Small leaves and low specific leaf area (SLA) have long been viewed as adaptations to Mediterranean-type climates in many species of evergreen woody plants. However, paleobotanical and floristic evidence suggests that in many cases these traits originated prior to the advent of the summer-drought climate regime. In this study, molecular phylogenies and ancestral state reconstructions were used to test the hypothesis of adaptive leaf evolution in 12 lineages of evergreen shrubs in the California chaparral. Across all lineages there was a small but significant shift toward lower SLA, but there were no trends in leaf size evolution. For individual lineages, adaptive changes were detected in only three cases for SLA and in one case for leaf size. Three of these cases of evolutionary change were observed in taxa derived from cool temperate ancestors (e.g., Heteromeles). In contrast, most lineages originating from subtropical ancestors exhibited relative stasis in leaf trait evolution (e.g., Ceanothus). The absence of change suggests that ancestors of chaparral taxa had already acquired appropriate traits that contributed to their success under Mediterranean-type climates. These results illustrate how biogeographic history may influence patterns of trait evolution and adaptation and highlight the contribution of ecological sorting processes to the assembly and functional ecology of regional biotas.

https://www.jstor.org/stable/10.1086/383062

Adaptation, niche conservatism, and convergence: comparative studies of leaf evolution in the California chaparral.

Although water is the most abundant molecule on the Earth’s surface, the availability of water is the factor that most strongly restricts terrestrial plant production on a global scale. Low water availability limits the productivity of many natural ecosystems, particularly in dry climates (Fig. 3.1). In addition, losses in crop yield due to water stress exceed losses due to all other biotic and environmental factors combined (Boyer 1985). Regions where rainfall is abundant and fairly evenly distributed over the growing season, such as in the wet tropics, have lush vegetation. Where summer droughts are frequent and severe, forests are replaced by grasslands, as in the Asian steppes and North American prairies. Further decrease in rainfall results in semidesert, with scattered shrubs, and finally deserts. Even the effects of temperature are partly exerted through water relations because rates of evaporation and transpiration are correlated with temperature. Thus, if we want to explain natural patterns of productivity or to increase productivity of agriculture or forestry, it is crucial that we understand the controls over plant water relations and the consequences for plant growth of an inadequate water supply.

Plant Water Relations

Five regions of the world — the Mediterranean Basin, California, central Chile, the Cape Region of South Africa, and Western and South Australia (Fig. 1.1) — share a unique climatic regime with their characteristic conditions of mild wet winters and warm dry summers. This mediterraneantype climatic regime typically has 90% or more of annual precipitation falling in the six cool season months, mild winter conditions with infrequent and relatively mild frosts, and frequent periods of extended summer drought.

Landscape Disturbance in Mediterranean-Type Ecosystems: An Overview

Effect of NaCl on the growth and the ionic balance K+/Na+ of two populations of Lotus creticus (L.) (Papilionaceae)

Historically, since fire is a recurrent catastrophic disturbance, mediterranean-climate shrubs have been classified by their mode of postfire regeneration, i.e., obligate seeders, facultative seeders or obligate resprouters. While these terms are useful, they are too restrictive in that they only refer to a species’ response to fire and do not adequately describe modes of reproduction for all taxa.

J. Aronson

Papers

Time scales of biological responses to water constraints: the case of Mediterranean biota.

Adaptive significance of some morphological and physiological characteristics in Mediterranean plants: facts and fallacies.