Environmental Engineering Reference
In-Depth Information
Figure 25.13
Tropical mountain vegetation zones,
Venezuelan Andes.
Source: Huggett (1995).
communities in rain-shadow areas, replacing larch with pine, for example. By contrast,
the moisture-source effect of high mountains permits a forest girdle to exist 0.5-1.0 km
above the zonal timberline in arid mountains, especially in the Himalayas.
Topoclimate may increase the frost-free or accumulated degree days substantially
through shelter and raise the local timberline, or depress it by acting as inversion frost
traps and cold-air drainage channels. This may lead to higher timberlines on exposed
ridges, especially in mid-latitudes, unless checked by wind stress or physiological
drought. Conversely, moist updraughts in tropical mountains elevate timberlines through
the development of
cloud forest
around the condensation level. Human impacts on
timberlines should not be overlooked. In general, forest clearance or environmental
disturbance, including atmospheric pollution, serves to depress them. In Britain, for
example, the natural treeline lies between 650 m and 800 m but most of that zone now
supports remnant arctic-alpine flora and montane grassland.
Silviculture
drives
timberlines upwards artificially.
FOREST-TUNDRA ECOTONE
Modern timberlines and Pleistocene snowlines are separated vertically by up to 1 km in
tropical and arid mountains but are almost identical outside the tropics. Since modern
timberlines and snowlines constrain the alpine vegetation zone, it cannot be older than the
Holocene, and slopes which it has colonized are still adjusting to Holocene
environmental change. The general location of this geoecological zone is shown in Figure
25.8. Edaphic, micro-relief and microclimatic factors including snow cover duration
assume increasing importance at the timberline. The ecotone is usually marked by a
lower forest limit and an upper, absolute alpine timberline. Sandwiched between them is
the
alpine meadow
and/or
alpine heath
, dominated by alpine tundra flora interspersed
