According to the CNPS Inventory of Rare and Endangered Plants, nearly 14% of the California native flora is either exceedingly rare or seriously endangered. While many of these rare plants owe their rarity to habitat destruction and alteration, many have never been widespread and some of them have, no doubt, always been rare. Though there are many reasons for a species abundance and distribution a significant factor is the nature of its endemism. Roughly 50% of the plants native to the California Floristic Province are endemic to the province and about 30% of the plants native to the state as a whole, are endemic. This high degree of endemism is unusual for a continental land mass.
The term endemic is used to describe a species natural distribution. Endemics can be broad or narrow. The genus Penstemon, for instance, is native to, or naturally occurring, only in North America and would be considered a broad endemic. Narrow endemics would include species found only in a few locations or only on a certain soil type such as the Kneeland Thaliapi, a serpentine mustard found only at the Kneeland Airport, or the Menzies wallflower found only on our local dunes. Three major types of endemism occur in California; paleoendemism and neoendemism which are based on the age and past and current distribution of the taxon, and edaphic endemism which is based on a species affinity for a certain soil type
Neoendemics are species that have speciated relatively recently in or near their present locations. California neoendemics tend to be narrowly distributed and are the result of in sutu speciation. Neoendemics are in part the result of Californias changing climate during the late Tertiary and early Quaternary periods. Many have occurred since the Pliocene Epoch, 5-7 million years ago when the Sierra Nevada, Coast Ranges, and Transverse Mountain Ranges were uplifted and the climate of California was profoundly changed. During that time much of California became isolated from the rest of the continent by deserts and mountain ranges and the climate slowly changed to a Mediterranean type characterized by wet winters and warm dry summers. Repeated climatic fluctuations with the eventual advent of a Mediterranean climate combined with Californias great habitat heterogeneity, large topographic relief, and diversity of edaphic conditions (soil types) to fuel the outburst of speciation that created Californias flora.
A large percentage of California neoendemics are annual dicots, cf. members of the genera Amsinckia, Collinsia, Eriogonum, Lasthenia, Layia, Lupinus, and Mimulus. Bursts of neoendemic speciation are also evident in the speciose shrubby genera Ceanothus and Arctostaphylos. According to Raven & Axelrods Origin and Relationships of the California Flora, the vademecum for the subject, several large families are quite outstanding for their degree of endemism to the state; among them are: Malvaceae (53%), Rhamnaceae (47.5%), Scrophulariaceae (41.1%), and Compositae (51.6%).
Californias recently evolved and narrowly distributed neoendemics came about by a very different process than its paleoendemics. Paleoendemics tend to be of a more ancient lineage, and in many cases, are more widely distributed outside of California. Neoendemics, having evolved here tend to have their nearest relatives in close proximity, while paleoendemics tend to be disjunct from close relatives. Many paleoendemics have a boreal affinity. They were either pushed south to California during the ice ages or were simply more or less widely distributed in the cool temperate regions in the past. The waning of the last ice age and the cessation of cool, wet summers caused many of these species to migrate northward, upward in elevation, or to other cooler moister habitats where they could survive Californias hot dry summers. Sequoia sempervirens and Sequoiadendron giganteum are probably the best known California paleoendemics. Both of these species were much more widely distributed in the past and have since gone extinct every where but here. Unlike redwoods, many California paleoendemics occur outside of California, usually to the north in the Cascades of Oregon and Washington and also in Canada and Alaska. The now disjunct holdout populations, called relicts, have managed to survive in small pockets of suitable habitat called refugia. The California populations for many of these species are now the southern terminus of their distribution. Though a number of these paleoendemics are more widespread outside of California, within California many are rare remnants of a past vegetation, and therefore an important component of the species genetic diversity. For both these reasons these species deserve protection. At least three examples of rare plants found near Trinidad are paleoendemics. The black crowberry (Empetrum nigrum ssp. hermaphroditum), found from Oregon up to Alaska, appears to have its southern terminus at Elk Head. Both the bog club-moss (Lycopodiella inundata), found at Big Lagoon Bog, and running-pine (Lycopodium clavatum), though widely distributed in both the northwest and the northeast, are rare in California. These three species are on the CNPS List 2.
Edaphic endemism is a situation where a species has a strong fidelity to a specific soil type and is found nowhere else. California has a great many edaphic endemics owing to the states great geologic complexity. Serpentinite endemics are probably Californias most familiar endemics. The CNPS inventory lists 282 rare species endemic to serpentinite. However, many other types exist. The CNPS Inventory also lists 103 rare California species found on granite, 89 species on carbonates, 77 on volcanic rock, and 61 rare species on alkaline soils. While Californias edaphic endemics also tend to be neoendemics, exceptions exist. The recently discovered Shasta snow-wreath (Neviusia cliftonii), for instance, is found only on limestone outcrops, a soil type rare in California, but its closest relatives are found in the southeastern US. This species is therefore considered a relict species.
In summary, the great diversity of the California flora, and specifically the floras high degree of rarity is due in large part to endemism. Californias topographic, geologic, and especially climatic diversity are primarily responsible for this endemism. But climate has acted in two opposing ways. It is interesting to note that the Mediterranean climate of cool wet winters and hot dry summers has much to do with the diversification of many neoendemics, but this dry summer climate is also responsible for the rarity of many of the species found here before the advent of a dry summer climate. It is mostly where the climate is more moist in the summer, such as the Klamath Region, and in small pockets of suitable habitat where the paleoendemics have managed to persist. For more information on endemism and the California flora I suggest Raven & Axelrods Origin and Relationships of the California Flora, recently reprinted by and available from CNPS, and Smith & Sawyers Endemic Vascular Plants of Northwestern California and Southwestern Oregon, Madrono Vol. 35, No. 1, 1988.