Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 360

Arctic and alpine vegetation share many of the same species but become increasingly
dissimilar with decreasing latitude. Alpine areas in the mid- and low latitudes often have
less than one-half of the tundra species found in the Arctic (e.g., Billings 2000). Floristic
differences increase in the tropics: The flora and life forms differ almost entirely, with
few arctic representatives present (Van Steenis 1935; 1962; Hedberg 1951, 1961, 1965,
1995; Troll 1958; Cuatrecasas 1968). These differences mirror the global latitudinal di-
versity gradient, the diversity of mountain environments (Kikvidze et al. 2005), the isol-
ation of alpine areas (Hadley 1987), and differences in their evolutionary histories (e.g.,
Weber 1965; Agakhanjanz and Breckle 1995). Floristic dissimilarity between the arctic
and alpine floras is intensified in floristically rich mountain ranges, where the number
of alpine plants can exceed the total number of plants found in the entire arctic tundra
(Körner 1995; Virtanen 2003).
The total area of alpine tundra is comparatively small in the southern hemisphere,
and it supports a dissimilar flora with the exception of a few bipolar plants. These com-
munities remain, however, similar in function and structure to the arctic tundra (Ward
and Dimitri 1966; Billings 1974a). Nevertheless, most writers have preferred the term
alpine vegetation, although some prefer the term “oreophytic” (Körner 1995), to refer
to plant communities growing above the climatic timberline in southern hemisphere
and tropical mountains (Van Steenis 1935; Hedberg 1965; Costin 1967; Mark and Bliss
1970).
Arctic and alpine tundra are commonly divided into low, middle, and high tundra
zones (e.g., Nagy and Grabherr 2009). The low alpine tundra is the zone immediately
adjacent to the timberline, consisting of a nearly complete cover of low-lying shrubs,
herbs, and grasses. The greatest number and diversity of species are found in this zone;
it is generally the most productive area within the tundra. Most grazing above tim-
berline takes place in the low alpine tundra (Bliss 1975). This zone is characterized
by weakly developed soils stabilized by alpine turf under natural conditions, but sens-
itive to erosion under high grazing, turf extraction, and harvesting of alpine shrubs
for fuelwood (Byers 2005; Nagy and Grabherr 2009). The lower alpine zone contrasts
markedly with the highest alpine or nival zone, with bare and rocky ground with occa-
sional mosses, lichens, and dwarfed vascular plants, similar to that found in high Arctic
or polar deserts.
Similar to treeline, the highest elevations reached by vascular plants (5,800-6,400
m/19,100-20,100 ft) are located in the subtropics between 20° and 30° latitude in the
Himalayas and Andes (Zimmermann 1953; Webster 1961; Swan 1967). In the equatorial
tropics, vascular plants do not occur above about 5,200 m (17,100 ft) because of cloud
cover and lower temperatures. The massive nature of the Himalayas and Andes also lim-
its their exposure to moisture-laden air, resulting in higher snowlines and longer grow-
ing seasons (Swan 1967). Above the elevational limits of flowering plants is the eolian
zone (Swan 1961, 1963a, 1963b, 1967; Mani 1962; Papp 1978; Spalding 1979). Life in
this zone is dependent on organic materials—pollen, spores, seeds, dead insects, and
plant fragments—carried to high altitudes by the wind (Edwards 1987). A similar life
zone is thought to exist in the extreme desert and polar regions.
Floristics