TABLE 23 2 Climate 



lifferenf zone 



the west 



ape of 



Utah (Price 



Depth of Total Per cent 



Eleva- snow on precipi- precipi- Frost-free 



tion. March 1, tation, tation period. 



Community meters cm cm as snow days 



Tliose species marked with an asterisk are found 

 also in the petran husli of Utah and, in addition, 

 the broad-tailed hummingbird, gray vireo, Virginia's 

 warbler, and green-tailed towhee are characteristic 

 there (Hayward 1948). The petran bush, even more 

 than tlie coastal chaparral, contains many species 

 from the deciduous forest-edge and montane forest 

 biociations. 



Ko snake or lizard is particularly characteristic 

 of chaparral, although at times they may be nu- 

 merous. For the most part, the reptiles occurring in 

 chaparral belong more properly to the desert or 

 grassland and reach their upper altitudinal limits in 

 this biome. Among invertebrates in the petran bush, 

 mites, ants, leaflioppers, locusts and grasshoppers, 

 beetles, aphids, and flies are conspicuous, and large 

 numbers of parasitic and gall-forming hymenopterans 

 depend on the oaks for completion of their life-cycles. 

 A few millipedes and centipedes are to be found 

 under rocks, but snails are scarce (Hayward 1948). 

 In the coastal chaparral of southern California, the 

 period of greatest activity for most invertebrate spe- 

 cies comes in March and April, towards the end of 

 the winter rains, and is correlated with the flowering 

 season of plants and the period of greatest soil mois- 

 ture. During the hot dry summer many invertebrates 

 aestivate (Ingles 1929). 



Alpine tundra and coniferous forest at the higher 

 elevations represent southward dispersal of the Arcto- 

 tertiary flora, while woodland, chaparral, grassland, 

 and desert at lower elevations represent northward 

 dispersal from the Madro-tertiary flora. These dis- 

 persals doubtless began as these western mountains 

 became elevated in mid-Tertiary time. During the 

 Pleistocene period, glaciation occurred extensively in 

 the higher mountains and forced all communities to 

 lower elevations. Pluvial climates generally accom- 

 panied glaciation so that grassland and desert in the 

 lowlands were succeeded by coniferous forest. More 

 continuous zones of forest in foothills and through 

 valleys encouraged wide latitudinal dispersal of ani- 

 mals. With recession of the glaciers and rewarming 

 of the climate, the forest and alpine tundra again 

 withdrew to higher elevations, and mountain ranges 

 assumed the isolation from each other that we see 

 at the present time. This isolation has induced con- 

 siderable speciation, but the similarity or relation- 

 ship between animal life in different mountain re- 

 gions is explicable from the paleo-ecological history 

 of the area. 



SUMMARY 



ZONATION 



With increase in elevation there is a de- 

 crease in temperature and increase in wind velocity, 

 depth of snowfall, and total precipitation (Table 

 23-2 ) . Mean annual temperature tends to drop about 

 0.6°C (1.0°F) for each rise of 100 m (325 ft); 

 hence, by going up a mountain a few hundred meters 

 one encounters similar, but not identical, climates 

 and biota as occur at lower elevations many kilo- 

 meters northward on the continent. In deep valleys 

 and canyons, on the other hand, there is often cold 

 air drainage at night, so that colder types of vege- 

 tation and fauna occur than on the nearby ridges. 



Coniferous forests are largely confined to 

 the Northern Hemisphere. Over most of the area, the 

 summer growing season is short and the winter long 

 and cold. On the Pacific coast of North America, 

 however, precipitation occurs mostly during the win- 

 ters, which are mild. 



Some mammal and several bird species occur 

 widely through the biome, but the following animal 

 communities are recognized : North American boreal 

 forest biociation, North American montane forest 

 biociation, and Eurasian boreal forest biociation. 

 Appalachian and forest-tundra faciations of the North 

 American boreal forest biociation are well-marked 

 for birds. 



In early Tertiary, the boreal unit of the Arcto- 



Coniferous forest, woodland, and chaparral biomes 313 



