Petrified Forest National Park, Navajo, Apache, Arizona, United States

A 2005 survey found that 447 species of flora, of which 57 species are invasive, occur in the park. Although the park is known for its fossils and eroded badlands, its main environment is semi-desert shrub steppe. Protected from development and overgrazing for many years, the park has some of the best grassland in northeastern Arizona. In the northern part of the park, the volcanic soils of the Bidahochi Formation support abundant plant life along the Painted Desert rim. In contrast to the relatively bare badlands below, the rim is covered with shrubs, small trees, grasses, and herbs.
The dominant plants in the park include more than 100 grass species, many native to the region. Growing among the grasses are flowering species such as evening primrose, mariposa lily, and blue flax, and shrubs such as sagebrush, saltbush, and rabbitbrush. Among the wide variety of grasses are native perennial bunchgrass, blue grama, sacaton, sideoats grama, bearded sprangletop, and bush muhly. Invasive species that crowd out slower-spreading natives include annual lovegrass and brome (cheat grass).
Trees and shrubs grow in riparian zones along the park's washes. Willows and cottonwoods are the larger plants, joined by rushes and sedges. There the invasive Eurasian tamarisk, also known as saltcedar, threatens native plants by crowding, using most of the available water, and increasing soil salinity by exuding salt through its leaves.