Geoscience Reference
In-Depth Information
ratio of below-ground roots to above-ground shoots
favours moisture absorption by plants. Many Mediter-
ranean plants are also aromatic, giving off oils and scents.
This characteristic may serve several functions, including
the lowering of surface temperature through evaporation
and higher suction ability of the plant for soil water. A
widespread adaptation in Mediterranean regions is the
sclerophyllous
leaf type. This term refers to the small,
thick-walled, rigid leaf cells which result from a build-up
of sclerenchyma tissue around cell walls. The leaves do
not bend easily or flutter, which could lead to greater
water loss, and they are usually leathery and shiny. Many
sclerophyllous plants, e.g. evergreen oaks, rosemary,
thyme, Erica, will transpire actively when water is available
but will close their stomata during water stress to prevent
transpiration. However, if stomata are closed for long
periods, photosynthesis will be reduced and the plants will
be slow growers.
cover which will be observed. For an explanation of such
spatial variability reference should be made to
Figure
in their influence on vegetation.
Plant cover responds most clearly to total rainfall,
which will determine primary productivity. In coastal
areas there are variations in rainfall due to regional
position and local 'rain shadow' effects. Thus along the
southern and eastern coasts of Spain rainfall declines
eastwards from Malaga (447 mm) to Cabo de Gata (122
mm) and increases again northwards (Valencia 472 mm).
This trend reflects both the increasing protection from
onshore cyclonic storms in winter and the 'rain shadow'
of the Sierra Nevada and the Spanish Meseta. Therefore
the distribution of shrubland, heath, steppe and semi-
desert is climatically controlled in a regional sense. As
mountainous terrain is common in Mediterranean
countries and islands, these trends are significant.
Increased elevation as one travels inland from the coast
leads also to lower temperatures, reduced evapotranspi-
ration rates and increased moisture effectiveness. The
zonation of Mediterranean vegetation with altitude,
because of these changes in climate, is a well studied
feature of the ecosystem.
Figure 25.10
shows the altitu-
dinal gradients which are common to many upland
massifs. Below 750 m the typical Mediterranean tree
species will be found on land not cleared for cultivation.
Also in this zone the commonest Mediterranean conifer,
Pinus halepensis
, Aleppo pine, has been planted extensively
in afforestation schemes. Between 750 m and 1,500 m
deciduous trees replace evergreen, with chestnut occurring
COMMUNITIES
The overriding characteristic of the Mediterranean is
sparse, scattered remnants of the natural oak forest
surviving amid widespread areas of shrub (
maquis
) and
heath (
garrigue
) communities. The forest has the
ecological status of climatic climax; the lower shrub and
heath are plagio-climaxes after centuries of human
impact. However, the visitor to the region cannot fail to
be impressed by the many variations in plant types and
2500
Heaths
Hedgehog broom
2000
Silver fir
Pines
1500
Beech
Oak
1000
Chestnut
Maquis
Garrigue
Steppe
Holm oak
500
Cork oak
Kermes oak
Olive, citrus
Aleppo pine
0
