Geography Reference
In-Depth Information
FIGURE 7.4 Treeline locations as a function of mountain mass and prevailing winds (A) in the humid
tropics of Indo-Malaysia (Price 1981, adapted from Eyre 1968) and (B) a vegetation cross section
of the Northern Range, Trinidad, West Indies (horizontal distance: 29 km (18 mi). (Price 1981,
modified from Beard 1946.)
At high elevations, tropical forests become discontinuous and consist of patches in-
terspersed with grasses. Lower temperatures become an increasingly important limit-
ing factor for trees and, by 3,500-4,000 m (11,500-13,200 ft), grasses, shrubs, and bare
ground become the dominant surface cover. The exact location of this boundary is of-
ten difficult to interpret because of the long history of human-set fires (e.g., Horn 1998;
Young and Keating 2001) that have lowered timberlines well below their climatic limits
(Smith 1975, 1977; Young and León 2007).
The position of tropical vegetation zones is not exclusively a response to changes
in elevation, as shown by the shift of vegetation zones to lower elevations on smaller
mountains relative to their larger neighbors (Fig. 7.4). Historically, this has been ex-
plained by the “Massenerhebung” or “mountain-mass” effect, based on the supposition
that larger mountains exert a proportionally greater effect on their own climates. While
this concept may hold some validity, it fails to explain why zones would shift under re-
gionally constant climate conditions (Van Steenis 1961; Grubb 1971). A more recent
explanation suggests that tropical vegetation zones are controlled by cloud cover, and
that the lowering of elevational vegetation zones on smaller mountains results from
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