The basically temperate nature of the United States of America means that variety of flora is considerably less than that of Mexico and several of the other nations of the Americas to the south. Pine trees are common and the greatest diversity of plants is found in rain forests in Oregon.
There are about 7,000 species and subspecies of indigenous categorized US flora. The eastern forests contain a mixture of softwoods and hardwoods that includes pine, oak, maple, spruce, beech, birch, hemlock, walnut, gum, and hickory. The central hardwood forest supports oak, hickory, ash, maple, and walnut. Pine, hickory, tupelo, pecan, gum, birch, and sycamore. The spectacular Pacific forest consists of enormous redwoods and Douglas firs. In the southwest are saguaro (giant cactus), yucca, candlewood, and the Joshua tree.
The interior of the continent cannot support large forests and it is here that the central grasslands or prairie dominate.
Locations of deserts in the world straddle the Tropic of Cancer and the Tropic of Capricorn, between 15 and 30 degrees north and south of the equator. In the USA, three of the four major deserts are contained within a geological region called the Basin and Range Province, lying between the Rocky Mountains to the east and the Sierra Nevadas to the west. Desert plants include cactus, grasses, bushes, and flower species.