Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 359

consist of non-forest species and are genetically predisposed to a low stature. This dif-
fers from northern hemisphere timberlines (Wardle 1965), where few species other than
the dwarf mountain pine ( P. mugo ) found in the Alps, Pinus pumilio in eastern Asia and
Japan, and green alder in several mountain systems of Eurasia have established eco-
types.
Abrupt forestlines in New Zealand and South America are related to the sensitivity
of Nothofagus seedlings to sunlight beyond the closed forest (Wardle 1973b; Cuevas
2000). A similar pattern exists in the moist tropics of New Guinea, where treeline occurs
at about 4,000 m (13,200 ft) (Wade and McVean 1969). These trees exhibit some stunt-
ing, but maintain their umbrella-shaped crowns and a 3-6 m (10-20 ft) high canopy. This
forest boundary changes abruptly into grasses or shrubs with no marked tendency to-
ward krummholz, reflecting the relative lack of strong prevailing winds. The abruptness
of this forestline may also be the result of fire (Gillison 1969, 1970; Hope 1976, Smith
1975, 1977).
Summary
Although upper timberline occurs in every climate region, its exact cause remains elu-
sive. Nonetheless, some generalizations are possible. The most obvious of these is the
universal occurrence of short stature, which appears to be essential for high-elevation
trees to survive. This physical acclimatization or genetic adaptation allows these plants
to benefit from several near-surface conditions including warmer daytime temperat-
ures, lower wind stress, and insulation provided by snow (Grace 1989). These and oth-
er factors far outweigh the negative consequences of higher summer leaf temperatures
and increased evaporative stress experienced by shorter plants (Aulitzky 1967). Glob-
ally, treeline appears to be most strongly controlled by a common thermal threshold
(Körner 1998; Körner and Paulsen 2004), but it varies at the regional scale, as high-
er treelines occur in continental than in marine climates (Caccianiga et al. 2008). At
the landscape scale, timberline elevations exhibit a direct relationship with insolation;
the influence of microtopography becomes increasingly important at the scales of com-
munities and individual plants. All of these observations underscore the importance of
temperature in establishing the elevation of the upper timberline. However, several dec-
ades of research regarding the energy relationships between trees and their net assim-
ilation rates (e.g., Handa et al. 2005; Hoch and Körner 2005) have shown that the num-
ber of variables controlling local vegetation patterns increases with decreasing area (cf.
Gosz 1993).
Alpine Tundra
Tundra is a Russian word meaning “treeless plain” and originally referred to arctic
areas north of the timber-line. More recently, “tundra” has also been used to refer to
alpine vegetation because its appearance is similar to arctic plant communities. This
similarity partly explains why several earlier studies compared various attributes of arc-
tic tundra and alpine tundra vegetation (e.g., Bliss 1956, 1962, 1971, 1975; Billings and
Mooney 1968; Billings 1973, 1974a; Table 7.1).