Geoscience Reference
In-Depth Information
weakly developed, there is much bare rock, and catenas
are less clear than in the Low Arctic, owing to the
patchiness of the plant cover. Frequently the aridity
produces calcareous and saline horizons.
cold-air drainage channels. This leads to higher treelines
on exposed ridges, especially in mid-latitudes, unless
checked by wind stress or physiological drought. Similarly,
moist updraughts in tropical mountains elevate treelines
through the development of
cloud forest
around the
condensation level. Human impacts on treelines from
forest clearance, environmental disturbance, and atmos-
pheric pollution, serve to depress them. In Britain, for
example, the natural treeline lies between 650 m and 800
m but most of that zone now supports montane grassland
and heath with remnant arctic-alpine flora.
The sequence of changes with altitude in the Canadian
Rockies at 50
Treeline
Treeline in both arctic and alpine regions is one of the
Earth's major ecological boundaries. Traditionally it has
been defined by the 10
C July isotherm, whether in
Eurasia, or the Andes (see
Figure 24.3
).
Kenneth Hare (see
Chapter 1)
has shown that the significant factor is
radiation balance, which determines the summer position
of the Polar Front which helps to fix the position of
treeline in North America; the contrast between the high
albedo of tundra and lower albedo of taiga is a key control.
In mountains treeline is an ecotone which marks a
transition from closed forest to one with clearings of
arctic-alpine species. Regional or topoclimatic variations
reflect soil moisture variations. In rain-shadow areas, low
moisture status is reflected in xeric communities, with
pine replacing larch. In contrast, the moisture-source
effect of mountains in drier regions like Mediterranean
(e.g. Sierra Nevada) or continental interiors (e.g.
Himalayas) permits a forest girdle to exist 500-1,000 m
above the zonal treeline.
Topoclimate may substantially increase the frost-free
period or accumulated degree-days through shelter and
aspect, and thus raise the local treeline; alternatively it can
depress it by temperature inversion, frost hollows and
N latitude is shown in
Figure 24.19
.
Micro-
relief, soil and microclimatic factors including aspect,
slope, avalanche risk and snow-cover duration are
controlling factors at the treeline. Alpine meadows and
heaths are interspersed with clusters of dwarf conifers of
krummholz
. Wind pruning creates
flagged krummholz
(
Plate 24.14
),
and in extreme cases the conifers develop a
creeping or
cushion
habit at the ground surface, as no
shoots are able to survive above the winter snowpack
Arctic and alpine tundra
The term 'tundra' for the dwarf shrubs, herbs, mosses and
lichens of the arctic-alpine flora refers to cold-tolerant
plants, almost entirely perennial and ground-hugging,
which represent the three lowest orders of Raunkiaer's
physiognomic classification of plants, namely,
cryptophytes
SOUTH
NORTH
Alpine
2350m
Alpine meadows, bare rock,
nival zone
2250m
2125m
Engelmann spruce, alpine fir,
alpine larch
1950m
Lower Subalpine
Engelmann spruce, alpine fir,
balsam fir, lodgepole pine
1625m
1500m
White spruce, aspen,
black spruce
Montane
