The Piney Woods is a temperate coniferous forest terrestrial ecoregion in the Southern United States covering of East Texas, southern Arkansas, western Louisiana, and southeastern Oklahoma. These coniferous forests are dominated by several species of pine as well as hardwoods including hickory and oak. Historically the most dense part of this forest region was the Big Thicket though the lumber industry dramatically reduced the forest concentration in this area and throughout the Piney Woods during the 19th and 20th centuries. The World Wide Fund for Nature considers the Piney Woods to be one of the critically endangered ecoregions of the United States. The United States Environmental Protection Agency (EPA) defines most of this ecoregion as the South Central Plains. Setting The Piney Woods cover a area of eastern Texas, northwestern Louisiana, southwestern Arkansas and the southeastern corner of Oklahoma. They are bounded on the east by the Mississippi lowland forests, on the south by the Western Gulf coastal grasslands, on the west by the East Central Texas forests and the Texas blackland prairies, on the northwest by the Central forest-grasslands transition, and on the north by the Ozark Mountain forests. They receive of precipitation annually. Flora The region has heavy to moderate rainfall, with some places receiving over of rain per year. Longleaf, shortleaf, and loblolly pines, along with bluejack and post oaks, dominate sandhills. A well-developed understory grows beneath the sparse canopy, and includes yaupon holly and flowering dogwood. Pine savannas consist of scattered longleaf and loblolly pines alongside black tupelos, sweetgums, and in acid soils along creeks sweetbay magnolias. Other common trees in this ecoregion include eastern redbud, red maple, southern sugar maple, and American elm. American wisteria, a vine, may cover groves of trees. Two varieties of wetlands are common in the Piney Woods: bayous are generally found near rivers and sloughs are generally found near creeks. In bayous bald cypress, Spanish moss, and water lilies are common plants. Sloughs are shallow pools of standing water that most trees are not capable of growing in. Other species, such as the purple bladderwort, a small carnivorous plant, have found niches in sloughs. A baygall is another type of wetland found the Piney Woods and other forest of the Gulf Coast states in the USA. Hardy species of prickly pear cactus and yucca can be found in the forests where deep sands occur. The indigenous Texas trailing phlox (Phlox