Page 680 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

644 part IV Soils, Ecosystems, and Biomes
also be characterised by the structure of the canopy, especially in forested regions. Together, the dominant growth form and the canopy structure characterise the vegetation type; the dominant vegetation type then characterises the biome. (The dominant vegetation type extending across a region is also sometimes called the formation class.)
Biogeographers often designate six major groups of terrestrial vegetation: forest, savanna, shrubland, grass- land, desert, and tundra. However, most biome classifi- cations are more specific, with the total number of bi- omes usually ranging from 10 to 16, depending on the particular classification system being used. The specific vegetation types of each biome foster related animal as- sociations that also help define its geographic area.
For example, forests can be subdivided into several biomes—rain forests, seasonal forests, broadleaf mixed forests, and needleleaf forests—based on moisture re- gime, canopy structure, and leaf type. Rain forests occur in areas with high rainfall; tropical rain forests are composed of mainly evergreen broadleaf trees (hav- ing broad leaves, as opposed to needles), and temperate rain forests are composed of both broadleaf and needle- leaf trees (having needles as leaves). Seasonal forests, also known as dry forests, are characterised by distinct wet and dry seasons during the year, with trees that are mainly deciduous (shedding their leaves for some sea- son of the year) during the dry season. Broadleaf mixed forests occur in temperate regions and include broadleaf deciduous trees, as well as needleleaf trees. Needleleaf forests are the coniferous forests of Earth’s high-latitude and high-elevation mountain regions. Coniferous forests are cone-bearing trees with needles or scaled evergreen leaves, such as pines, spruces, firs, and larches.
In their form and distribution, plants reflect Earth’s physical systems (the abiotic factors discussed in Chapter 19), including energy patterns; atmospheric composition; temperature and winds; precipitation quan- tity, quality, and seasonal timing; soils and nutrients; chemical pathways; and geomorphic processes. Biomes usually correspond directly to moisture and temperature regimes (Figure 20.3). In addition, plant communities also reflect the growing influence of humans.
Biomes are defined by species that are native to a re- gion, meaning that their occurrence is a result of natural processes. Today, few natural communities of plants and animals remain; most biomes have been greatly altered by human intervention. Thus, the “natural vegetation”
identified on many biome maps reflects idealized po- tential mature vegetation given the environmental characteristics in a region. For example, in Norway, the former needleleaf forest is today a mix of second-growth forests, farmlands, and altered landscapes (Figure 20.4). However, the boreal forest biome designation for this region remains, based on idealized conditions before human impacts (discussed later in the chapter).
Ecotones Boundaries between natural systems, whether they separate biomes, ecosystems, or small habitats, are often zones of gradual transition in species composition, rather than rigidly defined frontiers marked by abrupt change. A boundary zone between different but adjoin- ing ecosystems at any scale is an ecotone. These are often “zones of shared traits” between different communities.
Because different ecotones are often defined by differ- ent physical factors, they are likely to vary in width. Eco- systems separated by different climatic conditions usually have gradual ecotones, whereas those separated by differ- ences in soils or topography may have abrupt boundaries. For example, the climatic boundary between grasslands and forests can occupy many kilometres of land, while a boundary in the form of a landslide, a river, a lakeshore, or a mountain ridge may occupy only a few metres. As human impacts cause ecosystem fragmentation, ecotones between habitats and ecosystems are becoming more nu- merous across the landscape (Figure 20.4).
The range of environmental conditions frequently found in ecotones can make them areas of high biodi- versity; often they have larger population densities than communities on either side. Scientists have defined cer- tain plant and animal species as having a range of toler- ance for varying habitats; these “edge” species are often able to occupy territory within and on either side of the ecotone.
Invasive Species
The native species of natural biomes have come to in- habit those areas as a consequence of the evolutionary and physical factors discussed previously in this chapter and in Chapter 19. However, communities, ecosystems, and bi- omes can also be inhabited by species that are introduced from elsewhere by humans, either intentionally or acci- dentally, as described for Tristan da Cunha in Geosystems Now. Such non-native species are also known as exotic spe- cies, or aliens.
Georeport 20.2 Plant Communities Survive under Glacial Ice
Glacial retreat has exposed communities of bryophytes that lived 400 years ago, during the warmer interglacial period known as the Little Ice Age. Recently, scientists collected and dated samples of these communities in the Canadian Arctic. They also successfully cultured the plants in a laboratory, using a single cell of the exhumed material to regenerate the entire original
organism. Thus, bryophytes can survive long periods of burial under thick glacial ice, and under the right conditions, potentially recolonize a landscape after glaciation.