Environmental Engineering Reference
In-Depth Information
The long drought during the summer months means that plants must adapt to a period of
severe water stress at a time when air temperatures are at a maximum. The xeromorphic
(adapted to drought) strategies which allow plants to survive these adverse conditions are
many and varied. Annuals are ephemeral plants which grow only when conditions are
favourable, i.e. in the cooler and moister Mediterranean winter. By germinating, growing,
flowering, setting seed and dying within one growing season these plants exhibit the
strategy of drought avoidance. Geophytes adopt a similar pattern but grow from a bulb or
corm which is the vegetative resting stage after flowering (e.g. tulip, scilla, asphodel).
Succulents
are able to store water in swollen cells (e.g. cacti); a special group are
halophytes (salt tolerators) such as salt-marsh plants which can survive saline soil
conditions. Halophytes cope with salinity by two mechanisms: filtering at the root surface
and expelling salt at the leaf surface. The succulence is able to dilute the salt within the
plant.
Phreatophytes
are plants with deep tap roots allowing them to reach ground water
at depth (e.g. the carob tree).
A range of structural modifications in plants favour drought tolerance: needle-leaf
form; the elimination of all leaves to give a photosynthesizing stem; sticky, waxy or hairy
leaf cuticles (surfaces); leaf stomata sunk in surface depressions; loss of transpiring
leaves in summer (drought-deciduousness); pale leaves and stems to increase reflectivity
(higher albedo). A high ratio of below-ground roots to above-ground shoots favours
moisture absorption by plants. Many Mediterranean 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 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.
DISTRIBUTION OF THE PLANT 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 (Plate 26.3).
However, the visitor to the region cannot fail to be impressed by the many variations in
plant types and cover which will be observed. For an explanation of such spatial
variability reference should again be made to Figure 20.2, which shows the effects of
regional and local factors 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
