Definitions and Boundaries

Ecosystems above the upper climatic limit of trees are termed 'alpine'. Scientifically, the alpine life zone is an altitudinal belt defined by climatic boundaries (Figure 1) and the term 'alpine' does not refer to the European Alps, but refers to treeless high-elevation biota worldwide (mostly grassland and shrubland). 'Alpine' supposedly roots in the pre-Indogermanic word alpo for steep slopes, still used today in the Basque language. By contrast, in common language, 'alpine' is often used for places anywhere in mountainous terrain, irrespective of altitude (e.g., alpine village, even alpine cities). If a city were truly alpine it would have to be above the climatic tree-line, but no such city does exist worldwide. Hence, a distinction must be made between the scientific, biogeo-graphic meaning of alpine (the issue of this text) and common (often touristic) jargon.

The upper limit of the alpine life zone or alpine belt is reached where flowering plants have their high altitude limit. This is often close to the snow line (the altitude at which snow can persist year-round), but commonly a few scattered flowering plants also grow above the snow line, in favorable, equator-facing, and sheltered places. The uppermost part of the alpine belt, where closed ground cover by vegetation is missing, is often termed 'nival', referring to sparse vegetation in rock and scree fields. The highest place on Earth where flowering plants have been found is in the Central Himalayas at 6200-6350 m above sea level.

Depending on latitude, the climatic treeline and hence the lower limit of the alpine belt can be anywhere between close to sea level in subpolar regions (>70° N, >55° S) and close to 5000 m in subtropical continental climates (trees >3 m at 4800 m in Bolivia and at 4700 m in Tibet). In the cool temperate zone (45-50° N), the alpine belt may start anywhere between 1200 and 3500 m (in the European Alps at 2000 m, the Colorado Rocky Mountains at 3400 m); that is, it is lower under strong oceanic influence and higher in the inner parts of continents. The common natural treeline altitude near the equator is 3600-4000 m. The altitudinal width of the alpine belt above treeline is roughly 1000 m. It covers c. 3.5% of the globe's terrestrial area, if cold and hot deserts are disregarded (Antarctica, Greenland, Sahara, etc.).

Given this convention on the two boundaries of the alpine belt, it is important to note that these boundaries are not sharp lines, but are centered across gradients which change from place to place and depend on topography and region. Usually these boundaries are obvious from great distance (an airplane), but hard to depict on the ground, hence depend on scale.

Nival

Alpine

Treeline

Treeline ecotone

Timberline

Montane forest

Figure 1 The altitudinal belts of mountain ecosystems. With increasing altitude these belts become fragmented and topography (exposure) plays an increasing role. (Example from the Swiss Central Alps with Pinus cembra forming the treeline at 2350 m.)

Nival

Alpine

Treeline

Treeline ecotone

Timberline

Montane forest

Figure 1 The altitudinal belts of mountain ecosystems. With increasing altitude these belts become fragmented and topography (exposure) plays an increasing role. (Example from the Swiss Central Alps with Pinus cembra forming the treeline at 2350 m.)

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