Natural Disturbance Regimes

Fires burn most Mediterranean-climate ecosystems with regularity. Image credit: Jeffery Wilcox

Although the five Mediterranean-climate regions share many environmental characteristics, they can have very different natural disturbance regimes. Their exposure to fire, and history of geological disturbance, has shaped the characteristics of MTE vegetation in many ways.

By definition, all five Mediterranean-climate regions experience characteristic summer drought. But the magnitude of this drought is particularly severe in California and Chile, where more than 6 to 8 months may pass without measurable rainfall. Such extreme drought is rare in South Africa and southwestern Australia, where summer frequently brings light showers.

Fire is an important component of disturbance in most MTEs. Flames consume a significant amount of vegetation in four of the five MTEs, and provide opportunities for other species to emerge. Central Chile, however, is an exception to this rule. Fire is a rare event because this region is protected by the high Andean Cordillera from summer storms and lightning moving westward across Argentina. There is little evidence of fire influencing the evolution of life history characteristics in Chile’s native vegetation.

A volcano in Chile’s Lake Region. Geologic upheavals by earthquakes, glaciers, and volcanoes have renewed soils in the Mediterranean Basin, California, and Chile. Image credit: Violet Nakayama

Natural fire frequencies are quite different among the other four MTE regions. In South Africa, fynbos vegetation in the Cape Region commonly burns at intervals of 10–15 years, while in California, natural frequencies are thought to range from 30–50 years to as much as a century or more.

Other strong environmental differences make the Cape Region of South Africa and southwestern Australia distinct from the other three MTEs. Both areas lie in geologically ancient and stable landscapes which have highly leached and nutrient-poor soils. In contrast, earthquakes, volcanic activity, orogenic uplift, and other dynamic processes create natural disturbance and renew soils in California, Chile, and the Mediterranean Basin. Unlike South Africa and southwestern Australia, the younger landscapes of these three regions have experienced tremendous changes in climate regime and landscape structure in Quaternary and Holocene times. These changes have had profound impacts on patterns of speciation in fire-sensitive shrub lineages in the Cape Region of South Africa and southwestern Australia. These patterns are likely the result of a combination of relatively mild and stable Quaternary climatic conditions coupled with high fire frequencies in nutrient-poor habitats.