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.

                            Figure 1