Soils and Nutrients Availability

The five Mediterranean ecoregions are characterized by different geologies and soils characteristics due to their contrasted physiographic histories (Table 1). Landscapes of SW Australia and SW Africa consist of inland mass of geologically older origins than the three other ecoregions where mountain-building events occurred as recently as the Tertiary and the Quaternary.

In the two South Hemisphere ecoregions, soils on older substrata of uplands are generally highly leached lithosols in South Africa and by laterites and the process of podzoliza-tion in southern Australia; they have been exposed to weathering since the Paleozoic or even the Precambrian. Coastal deposits are younger and determine calcareous sands, decalcified humus podzols, and bleached sands. Calcareous soils of limestone origin are scarce and occur only in few places of the South African coasts and in the central-southern region of Mediterranean Australia.

In California and Chile, the violent tectonic activity down the west coasts during the Late Tertiary and Early Quaternary has given rise to rugged landscapes of the Cordilleran mountain chain. Upland areas possess generally coarse-textured lithosols, whereas the major inland valleys (Great Valley of California and Central Valley of Chile) have more fertile soils linked to alluvial deposits.

In the Mediterranean Basin, the diverse tectonic and orogenic activities, and also the consequences of Pleistocene glaciations, induced a complex patchwork of landscapes and a mosaic of soil types. The predominantly limestone rocks have given rise to the terra rossa soil, a clay-rich and relatively fertile soil of the lowland areas. Soils of the humid uplands are often leached podzols or brown forest soils occurring within forested landscapes.

In spite of these sometimes large differences in substratum geology, there are several similarities between soils of the Mediterranean ecoregions, due to similar pedogenic processes linked to water-driven erosion and

Table 2 Vegetation levels showing the correspondence between thermal variants and dominant woody types of the Mediterranean Basin

Thermal

Vegetation level

variant

m(°C)

T(°C)

Dominant woody species

Infra-Mediterranean

Very hot

> +7 °C

>+17°C

Argania, Acacia gummifera

Thermo-Mediterranean

Hot

+3 to +7 °C

>+17°C

Olea, Ceratonia, Pinus halepensis and P. brutia,

Tetraclinis, (Quercus)

Meso-Mediterranean

Temperate

0 to +3 °C

+13 to +17 °C

Sclerophyllous Quercus, Pinus halepensis and P. brutia

Supra-Mediterranean

Cool

-3 to 0 °C

+8 to +13 °C

Deciduous Quercus, Ostrya, Carpinus orientalis (Pinus

brutia)

Mountain-Mediterranean

Cold

-7 to -3 °C

+4 to + 8°C

Pinus nigra, Cedrus, Abies, Fagus Juniperus,

Oro-Mediterranean

Very cold

<-7 °C

< +4 °C

prostrate spiny xerophytes

m, mean minimum temperatures of the coldest month; T, mean annual temperature.

Modified from Quezel P and Medail F (2003) Ecologie et biogéographie des forêts du bassin méditerranéen. Paris: Elsevier.

Table 3 Main types of bioclimates and their theoretical correspondence with the dominant vegetation types of the Mediterranean Basin

Mean annual rainfall

Number of months

Bioclimate

(form = 0 ° C)

without rainfall

Main vegetation type

Per-Arid

< 100mm

11-12

Saharan

Arid

100-400 mm

7-10

Steppe and pre-steppe (Juniperus turbinata, Pinus halepensis, Pistacia atlantica)

Semi-Arid

400-600 mm

5-7

Pre-forest (Pinus halepensis, P. brutia, Juniperus spp., Quercus)

Sub-Humid

600-800 mm

3-5

Forest (Mostly sclerophyllous Quercus, Pinus halepensis, P. brutia, P. pinaster, P. pinea, P. nigra, Cedrus)

Humid

800-1000 mm

1-3

Forest (Mostly deciduous Quercus, Pinus brutia, P. pinaster, P. nigra, Cedrus, Abies, Fagus)

Per-Humid

>1000mm

<1

Forest (Deciduous Quercus, Cedrus, Abies, Fagus)

m: mean minimum temperatures of the coldest month.

Modified from Quezel P and Medail F (2003) Ecologie et biogeographie des forests du bassin méditerranéen. Paris: Elsevier.

leaching. These seasonally droughted and moderately to strongly leached soils are indeed characterized by a low availability of several nutrients, especially phosphorous, and nitrogen which is greatly affected by fire.

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