Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 355

FIGURE 7.9 (A) Flag and (B) krummholz mat, Colorado Front Range. Note exposure of upper
branches and snow accumulation within the krummholz mat relative to the surrounding surface.
(Photos by K. S. Hadley.)
Regardless of the role wind plays in shaping local treelines, it also cannot be con-
sidered the universal factor determining treeline positions, for two reasons. First, wind
is relatively unimportant in tropical timberline areas. Second, any correlation between
latitude, continentality, and wind velocity appears to be much weaker than that associ-
ated with latitude and timberline (Fig. 7.6). Nonetheless, wind is clearly a major factor
controlling local timberlines, as indicated by the marked depression of timberlines on
the windward side of mountain ranges (Fig. 7.4).
SOLAR RADIATION
Several theories argue that the location of timberline is a consequence of too much or
too little solar radiation (e.g., Bader et al. 2007). For example, high levels of incident
UV impair the photosynthetic process and have other deleterious effects in plants (e.g.,
Billings 2000; Bader et al. 2007). High insolation levels can also lead to excessive heat
build-up at the soil surface, limiting tree growth (Aulitzky 1967). Nevertheless, major
problems with the light intensity and heat build-up hypotheses include the presence of
trees above 4,000 m (13,100 ft) in the dry sub-tropics of Tibet and South America and
species-specific responses to both direct sunlight and clear-sky exposure that result in
lower minimum nighttime temperatures (Germino and Smith 1999). Cloud cover has
similarly been invoked to explain the upper limit of forests. Cloud cover does appear
to correspond to low timberlines, since timberlines are invariably lower in marine than