The Boreal Forest
Glen M. MacDonald
The boreal forest of North America, extending unbroken from the Pacific coast of Alaska to the Atlantic coast of Newfoundland, encompasses some 356 million hectares (ha) (fig. 14.1). The forest reaches a latitude of almost 69°N in western Canada and has southern limits near 48°N in the Great Lakes region. The North American boreal forest is part of a larger circumpolar coniferous forest that extends across Eurasia. The global boreal biome covers over 1100 million ha and represents one-third of all forested land on the planet (Kauppi and Posch, 1985; Apps et al., 1993). The North American boreal forest is sparsely populated with an average of less than one person per square kilometer, and much of it remains in a natural state. Despite its massive geographic extent, the species diversity of the forest is very low. The tree flora is largely dominated by a dozen species, seven of these being conifers of the family Pinaceae. The modern distribution of the boreal biome can be related to a specific set of climatic conditions. As climate has changed in the past, so too has the location and nature of the boreal biome. Indeed, because much of the area now occupied by boreal forest was glaciated during the late Pleistocene and early Holocene, the present boreal forest is a relatively young feature. However, the forest itself exerts a strong influence on regional and global climate, and it likely was an important factor in past climatic changes. Clearly, changes in the boreal forest caused by the greenhouse effect will be an important aspect of future global warming.
This chapter will review the climate, soils, and vegetation of the boreal biome. The Quaternary history of the forest will be broadly outlined. Finally, the relationship between the forest and future climatic change will be considered.
Although to the casual observer the boreal biome appears monotonous from Alaska to Labrador, the boreal climate does display some diversity (fig. 14.1). Central regions, such as interior Alaska and western Canada, experience an increasingly severe continental climate toward the interior of the continent. The mean January and July temperatures can vary by as much as 50°C. The coldest temperature recorded at a permanent weather station in North America occurred at Snag, Yukon, where the extreme minimum daily temperature in February is –62.8°C (Canada, 1982b). In contrast, the average extreme maximum daily temperature during July at Snag is 31.7°C (Canada, 1982b). Despite the occurrence of warm summer days, the growing season is generally less than 6 months in the south and as short as