Mediterranean climate and vegetation
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Mediterreanean climate
- Each hemisphere of Earth has three wind belts: in the northern
hemisphere, easterly 'Trade' winds extend to about 30°N, 'Westerly'
winds to about 60°N; and polar easterlies to the pole; although this is
complicated by the effects of continental land-masses, etc.
- The shores of Provence are at 42-43°N, and thus close to the
boundary between the Trade wind belt and the Westerly wind belt. This
boundary is a region of descending air and high pressure, and thus
typically fine dry conditions.
- However, the wind belts shift seasonally, with the result that spring
and summer is a hot dry season (with no wind or easterly winds) and autumn
and winter are cooler and wetter (with westerly winds and depressional
rain).
- A local feature of Provence is the 'Mistral', a north wind that blows
in winter when there is cold high pressure air over central France and
lower pressure over the Mediterranean; the wind is funnelled by the
Rhône valley instead of being diverted by Coriolis.
Mediterreanean vegatation
- A biome "consists of a distinctive combination of plants and
animals in a fully developed or climax community." Each biome "is
characterised by a uniform life form of vegetation, such as grass or
coniferous trees. It also includes the developmental stages, which may be
dominated by other life forms". (Smith, R.L., 1992. Elements of Ecology'
3rd ed.)
- The Mediterranean biome is characterised by evergreen trees, which can
grow during the wetter but well-illuminated winter, and which can resist
dry conditions during summer. The main tree species under natural climax
conditions are:
- Quercus ilex, the (ever)green oak;
- Pinus pinea, the parasol pine;
- Authorities differ as to which was the most important species, and
this depend on local conditions. The downy-leaved, deciduous, oak
Quercus pubescens might also be present and certainly would be
increasingly important inland.
- Rock on Provence is mainly limestone; this is porous, and water drains
away rapidly;
- However, the evergreen trees have deep roots that penetrate the rock
and tap water reservoirs here during summer; the leaf-fall from the trees
enriches the soil, and the shade of the trees, and the shelter they provide
from wind, keep the soil moist.
- During the last few thousand years, most of the original Mediterranean
woodland has been eradicated by humans, using it for timber and replacing
it by agriculture;
- Grazing by domesticated animals prevented tree regeneration, and,
without the shelter of trees, heavy autumn rainfall washed away much of the
soil;
- fires caused deliberately or accidentally by humans burns trees and
bushes and further damages the soil; it favours annuals and plants that can
regenerate rapidly from underground or near-ground buds;
- During the walk at Luminy we saw two main plant communities:
- near the coast was an open community of shrubs or bushes, mostly
forming low mounds separated by bare rock; this is the garrigue, and
some of the characteristic plants we saw were:
- the kerm oak Quercus coccifera, a spiny-leaved evergreen, the
characteristic shrub of this community, forming low mounds, or larger
bushes in sheltered conditions;
- rosemary Rosmarinus officinalis
- white rock rose Cystus albida
- the tall heather Erica arborea
- the gorse or whin Ulex europaeus.
- sheltered by the hill from sea winds and protected from grazing and
fire during recent years, was a young wood in which the main tree was:
- the aleppo pine, Pinus halepensis, originally from the eastern
Mediterranean;
- there were also a few Pinus pinea;
- underneath the pine trees were many of the plants of the garrigue that
had originally covered the land here but which are now gradually being
shaded out as the pine trees grow;
- there were also a few small Quercus pubescens and
Q.ilex, which may, given a long period of stability, come to grow
large and hence take over the wood;
- however, the accumulation of pine needles potentiates fires, which are
more damaging to the tree oaks than to the thick-barked pines;
- the garrigue and pine wood may be seen as successional stages leading
back to the Mediterranean evergreen oak wood; disturbance by humans, their
grazing animals, and fire, prevents this succession, and so garrigue is now
the typical vegetation of poor land in much of Provence;
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Page made by Paul Tett, School of Life Sciences, Napier
University, on 8 December 2003.