduous canopy trees including water oak (''Quercus nigra''), chestneu or basket oak ('' Quercus michauxii''), willow oak (''Quercus phellos''), cherrybark oak ('' Quercus pagoda''), and red oak ('' Quercus falcata''). Other important trees include the overcup oak ('' Quercus lyrata'') which can grow in and around standing water, sweetgum (''Liquidambar styraciflua''), blackgum (''Nyssa sylvatica''), water hickory ('' Carya aquatica''), and red maple (''Acer rubrum''), often with a dense mid-story of ironwood ('' Carpinus caroliniana''). Among the many wildflowers are three irises, yellow iris (''Iris pseudacorus''), southern iris ('' Iris virginica''), and short-stem iris ('' Iris brevicaulis''), as well as the fragrant ladies tresses orchid ('' Spiranthes odorata''), blue jasmine ('' Clematis crispa''), and the aquatic spatterdock ('' Nuphar luteum''). Although the floodplains in the region share much in common with one another, each river varies with some unique elements and plant species. The Trinity River basin in particular stands out in contrast to others with calcareous, relatively high pH, alkaline soils supporting vegetation that is seldom seen in other watersheds, including bois d'arc also called horse apple (''Maclura pomifera''), blueberry hawthorn ('' Crataegus brachyacantha''), cedar elm ('' Ulmus crassifolia''), and Texas sugarberry ('' Celtis laevigata'').

Baygalls (aka: bay-gallberry holly bogs ''in part''; Acid bog baygalls): With dense undergrowth and twisted vines growing in stagnant, blackwater, acid bogs, under a shadowy canopy of swamp tupelo and cypress, baygalls are often said to exemplify and epitomize the Big Thicket. Baygalls typically form at the base of slopes where seeps, springs, and rainwater drain onto the margins of the flat floodplains and bottomlands, away from the main channels. The drainage from the slopes maintain saturated, muddy soils in these already damp and humid areas, where pools, bogs, and stagnant drainages form in depressions on the flat landscape. With sphagnum mosses and high peat content, baygall soils and water have very high acidity, pH levels of 4.5 are common. The high acid levels are a significant factor in distinguishing baygalls from the floodplain and flats ecosystems. Although baygall waters are generally shallow and torpid, they sometimes form small, highly acidic blackwater streams, slowly moving into the larger creeks and bayous. Hanging bogs occur where water pools on irregular flat and low areas on the slopes above the bottomlands. The name baygall is derived from sweetbay magnolia (''Magnolia virginiana'') and sweet gallberry holly ('' Ilex coriacea''). These and swamp titi (''Cyrilla racemiflora'') are dominant and other common shrubs include southern bayberry (''Myrica cerifera''), water willow ('' Decodon verticillatus''), red bay (''Persea borbonia''), and Virginia sweetspire ('' Itea virginica''). Vines like muscadine grape (''Vitis rotundifolia''), supplejack or rattan-vine ('' Berchemia scandens'') can grow impressively large. Larger trees include swamp tupelo ('' Nyssa biflora'') and bald cypress (''Taxodium distichum''). In addition to the prominent fragrant water-lily ('' Nymphaea odorata''), baygalls harbor many small and obscure wildflowers such as four species of carnivorous bladderworts (''Utricularia''), and saprophytic species like burmannia ( Burmannia biflora) and nodding-nixie ('' Apteria aphylla''). Some baygalls and hanging bogs are no more than a small pool, while others can be a mile across. One author states that the Jack Gore Baygall Unit of the BTNP, at 12 square miles, is the largest baygall in the world.

Flats (aka: palmetto-oak flats; wet forests; palmetto-hardwood flats): Dense stands of dwarf palmettos (''Sabal minor'') are an indicator of this ecosystem and it can have an exotic and tropical look with epiphytes draping from the trees, like Spanish moss (''Tillandsia usneoides'') and resurrection fern (''Pleopeltis polypodioides''). Some of the most extensive and well preserved palmetto-oak flats are found around Pine Island Bayou and Little Pine Island Bayou, the area known as the "Traditional Thicket" or "Hunter's Thicket". Flats, often several miles wide, are old river channels and floodplains and their associated bars and levees, filled with centuries of sediments and alluvium deposits. They are some of the flattest and lowest areas in the Big Thicket. Flats are typically poorly drained areas with very deep calcareous soils of a high clay content having vertisols properties, meaning the soil moves, shrinking and swelling with moisture content. Palmetto-oak flats alternate between flooded and dry conditions, with a few inches of water standing for days, weeks, even months after rains, to dry periods in which the soils dry, leaving large and deep cracks in the hard baked surface. The canopy is made up of several hardwoods species of which the swamp chestnut oak or basket oak ('' Quercus michauxii''), laurel oak ('' Quercus laurifolia''), overcup oak ('' Quercus lyrata''), swamp post oak ('' Quercus similis''), and cedar elm ('' Ulmus crassifolia'') are dominant species. Other trees and shrubs filling in the canopy and understory include sweetgum (''Liquidambar styraciflua''), Carolina ash (''Fraxinus caroliniana''), blueberry hawthorn ('' Crataegus brachiacanthua''), and arrowwood viburnum ('' Viburnum dentatum''). The shade of the canopy and the severity of the wet and dry conditions limit the species that grow in the understory. However the dwarf palmetto (''Sabal minor'') thrives, typically growing to about , although occasional specimens can be found up to tall. Some regard the larger specimens as a different species or variety, the Louisiana palmetto ('' Sabal louisiana''), others do not recognize it a valid species. Other understory species found on the flats are creeping spot-flower (''Spilanthes americana''), Missouri ironweed ('' Vernonia missurica''), lance-leaved water-willow (''Justicia lanceolata''), and inland sea oats ('' Chasmanthium latifolium''). The ancient levees and bars deposited along the old river channels, varying in size from less than an acre up to , are slightly higher with better drainage supporting stands of loblolly pine (''Pinus taeda'') and water oak (''Quercus nigra'') with understory species such as Hooker eryngo ('' Eryngium hookeri''), and sharp-sepal penstemon ('' Penstemon tenuis''). Bald cypress (''Taxodium distichum'') and water tupelo (''Nyssa aquatica'') grow in and near permanent or semi permanent water and the disc water-hyssop ('' Bacopa rotundifolia'') may be found floating or growing at the water's edge. Drainage patterns are significant in shaping and maintaining these ecosystems and drainage alterations have reduced the historical size of this community.

Cypress slough (aka: sweet gum - oak floodplains ''in part''; swamp-cypress tupelo forest; slough, oxbows, and cypress-tupelo swamps): Recognized as a distinct ecosystem by some, regarded as a component found within the flats, floodplains, and baygall bogs by others, cypress sloughs with older bald cypress (''Taxodium distichum'') and their buttresses and protruding knees are impressive environments. Cypress sloughs occupy low laying areas in the floodplains of creeks and rivers set back from the main channels, with still or very slow moving permanent, or semi permanent, water, like secondary channels, ox-bow lakes, sloughs, and ponds. Cypress sloughs are found throughout the region but, are perhaps more extensive in the low laying south. The dominant trees are the bald cypress (''Taxodium distichum'') and the water tupelo (''Nyssa aquatica''). Water tupelo often grow in deeper water and bald cypress in shallower parts or at the periphery, although both are adapted to growing in permanently submerged areas or on dry land with sufficient moisture. Understory trees and shrubs found growing around cypress sloughs include water or Carolina ash (''Fraxinus caroliniana''), water elm ('' Planera aquatica''), river birch (''Betula nigra''), eastern hop-hornbean (''Ostrya virginiana''), and buttonbush (''Cephalanthus occidentalis''). Plants like lizard's tail ('' Saururus cernuus'') and burhead ('' Echinodorus cordifolius'') can be seen growing in shallow water with cardinal flower ('' Lobelia cardinalis'') and sensitive fern (''Onoclea sensibilis'') at the water's edge. Bald cypress trees typically live for 600 years or more and some are known to have lived 1,200 years. Cypress is a valuable hardwood, particularly prized for its resistance to rot and highly valued for shipbuilding, docks, and bridges. Consequently, this ancient swamp forest, once common in the region, have been greatly reduced because of logging in the nineteenth century. Trees over 100 years old are rare in the area now.

Mixed-grass prairies: Prairies (small patches are sometimes called coves), are sometimes included as a Big Thicket ecosystem. These are isolated fragments of coastal prairie more typical of Chambers and Jefferson counties to the south, in areas mostly found where the forest and coastal prairies transition. Prairies are dominated by grasses and herbaceous plants, some are the same as those found longleaf pine savannas, but others are distinctive like Indian grass ('' Sorghastrum nutans'') and prairie bluebell ('' Eustoma grandiflorum''). Common grasses of these areas include Indian grass ('' Sorghastrum avenaceum''), eastern gama-grass (''Tripsacum dactyloides''), and tall dropseed ('' Sporobolus asper''). One author distinguished prairies from longleaf pine savannas by the presence of pimple mounds or mima mounds. Prairies and coves were once found in the Big Thicket with some consistency, particularly in higher elevations in the south, and specifically in higher southwestern areas between the Pine Island Bayou and the Trinity River drainages. However, natural wildfires are an important factor in maintaining these areas and when fire is suppressed, trees and shrubs quickly grow in and fill in these areas. The Chinese tallow tree ('' Triadica sebifera'') is a particularly aggressive and problematic invasive species in prairie habitat in the region. Prairies are also often the first areas to be developed for farming and residential housing. Unfortunately, prairies in the Big Thicket are now rare.

Roadsides and river edge: Although not a natural or even stable plant assemblage, some have recognized roadsides as an ecosystem in the Big Thicket. Proponents of the concept have identified a number of distinguishing characteristics of roadsides from the other ecosystems. Sunlight and heat can reach the ground along the clear highway easements, unlike most areas where the light is filtered by the forest canopy before it reaches the ground. Roadsides receive more rainwater, diverted from the pavement and often retained for a time in roadside ditches. Soil types not normally found at the surface are often exposed with the construction of roads, particularly deep road cuts in the low hills in the north that expose calcareous soils or where the formations are thin near transition zones supporting the growth of plants not normally occurring in the surrounding habitat, allowing for more diverse plants species to grow. The "corridor effect" of traffic, both human (vehicles, tourists, and road crews) and birds and other animals, inadvertently dispersing seeds and roots along roadsides from other areas. And finally, regular cycles of mowing and cutting, as well as the use of herbicides can both inhibit and benefit various wildflowers and plant species growing there. Most plant species at any given roadside represent the association immediately adjacent to that locality, but often have a few odd of species from other areas. Roadsides are a convenient and accessible places to view wildflowers, regardless of whether they are true ecosystems. In the past some (including the National Park Service) have discussed the river edge as an ecosystem in the Big Thicket however, most ecologist do not recognize river edges as an ecosystem distinct from the one a river runs through.

File:Pine uplands after a fire, Big Thicket NP, Hickory Creek Unit, Tyler Co. TX; 23 Mar 2020.jpg, Longleaf pine (''Pinus palustris'') upland habitat after a recent fire. Big Thicket National Preserve, Hickory Creek Unit, Tyler Co. Texas; 23 Mar 2020
File:Big Thicket National Preserve, Hickory Creek Unit, Tyler Co. TX.jpg, The same vicinity as previous image five months after the fire. Big Thicket National Preserve, Hickory Creek Savannah Unit, Tyler Co. Texas; 20 Aug 2020
File:Sphagnum Bog, Big Thicket NP, Turkey Creek Unit, Tyler Co. TX; 1 May 2020.jpg, Sphagnum bogs or "acid bogs" with pH levels as low as 4.5 occur in the region. Big Thicket National Preserve, Turkey Creek Unit, Tyler Co. Texas; 1 May 2020
File:Blackwater and muddy water, Big Thicket NP. Jack Gore Baygall Unit, Hardin Co. TX; 3 Apr 2020.jpg, Baygall blackwater (left) mixing with typical muddy water (right). Big Thicket National Preserve, Jack Gore Baygall Unit, Hardin Co. Texas; 3 April 2020
File:Village Creek floodplain. Roy Larson, Sandyland, Hardin Co. TX ; 30 Apr 2010.jpg, Lush vegetation in the Village Creek floodplain. Roy E. Larsen Sandyland Sanctuary, Hardin Co. Texas; 30 April 2010

Exact numbers of plant and animal species occurring in the Big Thicket region are difficult to state objectively due to a number of factors including: the imprecise boundaries of the area; which (if any) accidental and vagrant species to count; whether to count extinct and extirpated species; whether to count introduced and invasive species; migratory vs. resident species; frequent taxonomic changes and taxonomic instability. These and other factors all contribute to what sometimes appear as inconsistent or contradictory counts and numbers.

Flora

The area contains over 100 species of trees and shrubs, with longleaf pine (''Pinus palustris'') once dominating the region. Big Thicket National Preserve has introduced programs to re-establish this dominance, including one of the US's most active prescribed burn programs. With the National Park Service's centennial occurring in 2016, efforts were made to plant between 100,000 and 300,000 longleaf pines. The National Park Service lists more than one thousand species of flowering plants and ferns that can also be found in the thicket, including 20 orchids and four types of carnivorous plants.

File:Coral Bean (Erythrina herbacea) photographed in Tyler County, Texas, USA (12 May 2012).jpg, Coral bean ('' Erythrina herbacea'') Tyler County
File:False Gromwell (Onosmodium bejariense) photographed in Walker County, Texas, USA (26 April 2014).jpg, False gromwell ('' Onosmodium bejariense'') Walker County
File:Rose Mallow (Hibiscus sp.) photographed in Hardin County, Texas, USA (6 November 2019).jpg, Rose mallow (''Hibiscus'' sp.) Hardin County
File:Pinewoods Lily (Alophia drummondii) photographed in Hardin County, Texas, USA (11 May 2012).jpg, Pinewoods lily ('' Alophia drummondii'') Hardin County
File:Turkcap (Malvaviscus arboreus var. drummondii) photographed in San Jacinto County, Texas, USA (16 August 2014).jpg, Turkcap ( ''Malvaviscus arboreus'' var. ''drummondii'') San Jacinto County
File:Ladies'-tresses Orchid (Spiranthes sp.) photographed in Tyler County, Texas, USA (11 May 2012).jpg, Ladies'-tresses orchid (''Spiranthes

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