Iiiglier in Mexico, l)ut this is offset l)y a higher rate of 

 evaporation. West of the Sierra Nevada precipita- 

 tion comes principally during the winter months, as 

 it also does in the Mediterranean region. 



PLA.NT ASSOCIATIONS IN 

 NORTH AMERICA 



Pinus-Quercus association : pine-oak 

 woodland. On momitain slopes of central and 

 northern Mexico : oak scrub, mostly evergreen, at 

 lower elevations grades into pine-oak woodland with 

 some juniper at higher elevations and then into the 

 Mexican pine forest; contains a rich variety of spe- 

 cies (Gentry 1942. Leopold 1950, Marshall 1957). 



Quercus-Quercus association : oak woodland. 

 Broad-leaved mostly evergreen oaks with Digger pine 

 in certain habitats ( Aliller 1951 ) ; mostly west of the 

 Sierra Nevada but e.xteiiding iiortli into Oregon and 

 Washington. 



Pinus-Juniperus association: pifion- juniper 

 woodland. Pines and junipers of several species 

 from eastern slopes of Sierras and Cascades across 

 Great Basin to \Vyoming and New Mexico (Wood- 

 bury 1947, Woodin and Lindsey 1954). 



PALEO-ECOLOGY 



The various types of woodlands in North 

 America were probably derived from the mixed pines 

 and oaks of the Madro-tertiary flora. The pifion- 

 juniper woodland is a segregation that became 

 adapted to the cold winter climates of the Rocky 

 Mountains and Great Basin. It was more widely 

 dispersed and found at lower altitudes during the 

 pluvial Wisconsin glacial period than it is at present. 

 The oak woodland during lower Pliocene was widely 

 distributed over the central and southern portions of 

 the Great Basin but with the trend toward colder 

 winters and decreased rainfall, the oak woodland 

 came to be restricted to the moister mountain habi- 

 tats within the desert and to Pacific coast regions 

 with winter rain and mild temperatures (Axelrod 

 1950, 1957). There is also some evidence that wood- 

 land vegetation including oaks extended around the 

 north side of the Gulf of Mexico as far as Florida 

 (Pitelka 1951). 



WOODLAND BIOCIATION 



The animal life of woodland communities 

 in western North America is not highly distinctive. 

 The trees, being broad-leaved or needle-leaved, at- 



ePTc:^^*; 



%a^M*<i^ji^.iiXUf^.''. 





:%^i 



FIG. 23 7 Pinon-juniper woodland in Utah. 



tract species from the adjacent deciduous forest-edge 

 (riparian woodland) and montane forest biociations. 

 Since the trees are sometimes scattered, interspersed 

 with grass or shrubs, chaparral, grassland, and 

 desert species may penetrate well into the community. 

 In respect to species composition, therefore, the 

 woodland in North America to a large extent is an 

 ecotone. 



Mammals 



Of larger mammals, the mule deer, mountain 

 lion, and coyote commonly occur during the winter 

 months in the piiion-juniper woodland of Utah and 

 Arizona, although most of these species spend the 

 summer high in the mountains. The bobcat also oc- 

 curs and the grizzly bear was formerly not uncom- 

 mon. The rock squirrel, cliff chipmunk, desert and 

 dusky-footed wood rats and pifion mouse are found 

 in both the pifion- juniper and the petran bush but 

 show preference for broken country, rocky hillsides, 

 and cliffs (Woodbury 1933, Rasmussen 1941). In 

 southern New Mexico, four species of mice — deer, 

 brush, rock, and pifion — occur more or less together 

 (Dice 1942). The open floor of the oak woodland 

 in California is relatively devoid of mammal life, with 

 only the California and brush mice common in the 

 vicinity of brushy growth. The western gray squirrel 

 is probably most common in this community 

 (Vaughan 1954). 



Birds 



Certain bird species appear to be more charac- 

 teristic of woodland than are mammals. Species oc- 

 curring rather widely in northern Arizona (Rasmus- 

 sen 1941), Utah (Hardy 1945), Cahfornia (Miller 

 1951), and Mexico (Marshall 1957) are: 



Coniferous forest, woodland, and chaparral biomes 31 



