associated with typical Transition species, while the fourth is 

 frequently a characteristic species of the next life zone above, the 

 Hudsonian. The following herbs and shrubs may be considered 

 as belonging to this zone: Bitter Cherry (Prunus emarginata)\ 

 Pinemat Manzanita {Arctostaphylos nevadensis)\Nama lobbii; 

 Hesperochiron californicus; the monkey flower M/mw/w5 lewisii; 

 and the lousewort Pedicularis semibarbata. 



The Hudsonian Life Zone is the timber-line zone, and is fairly 

 well defined as to its upper borders. It has an average altitude of 

 7,000 to 9,000 feet (2,134 to 2,743 m), a mean annual tempera- 

 ture of 45° to 50°F (7° to 10° C), and an average rainfall of 50 

 to 55 inches (127 to 140 cm). The most important index species 

 are Whitebark Pine {Pinus albicaulis). Western Juniper (Juniperus 

 occidentalis), and various herbaceous species. 



The Boreal Life Zone is true alpine. Its altitudinal range varies 

 from 9,000 to 14,500 feet (2,743 to 4,420 m) with a mean 

 annual temperature of 40° to 45°F (4° to 7°C) and an average 

 rainfall of 60 to 70 inches (152 to 178 cm). The characteristic 

 species are almost exclusively herbs. The Boreal Zone presents 

 marked plant formations on the Salmon Mountains, Mt. Shasta, 

 Lassen Peak, and the high Sierra Nevada. It recurs (though 

 represented by few species) on Mt. San Gorgonio in the San 

 Bernardino Mountains and feebly on Mt. San Jacinto in the San 

 Jacinto Mountains. 



Merriam's classification of life zones in North America was an 

 early attempt to subdivide the biota of the continent in a bio- 

 logically meaningful manner. However, his assumption of the 

 all-controlling influence of temperature conditions is a naive one 

 in view of the other variables that affect plant distribution. In 

 other words, Merriam's scheme was too simple in its approach, 

 although it was a valuable pioneer attempt to classify the vege- 

 tation of North America according to ecological principles. 



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