Environmental Engineering Reference
In-Depth Information
Figure 24.1 The limits of the Arctic environment: the 10° C
July isotherm, the treeline and the Arctic marine boundary.
north by the air masses associated with the North Atlantic Drift (Scandinavia) and the
Japanese current (north-west North America), and where it is pushed south by the cold
Labrador current (Labrador, Quebec) and the Bering current (Bering Sea, Kamchatka).
The 10° C summer isotherm boundary has been popular with ecologists, as it
corresponds reasonably well with treeline . Treeline is the limit beyond which trees do not
grow, and is one of the world's major ecological boundaries. In some areas the transition
from boreal coniferous forest to tundra is abrupt, whilst in other places the boundary is
more gradual over hundreds of kilometres. In the latter case the forest-tundra zone
consists of open ground with sparse stunted trees, an ecotone which has plants from both
the forest and the tundra. Isolated stands of trees can grow north of treeline where local
conditions permit; topography providing shelter is the commonest reason, but better soils
and more available water may be locally important. Although generally a good fit, the
10° C summer isotherm lies 100-200 km north of treeline over much of North America
and Eurasia. This indicates that the precise position of treeline reflects mean annual net
radiation rather than temperature. F. K. Hare has studied the climatology of treeline in
Canada and points out that it corresponds to the average summer position of the Arctic
Front which separates maritime Polar (mP) air masses to the south from cold maritime
and continental Arctic (mA and cA) air masses to the north. There is a sharp decrease in
net radiation between boreal forest and tundra, and part of it is also due to the albedo of
the surface, more incoming solar radiation being reflected from tundra vegetation than
from coniferous forest vegetation. Thus treeline is a polygenetic ecological feature; its
primary control is macroclimatic, based on the Arctic Front, but its course in detail is
modified considerably by vegetation, soil surface and water bodies. Because of the
anomalies, some ecologists feel that a closer fit is given by the Nordenskjöld line , an
isopleth joining places where the mean temperature of the warmest month equals (9-0·1
K), K being the mean temperature of the coldest month in ° C.
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