Western Mediterranean base-rich scree

This is a calcareous and ultrabasic scree, constituted by boulders, rock debris and riverine gravel of the western Mediterranean, from lowlands through to the high mountains. It is composed by rock types that are sedimentary and metamorphic limestones and dolomites, further serpentinite and other ultramafic as well as silica-poor igneous volcanic rock such as basalt. Epilithic lichens and bryophytes may be very diverse, particularly in the mountains, where they are mostly found in crevices and other shady and humid microsites of immobile boulders. The vascular plant vegetation of western Mediterranean base-rich screes consists mainly of hemicryptophytes and chamaephytes adapted to the mechanical disturbance caused by mobile screes, shortages in water supply and lack of fine-grained soil. Many plants are disturbance-resilient and capable of regeneration even after being buried by moving stones. Prostrate stems, stolons, radicants, extensive root systems, storage tubers and rhizomes are common traits in the plants that are present in this habitat. Scree creeping, passive moving with mobile screes, and accumulation of scree through resilient tussocks and root stocks, thereby controlling erosion, are characteristic growth form functional strategies in Mediterranean screes. Species-rich genera of vascular plants are, among others, Campanula, Iberis, Linaria and Scrophularia. The species composition in the high mountains is particularly variable and includes many regional endemic taxa. Several phytosociological alliances restricted to subalpine and alpine levels of mountain ranges in the Iberian and Apennine Peninsulas or to the larger islands have been described. They reflect phytogeographical patterns of isolation and centers of speciation. Lowland scree and gravel vegetation is in comparison more uniform in the western Mediterranean. Most plant communities belong to the extremely variable class of Thlaspietea rotundifolii. Local habitat variation reflects differences in slope and substrate mobility, rock size and mineral composition, microclimate, aspect and solar radiation, humidity and precipitation.

Mediterranean base-rich screes cover extensive areas in the high mountains. In the lowlands and foothills, by contrast, they may be rare. Gravel banks occur along permanent or temporary streams. Western Mediterranean base-rich scree habitats occur from the Iberian Peninsula through southern France to the Apennines (Italy), on the Balearic Islands, Corsica, Sardinia, Sicily, and on many of the smaller islands and archipelagos of the Tyrrhenian Sea, such as the Tuscan, the Aegadian and the Aeolian, and further in the Mediterranean domain of northwest Africa (Morocco, Algeria and Tunisia).

Indicators of quality:

The dynamics of scree and gravel habitats depend on the natural constant supply of rock debris and riverine materials. When there is no supply from rock source areas above, the habitat and its vegetation will be subject to succession and gradually change in character towards grasslands, shrublands or woodlands. High-mountain screes in the western Mediterranean, underlying natural dynamics, are normally little or not affected by human impact. In contrast, riverine gravel fills in the lowlands, with their rivers, have commonly been subjected to quarrying, hydrological control or other drastic changes of the river regime. Human-made screes such as mining heaps may provide important and valuable secondary habitats, especially after long periods of abandonment. They should be considered and preserved when their quality is good, especially when primary scree habitats are absent in a wider area. The habitat quality must be assessed by taking into account the regional species pool. Scree and boulder specialists, with many endemics and relict plants among them, are useful indicators of good habitat quality.

The following characteristics may be used as indicators of favorable habitat quality:

• Occurrence of rare and phyto-geographically significant plants
• Presence of sizable areas of scree and gravel with adequate and ongoing supply of rock material through cliff, stream and river dynamics, and with local differences in slope, moisture, aspect, substrate mobility, and grain size
• Contact with natural habitats such as cliffs, high-mountain pioneer grasslands and plant cushion vegetation, or riverine scrubs and woodlands
• Absence of gravel quarrying and mining
• Absence of hydrological and traffic constructions affecting the river regime

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats