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Our
purpose here is to deal with the universal, and rather terrestrial
rules that govern ecosystems in altitude. What are the common points
of high altitude environments, them being in America, Asia, Africa,
and Europe?
 What
are the characteristics of high altitude environments ?
A little bit
of physics to begin with... The physical characteristics related
to altitude, and common to all mountains on earth, are the following:
- Decrease of
the atmospheric pressure with altitude
- Decrease of temperature, resulting from the decrease in atmospheric
pressure with altitude
- Increase in the flow of solar radiation with altitude, when the
sky is clear as well as when it is cloudy
- Increase in cloudiness with altitude
- Relief is an important characteristic of high altitude environments,
with a high occurrence of steep terrains
What
about latitude?
The physical conditions in the different mountain massifs of the
world depend on their latitude. The latter indeed influence the
seasonal variation, the amount of solar radiation and the intensity
of rain and wind.
What
is an alpine zone?
An alpine zone is defined as the vegetation zone situated above
natural tree line, whatever the continent. This definition is therefore
not restricted to the Alps, where its name comes from, as this zone
is found on all mountains on earth. The alpine zone is characterized
by low vegetation. All the plants inhabiting this zone have common
characters (shape, physiology) that result from the adaptation to
similar environmental conditions. Four % of all known plants live
in the alpine zone. All the families of superior plants have at
least one species living in the alpine zone.
Depending on regions, the alpine zone occurs at different elevations.
Indeed, the upper altitudinal border of the forest greatly varies
depending on latitude. It is minimum under sub Antarctic latitudes
(a few hundred meters in Terra del Fuego) and maximum under tropical
latitudes (up to 4000 m).
Why
does forest disappear above a given elevation?
The upper forest
border is gradual and ranges on a few hundred meters. There is nevertheless
a sharp transition between two worlds: the forest and the alpine
meadows. This is obviously related to the climatic conditions found
up there. This border is quite pronounced in tropical, artic or
alpine environments. The main factor explaining the disappearance
of the forest with elevation seems to be the low summer temperatures.
Indeed, it has been shown that the average air temperature in July
around the treeline was 10 degree Celsius, wherever on earth and
whatever the elevation where treeline is located.
But
what is a tree?
A tree is defined as a plant consisting of a trunk and reaching
a minimum height of 3 m. The physiological reasons that keep trees
from living at high elevations are the following:
1. Low temperatures do not allow the production of a sufficient
number of new cells.
2. Unlike the species which grow close to the ground, the slender
stature of trees do not favour the development of a micro-climate
around the apical zone (cyme), responsible for the growth. In addition,
the canopy, by reducing the amount of solar radiation reaching the
ground, limits the temperature around the tree's roots.
Some
adaptations of plants to altitude
The cushion stature, very common among high altitude plants, results
from adaptation to low temperatures. For a given volume, the lower
the surface in contact with air, the less energy lost. So, for a
given volume, a spherical shape has a smaller outer surface than
an elongated shape. If you camp during winter in the mountains,
you better be rounded and plump such as Inuit, than tall and slender
such as Massai. This is the same thing for plants living all year
around attached to the mountains.
Imagine the
classical meteorological condition in mountains during a sunny day:
Air temperature: 4 , wind speed: 5m/s, 40% humidity
Can you imagine
which climate prevails at the same moment inside a cushion shape
like plant at 1-2 cm above the ground?
Temperature: 27, No wind, 98% humidity, i.e. the climate of a tropical
forest.
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High
altitude environments, a richness!
The number a living species gradually decreases with altitude.
However, the biological richness on a given area is more important
in altitude than on most lowland areas. This phenomenon is
due to the fact that mountain terrains are highly fragmented
and present large topographical variations on small areas.
This leads to a mosaic of highly diverse habitats inhabited
by a rich fauna and flora. In Chile for example, 4000 plants
have been described. Among them, 1900 species, i.e. 47 % of
all plants, can be found in the alpine zone, while this zone
only represents 19% of the country's area. Likewise, in Switzerland,
50% of the plants live above treeline, while this zone only
covers 23% of Switzerland. The same patterns occur in all
other mountains.
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