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Ecosistemes de Catalunya E. Carrillo. Ecosistemes
d’alta muntanya: prats alpins i boscos
de pi negre. The high mountain landscape in Alpine ranges is shaped as a result of the altitudinal variation of environmental factors (decrease in temperatures, growth period and soil-forming processes, increase of snow fall and wind incidence, etc.), which are reflected in a wide range of adaptive plant trends. These causes and trends are well exemplified in the Pyrenees, a typical Alpine system with marked particularities. The Pyrenean subalpine belt is the domain of Pinus uncinata woodlands. This plastic conifer may form various forest communities, differing in structure and understorey composition according to main exposure and substratum. In the central part of the range, from 2,300 m a.s.l. upwards, the extant pinewoods gradually become more open, and end at 2,400 m, giving way to the alpine belt. The transition zone between subalpine and alpine belts, or timberline, is an appropriate subject for the study of vegetation dynamics under global change – both climatic change and shifts in land use. The main feature of the alpine belt is the sharp partition of the landscape into dense, contrasted vegetation mosaics. Alpine pastures, mainly formed by short grasses and sedges, occupy the most favourable sites, whereas particular plant communities settle on a variety of habitats: scree, rocky surfaces, snow beds, fens, etc. The alpine mosaic also extends into the subalpine belt, where particular ground conditions prevent forest development. Above 2,700 m, rocky areas are dominant, and harbour sparse specialised communities and small spots of open, poor pastures. This zone, formed by a number of summit areas, corresponds to the subnival belt, while the uppermost peaks (above 3,000 m), bearing just a few sparse plant species, are an example of the nival belt. Both altitude belts are clearly greater in the Alps, where there are also large glaciers. As the mean temperatures continue to rise, glaciers and snow beds shrink and disappear. Thus, a rise of the alpine flora through the subnival and nival belts and into the vanishing snow beds may be anticipated, as well as the rarification –and eventual disappearance– of chionophilous plants |
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