Mexican Blue Oak Shrub Live Oak Pinyon Pine Mountain-mahogany Manzanita Skunkbush Coniferous Forest 68 above 6,000 18 to 30 Transition and start of Canadian Ponderosa Pine Gambel Oak Buckbrush Mountain Muhly Douglas-fir White Fir Aspen Snowberry Mexican White Pine [1]TUCSON MOUNTAIN SECTION Biologists have developed several systems for classifying assemblages of plants and animals over broad regions, and one of the most widely known is that of C. Hart Merriam. Around the turn of the century, he conceived a system of “life zones,” named for the parts of the continent where they are best developed: Tropical, Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic. In Saguaro National Monument, the Lower Sonoran Zone corresponds to the desert scrub type; the Upper Sonoran includes desert grassland transition, oak woodland, and oak-pine woodland; the Transition Zone is equivalent to the ponderosa pine forest; and the poorly represented Canadian Zone has Douglas-fir and white fir. Because Merriam’s system is so widely used, his terms are included in the tabulation on page 20, which summarizes the main characteristics of plant zones in the monument. The best way to appreciate the biotic changes that occur with elevation is to walk or ride a horse to the top of the Rincons. But if your time or energy is limited you can get a quick view of these sequences by driving up nearby Mount Lemmon, in the Santa Catalina Mountains. [Illustration: Saguaro Forest landscape from the scenic drive.] desert plants