Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 341

lower cloud levels, higher soil moisture, and slower decomposition rate of organic mat-
ter. These conditions are thought to reduce plant nutrients, causing plant communities
to adjust their location downward (Grubb 1971, 1974, 1977). Regardless of the cause
of lower vegetation zones on smaller mountains, mountain zones on tropical islands
are typically depressed in the direction of the prevailing trade winds (e.g., Leuschner
1996; Fig. 7.4). A similar pattern of forest-zone depression can be seen on windward
and coastal mountains, compared to leeward or more continental mountains (e.g., Cac-
cianiga et al. 2008).
Not all tropical mountains support well-developed forests (e.g., Wesche et al. 2000;
Burke 2005). Examples include Kilimanjaro and Mount Kenya in East Africa, which rise
steeply above dry plateaus covered by scattered forests and savanna grasses. A mossy
forest occurs at approximately 2,400 m (8,000 ft) but fails to support the typical wet
rainforest vegetation sequence above this level. Bamboo forests are a distinctive plant
community on the mountains of East Africa, except on Kilimanjaro, and are important
vegetation components on other tropical and midlatitude mountains (e.g., Veblen, 1982;
Taylor 1992).
Central Himalayan forests share many of the angiosperm families and genera char-
acteristic of tropical (e.g., Shorea ) and temperate forests ( Quercus, Betula ), but have
evolved several distinguishing traits in response to the climatatic influence of the Asian
Monsoon and a complex climate history associated with the rapid tectonic rise of the
region (Zobel and Singh 1997). Consequently, this region exhibits a high degree of
endemism, and important traits include a high proportion of broadleaf tree species with
multiyear leaf longevity, high concentrations of leaf nitrogen, and a range of flower-
ing and late reproductive stages corresponding to different monsoonal phases (Zobel
and Singh 1997). In general, these forests are highly productive, as measured by net
primary productivity and biomass (the dry weight of organic matter per unit area), and
well adapted to the variations in insolation, temperature, and precipitation character-
istic of monsoonal climate conditions.
The most xeric (dry) mountain areas are located above tropical deserts. Many of
these mountains, such as the Ahaggar and Tibesti in the Sahara Desert, exceed 3,000 m
(10,000 ft) and support scattered xerophytic (drought-adapted) shrubs and trees above
1,500 m (5,000 ft), but lack forests (Messerli 1973). Several regions on the western
slopes of the Andes are also treeless.
The Andes display a wide range of environmental conditions and illustrate how latit-
ude, elevation, and moisture availability interact to shape montane forests (Veblen et al.
2007). Near the equator in northern Ecuador and Colombia, the rainforest ascends to
similar elevations on both the lee and windward sides of the mountains, in response to
convectional processes that operate on both slopes, creating similar precipitation pat-
terns (Fig. 7.5). Within a few degrees to the north or south, the Andes come under the
influence of the northeast and southeast trade winds, creating humid conditions on their
east side and a pronounced rainshadow and desert conditions on the western slopes.
This shift in air flow results in an asymmetrical distribution of vegetation zones across
the range (Fig. 7.5) (Troll 1968).
Despite the general lack of moisture, trees attain their highest elevations in the dry
subtropics. This paradoxical situation is possible because of the high surface heating
and development of convection storms. Examples of these high-elevation forests include