vada, heavy winter precipitation falls as snow that 

 accumulates to several meters in depth ; winter tem- 

 peratures are considerably lower. Snowfall is not as 

 heavy in the central Rockies, and declines steadily, 

 southward. 



PLANT ASSOCIATIONS OF 

 NORTH AMERICA 



Pinus-Tsuga association (pine-hemlock 

 forest) : Eastern hemlock is the climax, but eastern 

 white, red, and jack pines are of wider distribution; 

 northern white-cedar and yellow birch are prominent. 

 The forest has been badly disturbed by logging and 

 fire, factors which, with climatic succession, have 

 permitted a wide penetration of hardwoods to form 

 an ecotone between deciduous forest and boreal for- 

 est. The association extends from Minnesota to New 

 England, and south into the Appalachian Mountains. 



Picea-Abies association (boreal forest) : White 

 spruce and balsam fir most prominent (related spe- 

 cies in Appalachians ) , but black spruce and tamarack 

 also prominent ; extends across southern Canada to 

 the northern Rocky Mountains, north into Alaska, 

 and south in Appalachian Mountains ; alder thickets 

 common in wet areas and heath shrubs in forest 

 openings ; quaking aspen and paper birch occur ex- 

 tensively as serai stages. Aspen groves, or parklands, 

 form a broad ecotone between forest and grassland 

 from Minnesota to the Rocky Mountains (Bird 

 1930). In the northern coniferous forest reaches lies 

 a zone extending to the tree line in which the forest 

 decreases in height and density, its floor carpet of 

 lichens and mosses increases in depth and extent, and 

 it becomes interspersed with numerous bogs or mus- 

 kegs. Lichen woodland is especially well developed 

 east (Hare and Taylor 1956) and muskegs west of 

 Hudson Bay. This whole area is forest-tundra, as 

 distinguished from the denser, taller boreal forest ; 

 it is equivalent to the Hudsonian zone of Merriam 

 et al. (1910). 



Picea-Pinus association (petran subalpine for- 

 est) : Extends southward at higher elevations in 

 Rocky Mountains to Arizona, New Mexico, and 

 higher peaks of Mexico ; contains Engelmann and 

 blue spruces, subalpine fir, and several species of 

 pine. 



Tsuga-Pinus association (Sierran subalpine 

 forest) : Occurs chiefly in Cascade Mountains and 

 Sierra Nevada ; mountain hemlock as well as various 

 pines, subalpine larch, and red fir prominent ; trees 

 tall and narrowly cylindrical at lower elevations but 

 dwarfed, gnarled, and misshapened at tree-line ; aspen 



and lodgepole pine extensive as serai stages after fire 

 in both Sierran and petran subalpine forests. 



Pinus-Pseudotsuga association (petran mon- 

 tane forest) : At lower elevations in the Rocky Moun- 

 tains ; ponderosa pine, Douglas-fir, and white fir most 

 important ; ponderosa pine most aridity-tolerant; trees 

 often widely spaced with grass stratum underneath, 

 .sometimes forming savannas. 



Pinus-Abies association (Sierran montane for- 

 est) : Contains species listed for petran montane 

 forest and also sugar pine, incense-cedar, and giant 

 sequoia (central Sierras) ; chaparral develops after 



Pinus-Pinus association (Mexican pine for- 

 est) : An extension of montane forest, chiefly pines, 

 at higher elevations in Mexico. 



Thuja-Tsuga association (coast forest) : A 

 luxuriant humid forest on the Pacific slope of moun- 

 tains from northern California to Alaska ; western 

 hemlock, western redcedar, Alaska-cedar, Douglas- 

 fir, Sitka spruce, and redwood most characteristic ; 

 trees sometimes 90 m (300 ft) high and to 6 m (20 

 ft) diameter ; deep shade in climax forest but in open- 

 ings there may be dense tangles of shrubs, lianas, tall 

 ferns ; moss often thick over ground and fallen logs ; 

 forest in North extends to west slopes of Rocky 

 Mountains in Idaho, Montana, and British Columbia 

 to form a Coast forest ecotone with petran montane 

 and subalpine forests, in which grand fir, western 

 white pine, and western larch are prominent. 



ANIMAL COMMUNITIES 



There are three principal biociations in this 

 biome, two in North America and one in Eurasia. 

 There is overlap in their species compositions. Spe- 

 cies occurring in serai or climax stages of both North 

 American biociations, although less common in the 

 Mexican pine forests, include 



Mammals 

 Water shrew Deer mouse 



Snowshoe rabbit Porcupine 



Red squirrel Gray wolf 



Northern flying squirrel Black bear 



Birds 



Goshawk 

 Pigeon hawk 

 Ruffed grouse 

 Great horned owl 

 Saw-whet owl 



Yellow-bellied sapsucker 

 Hairy woodpecker 

 Black-backed three-toed 



woodpecker 

 Traill's flycatcher 



302 Geographic distribution of communities 



