Adaptation to the dry environment is key. For example, creosote bush and mesquite have tap-root systems that can extend down in order to take advantage of a year-round supply of ground water. The diversity of Death Valley's plant communities results partly from the region's location in a transition zone between the Mojave Desert, the Great Basin Desert and the Sonoran Desert. This location, combined with the great relief found within the park, supports vegetation typical of three biotic life zones: the lower Sonoran, the Canadian, and the arctic/alpine in portions of the Panamint Range. Based on the Munz and Keck (1968) classifications, seven plant communities can be categorized within these life zones, each characterized by dominant vegetation and representative of three vegetation types: scrub, desert woodland, and coniferous forest. Microhabitats further subdivide some communities into zones, especially on the valley floor.