Texas has between 255 and 281 species of native trees in ninety-one genera and forty-seven plant families. Tree experts rarely agree about the figures because of controversy over the identification of some trees. Does eastern sugar maple (Acer saccharum) actually occur in Texas? Many Texans say yes, but eastern taxonomists say that the Texas tree is the Florida sugar maple (A. barbatum). Dahoon (Ilex cassine) and myrtle (I. myrtifolia) holly were recorded in Texas almost 100 years ago and have not been seen since. The definition of a tree as a plant with a trunk three inches in diameter at 4½ feet in height, with one stem only, the stem attaining thirteen feet in height, is controversial and omits some plants that we normally think of as trees. The anacacho orchid tree (Bauhinia congesta), for instance, is not considered a tree by some experts, and yet in many landscapes it fits the definition of a tree. Almost 50 percent of the nation's arboreal species occur in Texas, a large number primarily due to diverse climate and soils. East Texas forests are a continuation of southern pine forests and are essentially oak-hickory deciduous forests planted to pines. The coastal prairies and plains have trees that can survive periodic flooding and poor drainage. The Rio Grande plains have elements of the Mexican flora, as does the southern Trans-Pecos. In the mountains and highlands of the Trans-Pecos are elements of the Rocky Mountain flora. The High Plains is a short-grass prairie and is essentially treeless. The plains from the foot of the Caprock to the Western Cross Timbers are now more or less covered by shrubby mesquite (Prosopis glandulosa), with other scattered trees along intermittent watercourses. The Cross Timbers, both east and west, and the post oak savannah are part of the true prairie association with tall grasses in the openings and understory. The dominant trees of these regions are post oak (Quercus stellata) and blackjack oak (Q. marilandica). In the past, trees occupied only the creek banks and river valleys of the blacklands and Grand Prairies (see GRAND PRAIRIES AND THE LAMPASAS CUT PLAIN), areas that were grasslands but are now mostly in cultivation to the east with ranching on the west. These prairie areas have remnants of the Midwest flora, such as bur oak (Q. macrocarpa). The Edwards Plateau, which was once a grassland, is now rich in tree species, the dominant tree being the escarpment live oak (Q. fusiformis). More species of oak-thirty-nine-than of any other tree grow in Texas. Five oaks are dwarf and classed as shrubs. Twenty-four are white oaks (i.e., they produce acorns yearly) and fifteen are black (acorns need two years to mature in most cases). There are fifteen oaks less than thirty feet tall, ten oaks less than fifty feet tall, and fourteen oaks over fifty feet in height. Nineteen of the oaks are "live" oaks; their leaves remain green through most of the winter. Nineteen of the oaks are adapted to the acid soils and high rainfall of East Texas, sixteen are adapted to the alkaline soils of western Texas, and four thrive on the igneous soils and high altitudes of the mountains. Any area in Texas has an oak that can be grown for ornamental purposes. Other genera with several species include: Crataegus (hawthorns, 16–33), Fraxinus (ash, 9), Juniperus (juniper, 9), Pinus (pines, 8, with 5 species from West Texas and 3 from East Texas), Carya (hickories, 8), Acacia (acacia, 7), Populus (cottonwoods and poplars, 7), Acer (maples, 5), Prunus (plums, 5), Yucca (yucca, 5), and Sabal (palms, palmetto, 2). Families of plants with several tree species include Fagaceae (beech family), Rosaceae (rose family), Leguminosae (legume family), and Salicaceae (willow family). For sheer numbers of a species, no tree can even come close to the ubiquitous mesquite. This tree has now occupied well over sixty million acres of the Texas landscape and shows no sign of declining. After the mesquite, three oaks would contest for second place: from Central Texas to the Sabine, post oak; in deep East Texas, the southern red oak (Q. falcata), and in the Edwards Plateau escarpment, live oak.