from Gondwana. By the Miocene, the rich subtropical semiarid Madro-Tertiary (or Madrean) flora dominated interior Southern California. Besides the above taxa, it included Palmae, Lyonothamnus, Ceanothus, Arctostaphylos, Heteromeles, and Rhus. Continued drying in the Pliocene caused Madrean chaparral to expand farther into the Sierra foothills and coastal California and to lose some of its more tropical elements such as Acacia sensu lato. Raven and Axelrod visualized the Madrean flora meeting the Arcto-Tertiary flora at relatively abrupt boundaries along climatic gradients. These boundaries fluctuated during Pleistocene climatic cycles and largely arrived at their present configuration during the early Holocene warm period.
Raven and Axelrod (1978) believed about one-third of species in the California Floristic Province were of Madrean origin. The majority of the neoendemics belong to Madrean genera that radiated extensively in the province and are almost completely endemic at the genus level (e.g., Clarkia, Hesperolinon, Lasthenia, Mimulus, Phacelia). The neoendemic genera are believed to have undergone most of their diversification in the late Pliocene and the Pleistocene as the mountains rose and the climate became fully mediterranean. In the view of Raven and Axelrod (1978) and other classic authors (e.g., Stebbins and Major 1965), much of the speciation was stimulated by the climate-driven advances and retreats of Madrean vegetation across rugged landscapes, which created many opportunities for fragmentation, divergence, reproductive isolation, and/or subsequent hybridization. Preadaptation to summer drought and fire helped to determine which genera thrived and speciated in the new climate. Most annual herbs, and most shrubs that obligately recruit by seed after fire, are Madrean. There are also Madrean paleoendemics in the mountains of Southern California, representing wetter elements of the Madro-Tertiary flora that survived under occasional summer rainfall.
Raven and Axelrod also identified a second group of drought-adapted species that they termed the “warm temperate desert” element of the flora, which they thought moved into the California Floristic Province from the south during the mid-Holocene warm period and colonized the interior Coast Ranges and southern Central Valley. This group comprises 604 species, or 13.5 percent of the flora of the province. Raven and Axelrod (1978) estimated there were 44 endemics in the Central Valley, most of which they considered young (dating to the mid-Holocene warm period) and of desert origin.
Desert floras are relatively poor in endemic species because of the recency of the desert climate, according to Raven and Axelrod (1978). The Mojave and Great Basin Deserts were largely covered by pinyonjuniper woodlands during the Pleistocene. The Sonoran Desert was subtropical woodland. Having undergone less cooling and having retained moderate summer rainfall, the Sonoran Desert contains more relictual subtropical taxa. Raven and Axelrod (1978) enumerated a total of 102 genera and 935 species in the Californian deserts, including 9 species endemic to the Great Basin, 44 to the Inyo region that includes the White Mountains and Death Valley, 22 to the Mojave, and 8 to the Sonoran (Colorado) Desert.
Raven and Axelrod (1978) cited the geography of Californian endemism in support of their conclusions, relying largely on an analysis by Stebbins and Major (1965). Especially high concentrations of neoendemics were found in “intermediately” warm and dry mediterranean-type vegetation, particularly in coastal Southern California, the Sierran foothills, and the central Coast Ranges. Paleoendemics were found to be most prevalent in the Klamath-Siskiyou, the northern Coast Ranges, the Channel Islands (as also described in Raven 1965), and northern Baja California. Scarcity of both neo- and paleoendemics was noted in the climatically youthful Central Valley, deserts, and high Sierra.
In summary, Raven and Axelrod portrayed California’s floristic history as a progressive shift from a largely mesic tropical and warmtemperate flora to a modern flora with a much more arid-adapted character, with new species arising as climatic oscillations across the rugged landscape produced a constant interplay between two distinctive assemblages. The generally equable climate of California enabled the survival of many mesic Arcto-Tertiary relicts. The cycles of cool/moist to warm/dry climates since the Pliocene triggered outbursts of speciation, mostly among the southerly Madro-Tertiary component of the flora, leading to especially high neoendemism within the fully mediterranean climates where the modern vegetation is chaparral.
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
