ago, marked the beginning of extreme oscillations, with more than 20 relatively long glacial periods interrupted by shorter interglacials (Figure 10; Zachos et al. 2001). The present interglacial period began at the Pleistocene-Holocene boundary 12,000 years ago and is expected to end with another ice age unless disrupted by anthropogenic additions of greenhouse gases.
Today’s five mediterranean climate regions with their characteristic winter rainfall and summer drought are found on west coasts between roughly 30° and 42°; latitude (Figures 2, 3). In these locations, the jet stream brings winter storm systems from maritime rather than interior sources, leading to cool, rainy winters instead of cold, snowy ones. The high-pressure systems that create the world’s major deserts (from 23° to 30° latitude) shift poleward in summer with the earth’s tilt, creating the desertlike summer drought. Upwellings of cold deep-ocean water along the coast, which produce a marine layer of cool air capped by warmer inland air, are also a key ingredient of both the summer drought and the relatively gentle winter weather. Timing and duration of the summer droughts vary considerably among the five world regions, with California’s being among the longest and driest (Dallman 1998).
FIGURE 10. Global relative temperatures from the Paleocene to the present, based on oxygen isotope measurements from deep-sea sediments and ice cores. (Based on data from J. Zachos; see also Zachos et al. 2008)
Because of its relevance to plant and animal evolution, the history of the mediterranean climate is of great interest. In today’s mediterranean zones, tropical-like climates began to give way to more seasonal ones as many as 40 million years ago. Precipitation began to peak in the winter by the middle Miocene. However, the fully mediterranean climate with its near-complete summer drought emerged only after the onset of the Pleistocene brought the development of cold offshore currents. While glaciation in the Northern Hemisphere was well under way 2.7 million years ago, the modern ocean current system was not in place until about 1.5 million years ago (Ravelo and Wara 2004). The uplift of the state’s major mountain ranges in the past 5 million years contributed to increasingly steep internal climate gradients. Based on plant fossil evidence, Raven and Axelrod (1978) argued that some summer rainfall persisted in California until one million years ago, but this has yet to be corroborated with geophysical evidence.
Another important question is how severely the climate fluctuated during glacial-interglacial cycles. The conventional wisdom, largely from plant-based evidence (see later sections of this chapter), is that the climate was colder and rainier but remained mediterranean during glacial periods. However, isotopes indicate that during the coldest parts of the last several glacial periods, expanded oceanic ice sheets blocked the cold oceanic current system, producing warmer and rainier conditions in California resembling a prolonged El Niño. Fossil pollen indicates that while this advance warming speeded the recovery of the interglacial vegetation, it caused declines in abundance of the fog-dependent coastal redwoods (Herbert et al. 2001).
FLORISTIC HISTORY
Origin of the Flora According to Raven and Axelrod
Building on their decades of research in plant evolutionary biology and paleobotany, respectively, Raven and Axelrod (1978) described the biotic history of the California Floristic Province beginning with the Eocene, when many modern plant families diversified and most of the world had a warm, wet, essentially tropical climate. The Sierra was a low coastal mountain range, and the Klamaths were offshore islands. The Coast Ranges had not yet emerged. Many parts of present temperate North America and Eurasia were covered in a tropical rainforest flora containing families and species that are now largely extinct north of the humid subtropics, including laurels (Lauraceae) and palms (Palmae), among others.
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
