Unusual edaphic conditions harbor unique plant associations often characterized by rarity and endemism. Such conditions also foster distinct morphological and physiological modifications leading to characteristic plant communities. One of the most remarkable edaphic habitats in which such unique plant communities are found is on serpentine soils derived from ultramafic and related rocks (i.e., rocks high in iron and magnesium silicates). Ultramafic rocks such as serpentinite and their associated serpentine soils are found throughout the world, concentrated, however, along continental margins and in regions of orogenesis (i.e., mountain building). Serpentine soils are unique in that they are often high in pH and heavy metals such as magnesium, nickel, and chromium, and generally low in essential
nutrients, Ca/Mg ratio, and water-holding capacity. The rocks are often on open, steep slopes exposed to high light and heat conditions, and resulting soils are generally shallow and highly erodable (Figure 4). The serpentine syndrome, the unique biological effects manifested by these extreme geoedaphic conditions, has led to research on plant physiology, ecology, and evolution in many parts of the world. Serpentine soils, although covering a mere few percent of the Earth's surface, host many endemic species. In the Californian Floristic Province, for example, 198 out of 2133 taxa endemic to that province are wholly or largely restricted to serpentine. Tropical islands of New Caledonia and Cuba provide even better examples of plant restriction to serpentine soils. In New Caledonia, 3178 taxa, roughly 50% of the native flora, are endemic to serpentine soils while in Cuba, 920 species, one-third of the taxa endemic to Cuba, have developed solely on serpentine soils. Similar restrictions and remarkable floristic associations are also found in serpentine areas of the Mediterranean, Africa, Australia-New Zealand, and Asia. Studies of metal hyper-accumulators (i.e., plants that accumulate at least 0.1% of their dry leaf weight in a heavy metal) of serpentine soils have not only led to the discovery of novel physiological pathways and their underlying genetic bases but have also laid the foundation for the development of innovative technologies such as phytoremediation (i.e., the use of hyperaccumulators to extract heavy metals from contaminated soils).
Gypsum (CaSO4 ? 2H2O), a substrate formed by the evaporation of saline waters, is also widely known for its distinctive indicator flora. The plant response to gypsum (gypsophily) manifests itself as unique communities consisting of gypsophilic endemics. While gypsum-associated plant communities are found in parts of Europe, deposits in xeric areas of the American southwest and adjacent Mexico are especially noted for their unique species composition.
Limestone forms by precipitation and lithification of CaCO3, also leads to the formation of unique plant communities. In fact, some of the earliest observations on edaphic-plant relationships were made on landscapes overlying limestone and, by the late twentieth century, studies of limestone plant ecology had yielded a plethora of published work in both North America and Europe. Limestone and associated materials such as dolomite (CaMgCO3) have exerted a profound influence on regional floras across the world resulting in unique vegetation compositions. Of interest are those temperate formations found on the White Mountains of eastern California, Mount Olympus of Greece, the European Alps, and tropical formations of Jamaica, Cuba, Turkey, and parts of Asia.
In addition to habitats formed on geologies with extreme chemical composition, other edaphically influenced habitats, such as savannas, barrens, guano-rich bird nesting rocks, coastal bluffs, alkaline flats, and vernal pools are also important sites that harbor unique communities of plants and animals.
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