Clima social de aula: concepto desde el criterio de varios

Forest biological diversity can be quantified at several scales, these include:

assessing the genetic components within species, counting the number of species per unit area (local, regional, national, continental, global), determining numbers and arrangement of forest types and their age, classifying types of forest ecosystems, determining communities of species associated with forest ecosystem and describing landscape structure (UNEP, 1995).

28 2.1.11 Boreal forests

Boreal Forests, including tundra woodlands, extend over about 1270 million hectares, or about one third of the world‟s forest cover. The boreal forest is the second largest terrestrial biome after tropical forests. This northern circumpolar biome is strongly characterized by coniferous ecosystems with low tree species richness, extensive and fairly uniform stands and relatively short-lived species (<200 years), which are under fire, wind and insect disturbance regimes. Extreme oceanic types with broad-leaved deciduous tress are found in northwestern Europe, where the tree limit is formed by Betula pubescens subsp.

czerepanovii. Similar ecological conditions prevail in northern Asia, Alaska, and northern Canada, with stunted Picea, Larix, Pinus pumila and Betula nana at the treeline.

Boreal landscapes are composed of a complex of plant communities that, aside from vast tracts of forest stands, include various wooded and open mires of bogs, numerous water bodies of varying size, rivers, rock outcroppings and natural grasslands and ferns (Walter, 1979; Barbour and Christensen, 1993).

The Wisconsin glacial events, 10,000- 14,000 years ago, forced plant and animal life south, followed by northward migration, in recurrent cycles.

The boreal forest biome is distributed across areas formerly covered by continental glaciers and, consequently, the land has supported forest cover for only 3,000 to 7,000 years (Ritchie 1987). The number of tree species that characterise these forests is therefore low, especially in the Euro-Siberian area, where major watercourses and mountain ranges run at right angles to the direction in which the species migrated northwards. As a result of the post-glacial history of the biota, many boreal and subarctic tundra species have wide distributions.

There are relatively few endemics at the species level; most of these occur these occur in the extreme eastern and western parts of the continents, close to ancient refuges. Due to wide distributions and varying environmental conditions, evolution at the level of ecotypes and

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subspecies is common and some genera, such as Carex and Betula, show wide-scale hybridization (Jonsell, 2000).

Boreal forest stands normally contain no more than a few species, primarily of the genera Picea, Pinus, Abies, Larix, Thuja, Betula, Prunus, Alnus and Populus, and they often form monocultures, particularly in the case of Picea, pinus and Larix. These genera are panboreal and members of the four deciduous genera (Betula, Prunus, Alnus and Populus ) grow more rapidly than conifers and tend to occupy sites immediately following stand disturbance. Tree richness in North American forests is greater than in the Euro-Siberia region. In North America, four of the six principle boreal forest species extend across the continent, though no single tree species is panboareal. Picea mariana grows on poor soils and forms the northern treeline continent-wide. Where fire is uncommon, Abies spp. often predominates in the eastern and continental North American boreal zone. In Eurasia, this genus is ecologically largely replaced by two species of Larix. Larch forest, mostly consisting of Larix gmelinii, covers 2.5 million km² in continental Siberia where much of the terrain has deep permafrost. Larix sibirica often forms monotypic stands following disturbance by fire (Schulze et al., 1996). While in North America, Larix laricina is rarely a dominant species, and is found mainly in cold, wet and poorly drained sites such as in sphagnum bogs and muskeg. In Europe, only Picea abies and Pinus sylvestri are true dominants of the boreal zone, and are often mixed in successional phases with broad-leaved deciduous tree species such as Betula pendula, B. pubecens, Populus termula and Alnus glutinosa and A. incana. In more eastern European regions, Picea abies is replaced by the closely related Picea obovata, with Abies sibirica, Larix sibirica and Pinus cembra subsp. sibirica. There is a broad belt of hybrids, Picea abies x P. obovata, between their natural regions. In Eurasia, the proportion of Picea gradually decreases eastward while that of Larix increases correspondingly. In northern Japan, the number of coniferous species increases again.

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Conifers comprise the bulk of the biomass in these boreal ecosystem, although most forests also include a variety of deciduous trees and shrub species, dwarf-shrubs (notably member sof the Ericacea), grasses, sedges and herbs. In general, species diversity in taiga communities increases with length of the growing season, increasing soil fertility and favourable drainage. A comparatively moderate richness of bryophytes, lichens and fungi occur in many boreal forest types, they are especially common in older forest with their volume of decaying wood.

Animal species richness generally declines with increasing latitude, and boreal forests maintain fewer species than do temperate or tropical forests. Studies have shown a longitudinal gradient in the species richness of herbivores, with the region near the Bering Sea being particularly species poor (Danell et al., 1996). The fact that this region supports the woody species most chemically defended against browsing suggests that such gradients of plant chemical defence in boreal forests may be also partly responsible for gradients of mammalian species richness (Pastor et al., 1996). An important and characteristic component of boreal fauna is migratory birds which breed in summer in the boreal forest and winter in more southern areas. In many cases, these tropical and neo-tropical migrants travel thousands of kilometers between their winter and summer ranges. Species which must over-winter in northern forests have developed a range of adaptations to cold climates including hibernation, thick fur, denning beneath the snow, and the ability to maintain life with reduced availability and quality of forage, such as by storing fat in the fall and then losing weight over-winter.

Caribou or reindeer (Rangifer tarandus) can make use of lichens, a group of species not fed upon by other boreal animals. Large predators still remain common in Canada, Alaska USA, (bears Ursus Americana, Ursus arctos, wolf Canis lupus) and Russian boreal forests (wolf and tiger Panthera tigris altaica), but are absent from Scandinavia, although wolves have been recorded over the past decade. The large ungulates species are panboreal, including

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moose (Alces alces) and caribou. Food webs are not complicated and are a few common herbivores dominate the deits of all predators, avian and mammalian. Small herbivores (and their predators) in boreal systems are well-known for their periodicity, or even cycling (e.g., Krebs et al. 1995, Stenseth et al. 1998), which appears related to both food availability and predation rate. The dominant cycle length for a wide variety of mammals and birds in North America appears to be about ten years, while in Fennoscandia its length is usually four years (Keith, 1963; Finerty, 1980; Erlien and Tester, 1984). Such fluctuations represent a temporally dynamic aspect of biodiversity. Cycles of herbivores may result in differential survival of their preferred food species, such as fir, aspen and birch, as well as their predators, such as warbles that prey on budworm (Choristoneura fumiferana), or Canada lynx (Lynx Canadensis) that prey on small mammals (Keith, 1963; Hansson, 1979; Haukioja et al., 1983;

Bryant and Chapin, 1986; McInnes et al., 1992; Stenseth et al. 1998; Thomas et al, 2007).

There appears to be relatively few vertebrate animal species with highly restricted habitats or niches in boreal forests, although several species relying on dead wood or cavities to nest or breed find old forest to be optimal habitat (Thompson and Angelstam 1999).

Relatively few boreal species are listed by IUCN (2000) as threatened, however, several of the large carnivores such as Siberian tiger and brown bear are threatened.