Nutrients and Water Relations in Mediterranean-Type Ecosystems1

The broad correspondence between the sclerophyllous shrub form and the climate in mediterranean-type ecosystems indicates a climatic control over the vegetation particularly with respect to the summer drought, but all five regions are noted for nutrient deficiencies, especially Australia and South Africa. As studies of these ecosystems are completed, divergences related to the control of community structure, function, and form are becoming apparent. Differences in species composition and growth form are apparent on nutrient poor and base rich sites (Specht and Moll 1981). The mediterranean type ecosystems in South Africa and Australia are generally considered nutrient poor relative to the mediterranean-type ecosystems in Chile, the United States, and the Mediterranean Area. In Australia phosphorus limitation is assumed to have selected for sclerophylly, which preadapted the flora to the more recent summer drought climate (Beadle 1954, Specht 1979 Moll and others 1981). In South Australia the overstory vegetation is believed to be evergreen because of climate, while the understory is evergreen because of nutrient impoverishment (Specht 1972). In California the overstory is believed to be evergreen because of climate, and the understory is deciduous because of microclimate near the soil surface (Miller 1981). Nutrient deficiencies have been suggested to explain other aspects of the community structure in the mediterranean-type ecosystems; such as succession following fire (Specht 1972), species richness in South Africa aid Australia (Kruger 1979), the distribution of fynbos and heathlands in South Africa aid Australia over a wide range of annual precipitation (300-3000 mm/yr) (Specht 1979), and specialized morphological structures (Lamont 1972, 1973, 1980, 1981). In contrast, in California and Chile the length of the summer drought is thought to control sclerophylly (Miller 1981). Evergreen sclerophyllous shrubs are believed to occur in California where the length of the soil drought is about 100 days or less (Miller and Mooney 1974). In California the leaf area index develops until water use by the chaparral creates a drought lasting about 100 days (Miller and Poole 1981). In Australia evergreen shrubs moderate water use so that the period of summer soil drought is minimized (Specht 1972); water is then available throughout the summer to support photosynthesis and growth in summer. In some regions of high annual precipitation in South Africa and Australia, the soils are either rocky and sandy or are underlain by a hardpan, so periodic drought is possible in spite of high rainfall.