14 



HUGO SJORS 



with different floras. Some species of small size, but not trees, occur in all 

 sectors. Alnus rugosa var. americana and Sorhus decora of northern Canada, 

 for example, are more similar to the Eurasian A. incana and Sorhus aiicitparla 

 than admitted by leading present-day dendrologists, and probably would have 

 been regarded as conspecific if they had been small plants. However important 

 from the point of view of historical plant geography, this longitudinal sub- 

 division is less important than the latitudinal zonation ecologically and also for 

 economic forestry. Such a zonal subdivision of the Boreal zone has been 

 independently proposed in most Boreal countries. 



Newfour 

 Peatland dominant 

 ■■Peatlands large and abundant 

 : The same, but distribution more loca' 



Fig. 1. Extension of the Boreal zone (between the full lines). South of, or below, the 

 tree-line, woodland-tundra or Sub-Alpine areas extend to the northern broken 

 line. South of the Boreal zone, Boreo-nemoral and Aspen-Birch Woodland 

 regions reach to the southern broken line. The abundance of peatland is indicated 

 on the figure. Most lowland in Iceland and the Shetlands is south of the tree-line. 



Boreal of North America 



In Canada propositions regarding a subdivision of the Boreal have been 

 advanced by Halliday (1937), Hustich (1949, 1951), Rousseau (1952). Hare 

 (1950. 1954, 1959). Ritchie (1956. 1959, 1960), Rowe (1959). and others. The 



