Mountain environments - Fundamentals of the Physical Environment - page 767

adaptations and longer-term (perhaps genetic) acclimatization . The partial pressure of
oxygen falls with altitude, in line with general atmospheric pressure, leading to
hypoxaemia or reduced blood O 2 concentration. The quantity of O 2 inhaled by our lungs
and bound by haemoglobin in red blood cells determines our aerobic working capacity .
This falls by 10% km −1 above 1500 m as reduced O 2 flow to body tissues causes anoxia
and its more severe form, hypoxia .
Apart from making any activity more tiring, they may trigger more serious disabling
consequences - often exacerbated by other mountain weather phenomena. Acute
mountain sickness is often first to appear, caused by a slow leakage of fluids into the
brain and resultant swelling as the heart tries to compensate by pumping more blood.
Rapid ascent heightens the risk above 3·5 km and, if not redressed by equally rapid
descent to lower altitudes, may seriously impair mental judgement and develop life-
threatening high-altitude cerebral oedema . Fluid may also build up in the lungs
(enhanced by water vapour condensed from cold inhaled air) and cause high-altitude
pulmonary oedema , which reduces O 2 intake further still and can also prove fatal.
Conversely, cold dry air can cause coughing severe enough to crack ribs.
Enabling responses may prevent or mitigate some of these conditions. Even at low
altitude, most of us experience a faster pulse, raised cardiac output and hyperventilation
on physical exertion as the heart seeks to raise its output of oxygenated blood or the lungs
increase ventilation (breathing volume). Two other responses involve the bloodstream. In
haeomoconcentration blood plasma (fluid) levels fall and thereby raise the red blood cell
concentrations, whilst in polycaethaemia bone marrow actually produces more red blood
cells. These are clearly longer-term responses and may become part of the genetic
adaptations which many mountain peoples possess - and explains why they produce such
good long-distance athletes!
MOUNTAIN ECOSYSTEMS
Mountain climate, geomorphology, pedology and ecology are more integrated than in
most terrestrial systems through their high degree of variability over short distances and
mutual sensitivity to instability and environmental change. Emerging from lowland
geomorphic and 'zonal' vegetation systems, integration focuses on the high mountain
environment and its montane forest- alpine-cryonival zones (Figure 25.12). It is not
prescribed by specific elevations so much as by the distribution of accordant
geoecological features and allows us to resolve the term alpine . Its use has both narrowed
and widened from Latin origins. It identifes the zone between permanent snowline and
timberline and in the term alp a bench overlooking a glacial trough or the high meadow
often developed there, but also refers descriptively to high mountains in general.
Ecologists may settle for the tree-line-snowline belt in locating an arctic-alpine flora,
synonymous with arctic tundra ecosystems although diversified by the greater range of
high mountain climates and their smaller individual areas. Net arctic-alpine primary
production is low, averaging 140 g m −2 yr −1 dry weight and ranging between 40 g m −2
yr −1 (nival belt) and