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over 380 C (1000 F) in the Inner Gorge. Average nighttime sumliler temperatures drop to about 40 to 60 C (low 40s F) on the Kaihab Plateau, the low 10s (jow 50s F) on the Coconino Plateau, and the r:lid-20s (high 70s F) in the Inner Gorge. Average wi nter daytime hi gh temperatures range froP.'! 1 to 60 C (mid-30s to low 40s F) on the Kaibah Plateau, 5 to 60 C (10\" 405 F) on the Coconino Plateau, to about 15 to 180 C (low 60s F) in the Inner Gorge ,wi th nighttime averages at abollt -10 to _70 C (mid-teens F) on the Kaibab Plateau, -4 to _80 C (high teens to mid-20s F) on the Coconino Plateau, to about I) to 20 C (mid-30s F) in the Inner Gorge. Hence, as temperatures fluctuate above and below freezing in the canyon duringr:1LJch of the year, frost action is a major conponent of nechanical weathering and mass wasting. r- t Vegetation Due to the dramatic el evation gradients in the canyon and the resulting v~ria~ions in temperature and precipitation, vegetation can be broadly sukdivided into a number of distinct, vertically differentiated 1 He zones (Merri am 1890; Lowe 1964). In general, the cool, humi d, boreal "Hudsoni on zone" that occurs above 2,500 m (8,200 feet) on the North Rim includes spruce-fir, White fir, and mountain grassland vegetati on cOf"\nuni ti es. The "Transi ti on zone," or ponderosa pi ne com~unity, occurs between about 2,195 and 2,500 m (7,200 and 8,200 feet) on the North Rim and above 2,134 m(7,000feet} on the South Rim. t1erriam's "Upper Sonora" zone" includes the pinyon-juniper and blackbrush scruh communities at between 1,219 and 2,225 m (4,000 and 7,300 feet) and 1,067 and 1,372 m (3,500 to 4,500 feet), respectively. The "Lower Sonoran zone" below 1,219 m (4,000 feet) consists of the r.10have Desert scrub vegetation coml11.lnity and a riparian woodland community along the Colorado River corridor. The vegetation of each site is described in more detail in Chapter 4. Late Cenozoic Clim}te and Vegetation Changes During the Pliocene, as the Grand Canyon \~as forming, climate is thought to havl7 ':leen harsh and dry, wi th the severest desert condi ti ons occurring at about 4 million years ago (Axelrod 1948). Temperatures probably dropperl significantly near the onset of the Pleistocene period 16