Environmental Engineering Reference
In-Depth Information
Figure 26.10 The zonation of Mediterranean plant
communities with altitude.
schemes. Between 750 m and 1500 m deciduous trees replace evergreen, with chestnut
occurring in the lowest sub-zone, and being replaced at higher elevations by deciduous
oak, beech and elm. Between 1500 m and 2000 m evergreen conifers become dominant,
chiefly pine, silver fir, cypress and cedar. Above treeline at about 2000 m the subalpine
and alpine zones consist of heaths and cushion vegetation. One further important
influence which mountains have on the flora is to provide refugia which plants have been
able to retreat to during glacial periods and have there evolved in isolation, giving rise to
many endemic species. Shrub and ground flora are much richer on Mediterranean
mountains than on the coasts because of the higher soil moisture contents. It is among
these plants that endemism reaches high levels.
In addition to elevation, but equally obvious to the local observer, is the influence of
aspect, which has an important effect on surface microclimates and vegetation growth,
owing to the annual radiation budgets (see Chapter 9). It has long been recognized that
aspect (compass bearing with regard to north and south) and topographical position
(location on the slope profile) interact to influence air temperature, soil temperature and
soil moisture. In mid-latitudes the north- and south-facing slopes show strong asymmetry
in total annual incoming radiation. Figure 26.11 shows the effects of latitude and slope on
the differences in total amounts of short-wave radiation received by north- and south-
facing slopes. Assuming clear skies and no shading effects, the differences are greatest in
middle latitudes and least in tropical and polar regions.
In reality, of course, a complication can be introduced at the bottom of deep valleys,
where there may also be an effect due to shading by surrounding hills. All these
influences can be studied by recording in the field the three parameters of aspect, gradient
and angles to the
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