However, the more northerly and mountainous interior regions of this Eocene world contained what earlier authors had called the Arcto-Tertiary Geoflora, a rich mixture of trees, shrubs, and herbs whose descendants are now found in the temperate forests of East Asia and eastern North America. This flora has sometimes been described as resembling a modern redwood forest, although with many more species of both angiosperms and conifers. It included the ancestors of today’s Californian coastal forests, which were found north of 44° latitude, as well as the ancestors of the drier montane Sierran forests, which were then found farther south. Both of these elements shifted coastward as cooling and drying accelerated in the Oligocene to Miocene, and many of the warm tropical forest elements became extinct. As revealed by fossil assemblages, these forests were enriched by the co-occurrence of species that are now segregated by elevation and habitat. Five million years ago, the mediterranean climate was clearly developing, but fossil floras indicate a less seasonal climate; the wide occurrence of Abies and Picea suggests cooler summers, while the presence of now-extinct Per-sea, Castanea, and Ulmus suggest wetter summers. Late Pliocene decreases in summer rainfall increasingly restricted conifers to high elevations, and cooling temperatures restricted broad-leaved evergreens to low elevations. Pleistocene vegetation was essentially modern, except that it was shifted downward in elevation and latitude compared to the present; conifers were more widespread due to cooler summers, and hardwoods survived in mild coastal climates. During the early Holocene warm period 8,000 to 4,000 years ago (called the “Xerothermic” by early authors), the remnants of the Arcto-Tertiary flora retreated toward their present coastal, riparian, and higher-elevation refuges.
Together with other authors, Raven and Axelrod (1978) considered California one of the most important areas for survival of the Arcto-Tertiary flora, second in the United States only to the considerably richer forests of Appalachia. The refuge existed because the climate remained consistently equable during and since the Tertiary, without widespread glaciation or extreme aridity. Within California the most significant Arcto-Tertiary refuge is thought to be the Klamath-Siskiyou region, where the greater total and summer rainfall, milder winters, and cooler summers amount to a climate resembling that of the Miocene and Pliocene. Elsewhere, Arcto-Tertiary forests became restricted to small patches during the mid-Holocene warm period, and monodominant stands of species such as Pinus jeffreyi, Pinus ponderosa, and Pseudotsuga menziesii developed in recent millennia through the progressive loss of other species.
Today’s Arcto-Tertiary flora, as defined by Raven and Axelrod (1978), comprises just over half the species in the California Floristic Province and is the source of most of its paleoendemics. Some Arcto-Tertiary taxa that meet the standards for very strict paleoendemism, such as having their closest relatives outside of western North America, are Quercus sadleriana, Picea breweri, Berberis nervosa, and Pinus albicaulis. Paleoendemics in a slightly broader sense, such as plants having few close relatives in western North America, include Chrysolepis chrysophylla, Taxus brevifolia, Torreya californica, Calycanthus occidentalis, Dirca occidentalis, Lithocarpus densiflorus, and Acer circinatum. Neoendemics in California are not normally thought of as being of Arcto-Tertiary origin. However, Raven and Axelrod (1978: 16) listed 50 Arcto-Tertiary genera that have undergone significant speciation in California (e.g., Allium, Aster, Bromus, Calochortus, Delphinium, Iris, Lomatium, Lupinus, Silene, Viola). Together, these genera comprise 645 taxa, of which 253 are strictly endemic to California (see Chapter 3 for further analysis).
As the climate became drier in the mid- to late Eocene and the Arcto-Tertiary forests shifted coastward, a more drought-adapted flora began to expand northward into western North America. Fossil floras containing sclerophylls (species with hard, thick, drought-adapted leaves), such as species of Quercus, Arbutus, Pinus, and various laurels (Lauraceae), have been found in deposits as old as 50 million years. Axelrod named this broad assemblage the Madro-Tertiary Geoflora, based on a resemblance to the flora of today’s Sierra Madre of northern Mexico. Axelrod speculated that this flora might have its ancient roots in Mediterranean Europe and/or in localized dry habitats such as rocky southfacing slopes in low-latitude North America. Some members, such as Arbutus, Cupressus, Erodium, Lavatera, Quercus durata, and Q. berberidifolia, were noted to have close relatives in the Mediterranean Basin; Raven and Axelrod termed these taxa “Madrean-Tethyan” after the ancient Tethys Sea that separated Laurasia 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