International Geology Review, Vol. 46, 2004, p. 235–242. Copyright © 2004 by V. H. Winston & Son, Inc. All rights reserved. Does the Age of Exposure of Serpentine Explain Variation in Endemic Plant Diversity in California? SUSAN HARRISON,1 Department of Environmental Science and Policy, University of California, Davis, California 95616 HUGH SAFFORD, U.S. Department of Agriculture, U.S. Forest Service, 1323 Club Drive, Vallejo, California 94592 AND JOHN WAKABAYASHI 1329 Sheridan Lane, Hayward, California 94544 Abstract We analyzed botanical, geologic, and climatic data in an effort to understand why there are many more serpentine endemics—i.e., plant species restricted to ultramafic substrates—in some regions of California than in others. We found that, in addition to elevational range (which subsumed the effect of serpentine area) and annual rainfall, the age of exposure of serpentine explained significant geographic variation in the diversity of endemic plant species. Estimated ages of subaerial exposure are generally higher in the Klamath and northern Coast Range serpentines than in Sierra Nevadan and southern Coast Range analogues, paralleling the trends in endemic plant diversity. However, the fit of the relationship is weakened by the southern Sierran serpentines, which are apparently long- exposed yet poor in endemic plant species. These results have significance for understanding local ecological processes, because we have also found that the diversity of endemic species within 50 × 10 m field plots is best predicted by the diversity of endemics in the surrounding region, which in turn is a function of the age of subaerial exposure. Introduction species restricted to ultramafic substrates (Brooks, 1987).2 THE CALIFORNIA FLORA is characterized by a Serpentine endemics contribute disproportion- remarkable number of plant species that are found ately to the distinctiveness of the state’s flora, com- nowhere else on earth (Myers et al., 2000; Stein et prising at least 10% of California’s unique plant al., 2000). The state’s rich flora is thought to be the species, even though ultramafic rocks and soils product of rapid evolutionary diversification brought cover only around 1–2% of the state’s surface area about by interactions between the Late Tertiary (Kruckeberg, 1984). In his comprehensive treatise development of a Mediterranean-type climate along on this flora, Kruckeberg (1984) estimated that Cal- the southwest Pacific coast, active tectonism, and ifornia has just over 200 strict serpentine endemics high levels of geologic and geomorphic complexity and another 200 serpentine “indicators,” or plant (Stebbins and Major, 1965; Raven and Axelrod, species partially restricted to this rock type. The 1978). An important facet of California’s geological California serpentine flora has thus been a long- 2 and botanical diversity is the > 5,000 km of ultra- standing subject of study for those interested in the mafic rocks found along ancient and modern tec- origins, distribution, and maintenance of biological tonic margins. Ultramafic rocks generate soils that diversity (e.g., Stebbins, 1942; Raven, 1964; Steb- are poor in essential plant nutrients (N, P, K, Ca) bins and Major, 1965; Raven and Axelrod, 1978; and high in potential toxins (Mg, Ni, Cr). These con- Kruckeberg, 1984; Coleman and Kruckeberg, 1999; ditions have led to the development of unique plant Kruckeberg, 2002). species and communities. California joins Cuba, New Caledonia, and Turkey as one of the world’s richest locations for “serpentine endemics,” or plant 2 We here use the term “serpentine” in the broad ecological 1Corresponding author; email: [email protected] sense to refer to ultramafic rocks and soils derived from them. 0020-6814/04/723/235-8 $25.00 235 236 HARRISON ET AL. Here we report part of an ongoing attempt to bet- sity on old islands is widely observed to be higher ter understand variation in numbers of serpentine than on younger islands (Rosenzweig, 1995). In endemics within California, as a small step toward Cuba, which has the world’s richest known serpen- better understanding why some areas of the world tine flora, with 920 endemic species and 24 are especially rich in unique biological diversity. endemic genera, the length of time that serpentine Kruckeberg (1984) quantified the observation by “islands” have been exposed is considered the earlier authors (e.g., Stebbins and Major, 1965; primary reason for strong geographic patterns in ser- Raven and Axelrod, 1978) that the numbers of ser- pentine endemism. Serpentine outcrops on the east- pentine endemics decline sharply from the north- ern and western ends of Cuba are believed to have western to the southern and eastern parts of the been exposed above sea level for as much as 30 m.y., state. Kruckeberg estimated that there are about 80 and these support 86% of the serpentine-endemic total (50 restricted) endemics in the Klamath- species and 100% of endemic genera. The serpen- Siskiyou region, about 100 total (40 restricted) in tines in central Cuba are believed to have been the northern Coast Ranges, 30 (20 restricted) in the exposed for only around 1 m.y., and although they San Francisco Bay Area, 60 (40 restricted) in comprise 36% of the serpentine area, they support the southern Coast Ranges, and 30 (15 restricted) in only 19% of endemic species and 8% of endemic the Sierra Nevada. Kruckeberg surmised that these genera (Borhidi, 1996, pp. 129–130). geographic trends were the evolutionary product of In their landmark monograph on the origin and the greater area of serpentine in northwestern Cali- relationships of the California flora, Raven and fornia (Fig. 1). In evolutionary biology, a larger area Axelrod (1978) considered the age of exposure as an of islands or island-like habitats is well known to influence on the distribution of Californian serpen- promote speciation by reducing the rate of extinc- tine endemics. They concluded from map interpre- tion of new lineages (Rosenzweig, 1995). tation that most serpentine exposure ages in To explore this and other explanations for vari- California were Late Pliocene to Early Quaternary. able levels of serpentine plant diversity in Califor- Because many shrub and tree species confined to nia, Harrison et al. (2000) assembled a geographic serpentine (e.g., leather oak, Quercus durata; Sar- data base with the numbers of total plant species, gents cypress, Cupressus sargentii) were known to be numbers of serpentine endemics, and various geo- older than this, Raven and Axelrod concluded that logic, topographic and climatic variables for each of these species once occurred on other soils, and the serpentine-containing spatial units (“subre- became restricted to serpentine following climate 3 gions”) of the state. The two most significant pre- change.4 In contrast, these authors concluded that dictors of endemic diversity are the average annual most herbs endemic to serpentine belonged to rap- rainfall and the elevational range (maximum minus idly diversifying taxa—e.g. the genera Navarretia, minimum) of the ultramafic areas within a subre- Streptanthus, and Hesperolinon—and had originated gion. The area of ultramafic outcrops in a subregion, recently on serpentine soils.5 However, Raven and which is highly correlated with elevational range, is Axelrod (1978) did not specifically address the rela- not statistically significant if elevational range is tionship between age of exposure and the geo- included in the model. Rainfall and elevational graphic variation in the numbers of serpentine range are also positively correlated with the total endemics in California. number of plant species in a subregion, and with the To explore this question, we generated a set of percentage of plants that are serpentine endemics. estimated ages of subaerial exposure to add to the Thus, there is a correlation between Californian database described above. We then examined the plant diversity and climate, as others have found partial correlation of this variable with numbers of (Richerson and Lum, 1970), but it appears to be even stronger among serpentine endemics than endemic plant species, employing statistical models plants as a whole. that also included climate, elevational range, and serpentine area. We report our tentative conclusions In this paper we consider another possible influ- ence, namely the length of time that serpentine has been available for plants to colonize. Species diver- 4 Such old and relictually distributed species are called “paleoendemics.” 5 Such newly evolved and narrowly distributed species are 3 See the Methods section for more details. called “neoendemics.” EXPOSURE AGE OF SERPENTINE 237 FIG. 1. Map of California showing counties and outcrop areas of serpentinized peridotite, after Jennings (1977). here in the hope of generating discussion and per- how biological diversity is shaped by the interaction haps filling gaps in the data. between large-scale physical variables such as cli- In another part of our work, not discussed in mate and geologic history, and smaller-scale ecolog- detail here, we are conducting plant, soil, and rock ical variables such as soil fertility, species sampling in 50 × 10 m field plots on serpentine out- interactions, disturbances, and the spatial distribu- crops at >100 sites well distributed across the 92 tion of available habitats (see Harrison, 1997, 1999; subregions. By combining our regional (GIS) and Harrison and Inouye, 2002; Safford and Harrison, local (field) data, our ultimate goal is to