| Summary: | Vegetation is the plant cover of a region, which usually refers to the potential natural vegetation prior to any intensive human disturbance. The description of vegetation for an extensive area involves the recognition and characterization of units called formations, which are named with reference to composition (e.g., coniferous), aspect of habit (deciduous), distribution (western North America), and climate, either directly (tropical) or indirectly (tundra). Further subdivisions are termed associations or series, such as the beech-maple association or series within the deciduous forest formation. Formations and associations constitute a convenient organizational framework for considering the development of vegetation through Late Cretaceous and Cenozoic time. For this purpose seven extant plant formations are recognized for North America: (1) tundra, (2) coniferous forest, (3) deciduous forest, (4) grassland, (5) shrubland/chaparral- woodland- savanna, (6) desert, and (7) elements of a tropical formation. Several summaries are available for the modern vegetation of North America, including Barbour and Billings (1988), Barbour and Christensen, Kuchler (1964), and Vankat (1979). The following discussions are based primarily on these surveys. Tundra (Fig. 1.2) is a treeless vegetation dominated by shrubs and herbs, and it is characteristic of the cold climates of polar regions (Arctic tundra) and high-altitude regions (alpine tundra). In the Arctic tundra a few isolated trees or small stands may occur locally, such as Picea glauca (white spruce), but these are always in protected habitats. The Arctic region experiences nearly continuous darkness in midwinter, and nearly continuous daylight in midsummer. There is a short growing season of only 6-24 weeks; this accounts, in part, for the fact that 98% of all Arctic tundra plants are perennials (Vankat, 1979). Strong winds are another feature of the Arctic landscape, often exceeding 65 km/h for 24 h or more. They likely account for the frequency of rosettes, ...
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