Biome: Boreal Forest

Definition and Geographic Extent

Boreal forest received its name after the Greek god of northern winds Boreas, indicating the geographic position of this biome in high northern latitudes. No equivalent belt exists in the Southern Hemisphere since large land masses at similar latitudes are not found there. The geographic boundaries of the boreal biome are frequently defined by climatological variables representative of the temperature-limited growing conditions. Several temperature-related metrics are commonly used to define the exact northern and southern boundaries of boreal forests. One of the approaches is to define the southern boundary of the biome at the average yearly +5 °C isotherm (i.e., a line connecting points of equal temperature) and the northern boundary at the average yearly −5 °C isotherm. Another approach uses mean monthly July +18 °C and +13 °C isotherms for southern and northern boundaries, respectively. Yet another approach defines the boreal zone using the length of the period with mean daily temperatures of at least 10 °C and draws the southern boundary at less than 120 days and the northern boundary at approximately 30 days. The area within these boundaries typically has 50 to 100 frost-free days per year. The amount of precipitation in the boreal forests depends on their proximity to the zone of oceanic influence and varies from 200 to 2,000 millimeters per year. However, relatively low temperatures and a short warm period lead to low evaporation rates and thus to generally humid climate even within areas that receive little precipitation.

A different approach to defining boreal forests is to use the dominance of a particular vegetative form. Under this approach, boreal forests are defined as tree-dominated landscapes in high [Page 215]northern latitudes with prevalence of needle-leaved species in primary (climax) forests. Although this approach is very different from the climatological definitions described above, the overall global extents of boreal biome produced by various schemes are similar and reflect a strong relationship between climate and vegetation distribution. In North America, the majority of boreal forests are found between 45° N and 55° N, whereas in Northern Eurasia boreal forests are rarely found below 55° N. The vegetation-based definition extends the northern boundary of boreal forest in Eurasia above 70° N; however, most ecologists agree that there it represents a forest-tundra transitional zone rather than traditional boreal forests. Boreal forests are also found south of their broad geographic boundaries presenting Alpine extensions of the biome range or mountain taiga. These extensions can penetrate deep into general ranges of other biomes stretching along mountains as in the example of the Appalachians.

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