Raven and Axelrod’s (1978) account is remarkable for its attention to detail. Their sweeping historical analysis is complemented by attention to the numbers of species belonging to each geographic region, life form, and biogeographic origin. Tables in their monograph give the identities of the taxa interpreted as having different biogeographic affinities. For virtually no other large region in the world is there such a comprehensive attempt to link the identities of modern species to their places in a broad account of biogeographic history.
Critiques of the Classic Story
Scientific progress leaves every ambitious accomplishment open to reconsideration. Perhaps the most outdated aspect of the Raven-Axelrod analysis is its reliance on the geoflora concept. Geofloras were envisioned in the early and mid-twentieth century as widespread and long-lasting assemblages that formed in the Tertiary and remained constant in their ecological requirements, changing little through either trait evolution or differential migration and moving as a unit in response to climate change. This is clearly out of step with the modern view that emphasizes the individualistic nature of species range shifts, the recent assembly of modern communities from disparate ancestry, and the significance of adaptation as well as migration in response to climate change (e.g., Davis and Shaw 2001). The existence of an Arcto-Tertiary Geoflora as classically defined was disputed by Wolfe (1978) on the basis of Eocene fossil floras from the Arctic that were predominantly broadleaf evergreen rather than deciduous. However, the notion of a north-temperate deciduous flora at Arctic latitudes in the Tertiary has been borne out in more recent analyses (e.g., Basinger et al. 1994; Brown 1994). More generally, it could be argued that the basic Raven and Axelrod story of the formation of the Californian flora, through range shifts and evolution in contrasting northerly mesic-adapted and southerly drought-adapted assemblages, could still be valid even if the shifts occurred in a less unitary fashion than these authors envisioned (Ackerly 2009).
Modern authors generally substitute more nuanced terms for the old geoflora names. This book follows Ackerly (2009) in using north-temperate” in place of Arcto-Tertiary and “subtropical semiarid” in place of Madro-Tertiary, except when specifically citing Raven and Axelrod.
Terrestrial plant fossils are uncommon in California, and Axelrod’s paleobotanical conclusions involved much interpolation from scarce data. In keeping with the geoflora concept and its principle that species have evolved little, Axelrod employed the assumption that a now-fossilized plant inhabited a climate much like that of its closest living relative. This method has been criticized for ignoring evolution and within-taxon diversity, although it may have some validity if large enough suites of species are used (Basinger et al. 1994). An alternative approach to inferring ancient climates is to use the physiognomic traits of entire fossil assemblages; for example, the proportion of plants with entire (smooth-margined) versus toothed and lobed leaves is strongly correlated with mean annual temperature in climates with year-round rainfall (Wolfe 1978). Another problem was that Axelrod assigned fossil taxa to modern genera using subjective matching of easily visible traits, such as leaf outline and major venation (Edwards 2004); newer approaches to fossil identification emphasize venation patterns and the microscopic examination of epidermal anatomy (Ellis et al. 2009).
More recently, stable isotopes have increasingly allowed paleoclimates to be reconstructed independently of plant fossils. One of the most important findings has been that “the Ice Age” was not a single cold event, as was once believed, nor was the early Holocene warm period an aberrant extreme. Rather, the Pleistocene consisted of many glacial cycles, interspersed by periods that often reached temperatures as warm as the early Holocene (Millar 1996). Complex changes in Californian plant distributions occurred during glacial-interglacial cycles, with both herbs and hardwoods tending to expand as the glacials ended (Edwards 2004).
Another problematic issue is that plants were designated as Arcto- or Madro-Tertiary by subjective methods that relied on species traits and contemporary distributions as well as fossil evidence. Thus groups such as Brodiaea and its relatives, having many species in the province and few outside it, were designated Madro-Tertiary because “the degree of radiation . . . in the California Floristic Province suggests a relatively great antiquity for that group in Madrean vegetation” (Raven and Axelrod 1978: 50). In other words, they are of Madrean origin because they are diverse in Madrean vegetation. While such inferences could well be correct, corroboration from independent evidence would lead to stronger inferences. Moreover, the concept of biogeographic “origins” has its problematic aspects, since (for example) a genus may be inferred to have arisen in one region or climate but its family in another. For adherents of the geoflora concept, the Eocene is regarded as the period when modern lineages acquired the traits defining their climatic niches, an assumption that has never been tested. Ackerly (2009) notes that Raven and Axelrod seem to equate the geographic region in which greatest diversification occurred with the region of origin, although the two need not be the same. An alternative approach would be to focus on traits, which unlike lineages originated at specific times; phylogenetic methods can be used to ask in what regions and climates a trait arose and how it affected the subsequent spread and diversification of a lineage (Ackerly 2009).
Raven and Axelrod’s classification of species in terms of biogeographic origin has been examined in a number of recent analyses. Ackerly (2003) found that woody species classified as Madro-Tertiary had larger seeds and lower specific leaf area (i.e., thicker leaves) than those classified as Arcto-Tertiary. Harrison and Grace (2007) and Ackerly (2009) found that the geographic distribution of the groups conforms as expected to climatic patterns within the state; Raven and Axelrod’s Arcto-Tertiary species are most numerous in the rainy and mountainous north, Madro-Tertiary species (including those “strongly associated with the California Floristic Province,” many of which are endemics) were most abundant in the Coast Ranges and Sierra Nevada foothills, and desert species tend to be found in the driest parts of the California Floristic Province. Damschen et al. (2010) found that over a six-decade period (1949–2007), as mean temperatures in the Siskiyou region increased by 2°C, the abundance of north-temperate (Arcto-Tertiary) forest herbs declined relative to other species. All these analyses bear out, to some extent, the existence of genuine differences between the lineages subjectively identified by Raven and Axelrod as Arcto-Tertiary and Madro-Tertiary.
Phylogenetic analyses of molecular data are a vast new source of evidence on evolutionary processes that were unavailable to Raven and Axelrod (1978). Chapter 3 considers what is now known about Californian plant endemism in light of this and other new evidence.
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The Californian landscape has come into existence over the past 200 million years, and the modern climate and flora have developed during around 50 million years of global cooling and drying. Rapid geologic and climatic changes within the past 5 million years have left an especially strong imprint on the present-day biota. In the historic account by Raven and Axelrod (1978), the Californian flora arose from the interplay of two main sources: a northerly, temperate assemblage (Arcto-Tertiary) and a subtropical, arid-adapted (Madro-Tertiary) assemblage.
The north-temperate assemblage is thought to have given rise to many paleoendemics with relatives in eastern North America or eastern Asia. They include many broad-leaved deciduous trees and shrubs, conifers, and perennial herbs, and are most prevalent in cool and wet environments. The subtropical seminarid assemblage is considered to have given rise to the majority of neoendemics in the California Floristic Province. These species are often evergreen shrubs, geophytes, or annuals. Their centers of diversity are in mediterranean-type chaparral and coastal scrub habitats. Most neoendemics are thought to have arisen in the past 5 million years or less, since the climate became fully dry in the summer, although recognizable relatives have been found in 20- to 30-million-year-old fossil