Endemism in the California flora, Part I

A couple of issues ago I wrote about some of the concepts of rarity in plants. One of the most fascinating aspects of California botany is the extent to which our native species are endemic, or restricted, to the state. California is well known as a center of floristic diversity and for exhibiting a high degree of endemism (Stebbins and Major 1965). In general, variations in climate, topography, and geology are strongly correlated with floristic diversity. In the United States, California is exceeded only by Hawaii in terms of the number of endemic plant species (Stebbins I 978a, b). The Jepson Manual (Hickman 1993) lists approximately 6,000 native plant species, subspecies, and varieties. Another 300 or so receive mention as minor taxa. Over 2.2% of the genera and 30% of the species of vascular plants in California are endemic (Raven 1988). Considering the California Floristic Province alone, which excludes the Great Basin east of the Sierra Nevada and the deserts, the percent of endemic genera and species increases to 7.5% and 48%, respectively (Raven 1988).

The extant California flora is composed of elements that evolved during three distinct geologic epochs; the northern temperate Arcto-Tertiary Geoflora, the Madro-Tertiary Geoflora, and the NeotropicalTertiary Geoflora (Axelrod 1968). With the onset of a global drying trend following the Eocene (38-54 million years before present), mesic (moist) Arcto-Tertiary and NeotropicalTertiary elements were gradually displaced by xeric (dry) elements of the Madro-Tertiary Geoflora (Axelrod 1958). Expansion of xeric habitats is thought to have exerted a major selective pressure leading to the development of the xeromorpbic features (e.g., thick cuticles, reduced leaf surface area, spiny stems, drought deciduousness, etc.) that are common in the present-day flora.In addition to climate, California’s mosaic of geologic substrates has also had a strong influence on shaping the state’s flora. It has been suggested that a majority of California’s endemism and populational disjunctions can be directly linked to diverse edaphic (soil) conditions (Raven 1964; Stebbins and Major 1965). Relictual elements of more mesic ancestral floras can be found in microhabitats protected from an increasingly arid climate. Particular edaphic properties, such as increased moisture-holding capacity, may create refugia for less xeromorphic species, allowing them to persist in the face of increasingly droughty conditions.

Humans have long been aware of sharp discontinuities in vegetation and that some soils are more productive than others. This fact alone has had a pronounced impact on civilization by determining where large population centers could persist. But it wasn’t until the nineteenth century that any direct correlations were drawn between geologic substrates and the vegetation they support. Franz Unger, an Austrian botanist, first emphasized the significant role of geology in plant distribution in 1836 (Kruckeberg 1969). Having observed the divergent species composing the vegetation of two facing mountain slopes in northeastern Tyrolia, Unger hypothesized that the primary influence on plant distribution in this region was the mineral content of the soil. Since then, chemical and physical properties of soils have been widely cited as the most critical environmentalfactors affecting the distribution of plant species and vegetation types.

More recently, the general discontinuity of the physical environment and its role in isolation, speciation, and natural selection of plant populations has become the focus of attention by ecologists and evolutionary biologists. Mason (1946, 1954) suggested that environmental diversity results in broad selective powers. In this way, he related edaphic discontinuities with genotypic differentiation and endemism. Raven (1964) proposed that marginal populations are often found on soil types unusual for the species as a whole and that substrates unfavorable to those species becoming dominant as the result of changing climatic conditions could permit the persistence of retreating species as relictual populations. Such thinking led to the concept of species adapted to a particular substrate being competitively superior to otherwise more widespread species. Drury (1974) thought of species restricted in this manner as being adapted to “stressed” sites and that preadaptation to these sites can be seen as a strategy for survival leading to ecotypic differentiation.

Kruckeberg (1969) reduced environmental factors affecting plant distribution to two broad groups – climatological and geological. He felt that the interaction of microclimate, biotic features, and geology creates a rich mosaic of microhabitats but that edaphic influences play the dominant role in determining diversity and the distribution of plant species. Raven and Axelrod (1978) and Kruckeberg (1986) agreed that geologic diversity begets biologic diversity and Kruckeberg and Rabinowitz (1985) stated that the overall magnitude of geological discontinuity may be the ultimate cause of local rarity.