BSC 314 Lecture Notes - Lecture 67: Alpine Tundra, Diagram, Northern Hemisphere
28 Jun 2018
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Distribution of Vegetation
Even the most cursory examination reveals that different kinds of plants grow in
different kinds of places. The vegetation of the world is aligned latitudinally in broad
bands circling the globe. As the climate changes from the equator northward and
southward, so too, does the vegetation. The lush tropical rainforests of the equatorial
band in the Northern Hemisphere give way to temperate deciduous forests, which in
turn are replaced by coniferous forests that, at their northern limit, are replaced by
treeless arctic tundra. More land is present north than south of the equator so the
banding pattern is less pronounced in the Southern Hemisphere. High mountains on all
continents also break the pattern.
The latitudinal bands of vegetation on the continents are replicated on a smaller scale
by the altitudinal bands of vegetation on mountains.
Much more than temperature and precipitation changes environmentally with altitude,
but the vegetational banding pattern remains: tundra on the tops of high mountains,
coniferous forests in middle slopes, and deciduous forests at the base of mountains.
The bands constrict and altitudinal limits become lower on mountains progressively
northward from the equator. Timberline, which is the upper limit of tree growth and
separates the alpine tundra from the coniferous forests, is at 10,000 feet in the southern
Rocky Mountains, but at the Canadian border, is at 6,000 feet. Farther north it is lower
still.
It is more than coincidental that vegetation and climate follow the same distributional
patterns: Plants have tolerance ranges—ranges of environmental conditions—in which
they can survive. Two environmental factors of great importance to plants are two that
also determine climate—temperature and precipitation. There's more to the story than
this, of course, but with available water and a moderate range of temperatures most
plants will grow. As extremes are reached in both temperature and precipitation, specific
kinds of plants disappear from the regional floras. In the tropics, there are over 40,000
species of vascular plants; in the forests of the southeastern United States, 5,000; in the
Canadian arctic, about 425.
Figure illustrates in diagrammatic form how vegetation types are related to temperature
and precipitation. Tropical rain forests, for example, occur in the hottest, wettest regions
of the world, deserts in the hottest, driest, tundra in the coldest, driest, and so forth.
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