CONIFKRODS FOHKST BIOMK 



I life 



runs lure: 



coiUimious, often 



dense, forest of needle- or scale-leaved evergreen 

 trees. The sclerophylloiis leaves prevent excessive 

 evaporation of water during winter and dry periods, 

 and are adapted to withstand freezing. The ever- 

 green leaves take full advantage for |)hotosynthesis of 

 short summer growing seasons, intermittent warm 

 periods of autumn and spring, and the warm winter 

 rains of tiie Pacific coast. The flexihle branches bear 

 snow-loads without breaking; snow-loads tumble 

 easily off the cone-shaped tree. The dead, dry nee- 

 dles which cling to the trees feed devastating crown 

 fires, much more common in coniferous tlian decidu- 

 ous forest. 



DLSTRIBUTION AND ORIGIN 



23 



Geographic 



Distribution of 



Communities: 



Coniferous forests are largely confined to the 

 northern hemisphere. They are transcontinental in 

 Canada (Halliday 1937) and in higher elevations on 

 the mountains through Mexico and Guatemala, into 

 Honduras and Salvador. In Eurasia there is also a 

 northern transcontinental coniferous belt with dis- 

 junct patches of coniferous forests on all higher 

 mountains southward. The main mass of coniferous 

 forest species is doubtless derived from the boreal 

 element of the Arcto-tertiary flora, and is much older 

 geologically than is the deciduous forest. There is 

 some evidence, however, that the eastern hemlock is 

 a segregate from the temperate rather than the boreal 

 unit of the Arcto-tertiary flora (Braun 1950, Costing 

 and Bourdeau 1955, Whittaker 1956), and that the 

 western arid-tolerant ponderosa pine and Mexican 

 pines come from the Madro-tertiary flora. 



CLIMATE 



Coniferous Forest^ 



Woodland, 



and Chaparral Bionies 



In the transcontinental forest of North Amer- 

 ica, precipitation varies between 38 and 100 cm 

 (15-40 in.) and is mostly summer rain. Mean 

 monthly temperatures vary from a winter low of 

 about — 30°C to a summer high of 20°C (—20° to 

 -(-70°F). The summer period between killing frosts 

 varies from 60 to 150 days. On the Pacific slope of 

 the high western mountains, because of the westerly 

 winds coming from the warm Japanese current, pre- 

 cipitation is higher ( 125 to 225+ cm, 50 to 90+ in.) ; 

 most of it falls as winter rain. Mean monthly temper- 

 atures are more uniform (2° to 18°C, 35° to 65°F) 

 and the frostless season is 120 to 300 days long. Hu- 

 midity is high, and fogs are frequent in this region. 

 In the northern Rockies, Cascades, and Sierra Ne- 



301 



