Main Ecosystem Types

The vegetation types usually considered as 'typically Mediterranean' are the evergreen and sclerophyllous shrublands or heathlands, named maquis and garrigue in the Mediterranean Basin, chaparral in California, matorral in Chile, fynbos in SW Africa, kwongan and mallee in SW Australia. But there exists considerable differences in the composition and structure of these shrublands (Table 1). Depending on the type of shrubland chosen, it is possible to find strong similarities or dissimilarities, between and within the diverse Mediterranean ecore-gions. The similarity most frequently cited is that between kwongan and fynbos which show a striking ecological convergence with an open shrub cover and a high shrub diversity with the quasi-absence of annuals, a dominance of postfire seeders, and serotinous shrubs, and the frequent occurrence of seed dispersal by ants (myrme-cochory). Shrublands occur generally in a mosaic with xerophytic grasslands, steppes, woodlands, or forests.

True Mediterranean forests are rare and they represent 1.8% of world's forest area. Northern Hemisphere Mediterranean forests show a higher structural and species diversity than those of the Southern Hemisphere, because the latter cover less extensive areas and, as in South Africa, may be outside the range of Mediterranean bioclimate. The Southern Cape forests are very patchy with mainly subtropical sclerophyllous trees and conifers (Afrocarpus, Podocarpus). Forests of Mediterranean Chile are more diverse due to the strong latitudinal gradient and the increase in rainfall from north to south; semiarid Acacia caven and Prosopis chilensis forests in the north are succeeded by subtropical broad-leaved and sclerophyl-lous forests in central Chile, and by deciduous Nothofagus forests farther south. Together with species-rich sclero-phyllous shrublands (kwongan and mallee), the forests and woodlands of SW Australia are dominated by Eucalyptus, Acacia, and Casuarina on poor sandy soils, where mean annual rainfall exceeds 400 mm; several types of Australian woody vegetation are distinguished according to the foliage cover of tallest stratum and the high of the trees.

In the Mediterranean Basin, the current diversity of forest structures can be organized into three major structural types based on bioclimatic and/or human impact criteria (Tables 2 and 3).

True forest vegetation types are related to metastable equilibrium of vegetation structures. They represent the potential structures at the end of a dynamic ecological cycle, which can be achieved where soil and climate conditions are favorable and where the impact of man is not too strong. Dominant species are sclerophyllous oaks in semiarid bioclimates and deciduous oaks in more humid conditions.

Preforest types can be divided into two categories. Under perhumid, humid, and subhumid bioclimates, they consist of vegetation structures that have undergone severe human impact, although their soil is still relatively well preserved. They are transitory structures from true forests to more open systems. Under semiarid bioclimatic conditions, or under particularly stressful conditions (e.g., ultramafic substrates) in any bioclimate, preforests are comprised of shrub-dominated vegetation structures with scattered trees (matorrals). Conifer species (Pinus, Tetraclinis) play an important role in these structures.

Presteppic forest types, very frequent in southern and eastern Mediterranean, consist of open-vegetation structures dominated by nonforest plant species under scattered trees. Nonforest species are steppe-type perennial species that can eventually be replaced by ruderal annual species when grazing occurs. Presteppes are most frequent under warm and hot temperature variants of arid (and sometimes semiarid) bioclimates. They gradually merge into steppes under hotter and drier conditions. On mountains, presteppes are a transitional vegetation structure from forests (or preforests) to high-elevation steppes dominated by low and scattered cushion-like spiny xerophytes.

Annual grasslands represent also key ecosystems in the two northern Mediterranean ecoregions. The composition and structure of grassland communities are strongly controlled by disturbances, which create a complex pattern of microsites and canopy gaps. Therefore, the heavily grazed grasslands of the Mediterranean Basin have probably the greatest alpha diversity of any temperate plant community, and annuals represent half of the total species found in this region. Nearly one-fifth of California is covered by grasslands, but most of them are dominated by non-native annuals (Bromus, Avena, Erodium, etc.) originated from the Mediterranean Basin.

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