California has long been known as a land of opportunity. In the botanical world, California is also renowned as the land of plenty, for its diversity of plants is unparalleled in the continental United States.
With over 5,000 species growing within its bounds, California's plant life is more varied than the flora of the entire central and northeastern United States, plus adjacent Canada- an area almost 10 times as large!
In addition, more than 30 percent of California plants occur naturally nowhere else in the world-that is, they are endemic to the state. Compare this with the Northeast where only 13 percent of the flora are endemic. Equally astonishing, at first glance, is the fact that only one percent of the plants of the British Isles, an area three-quarters the size of California, are endemic.
Serpentine areas, such as this, are often host to plants found nowhere else in the world. Photo by Susan Cochrane
Why are there so many species unique to California? What causes such species richness? The answer is habitats. The presence of so many varied habitats is the greatest reason for the abundance of rare plants in California.< p> A plant, or any other living thing, thrives only in a place where conditions required for its survival and reproduction are met. Those conditions define its habitat. The more important factors that influence creation of different habitats for plants include elevation, slope, soils, latitude, rainfall and temperature. Of course, a plant species' survival also depends upon its interactions with other species, such as its ability to compete with other plants for space in the same habitat or to protect itself from destruction by animals, as well as its genetic fitness-its ability to successfully reproduce and disperse seed.
Geographers divide the California landscape as it is today into units of similar habitat known as landform provinces. These are regions of topographic and climatic similarity, but within each province the number of unique habitats is enormous.
The wealth of habitats in California is a result of the geologic and climatic processes that have been at work for millions of years. Before the mighty Sierra Nevada rose to their towering present-day height, California was covered by shallow seas. The climate was moist and warm, and tropical forests dominated the landscape. As the climate cooled and dried, the forests were gradually restricted to northern coastal areas of moderate temperatures and higher rainfall. During warmer, drier periods, woodland and desert plants from the south replaced forest species throughout much of the southern part of the state.
About this time, volcanic activity and mountain-building changed the shape of California to its present-day visage. High mountains intercepted rain clouds traveling east from the coast, creating moisture- dependent forests on west slopes and rainshadow deserts on the east. These tall mountain ranges formed the barrier that now separates the more moderate maritime climate areas from the more extreme continental climate region of the interior United States. Subsequent periods of glacial activity alternating with warming trends caused further shifts in the types of vegetation.
As conditions changed, species evolved to fill newly-created habitats. Older species continued in restricted pockets of previous habitats, known as refugia. An excellent example is the desert fan palm, a member of a tropical family of plants that is now relegated to California's only remaining habitat that reflects those moister warm-climate times-desert oases. The coast redwood is also a relictual species, although its refugium still covers a relatively large area of the coastal part of the state.
The distribution of plants in California today has thus been heavily influenced by the past climates and geologic history. Many species are rare because they require habitat conditions that are no longer prevalent. But just as many, if not more, are rare because they occur only in highly restricted habitats that are of a more recent origin and w@ may never become common. These plants have specific needs met by a unique combination of habitat factors not often duplicated.
One of these unique habitats is a vernal pool. Vernal refers to spring. These small pools, or shallow lakes, fill during winter rains and dry during the spring to become covered with flowers. They form only in areas where clay soils ]each to make hardpan layers beneath the surface. This restricts water percolation into the soil so it remains until it is evaporated in the spring.
Many rare plants are found in California's vernal pools. These species have evolved to live in a habitat that is flooded part of the year, moist for a few months and bone-dry the rest of the year. They germinate underwater and flower around the edges of the drying pools. Their seed is then stored in the dried mud until the next season.
Sometimes spectacular rings of color can be seen, but the blooms last for only a few short weeks.
Because vernal pools are isolated from one another, different species have evolved in very localized areas. Crampton's Orcutt grass is known from only one pool in the Jepson Prairie of Solano County. In fact, all five species of Orcutt grasses depend on vernal pool habitat for survival and are found no other place in the world.
Unique habitat areas isolated in a sea of more widespread vegetation might be thought of as "ecological islands." Many ecological islands in California exist because of geology. Such areas of distinctive soils are often home to an ensemble of rare plants.
The soils at lone in Amador County are acidic with a pH below 4. The lone manzanita and lone buckwheat survive under these most extreme conditions. Highly mineralized soils near the hot springs and steam vents at the Geysers in Sonoma County harbor the only populations of the Geysers' panicum, and alkaline soils surrounding desert springs provide habitat for still other rare plants. The limestone and gabbro soils dotting the state are themselves dotted with sites of rare and endemic plants.
Serpentine is a rock type infused with large amounts of heavy metals, such as nickel and manganese. Serpentine is scattered along the western foothills of the Sierra Nevada and in the north and central coast ranges. Although its overall range is large, its patchy distribution provides restricted habitats to many local endemics.
Endemic plants are routinely found on serpentine areas. Although it is often thought that these plants could not grow in more normal soil conditions, in truth, they usually do not compete well enough with nonserpentine plant species to become established there.
Most of the rare plants, or habitat conditions supporting them, discussed thus far describe one type of rarity. That is the rarity of a species restricted to a small area, even though the number of individuals within that specific area may be quite large.
One of the vernal pool flowers, downingia add a purplish ring around the drying ponds.
A second type of rarity exists that is not as easily explained.
Some plants occur over wide areas, have broad distribution in more common habitats, but are seldom seen or occur in such low numbers as to be considered rare. The reasons for their rarity are not clearly evident. They may be relicts of earlier times, once more common, or they may just have poor mechanisms for propagating and dispersing themselves. Some, like the sandloving phlox, may require habitat conditions that are not produced every year. The sand-loving phlox grows only in sandy washes in the Mojave Desert, but is seen only during years of exceptionally high rainfall.
As should be obvious by now, California's climatic and geologic history has combined with our present-day Mediterranean climate of cool, moist winters, and warm, dry summers to produce the state's incredibly diverse flora. This habitat diversity is further enhanced by terrain which ranges from seashore to mountaintop and a latitudinal range for the state equivalent to the distance from Savannah, Ga., to Cape Cod, Mass. It is not hard to understand why this almost neverending variety of habitats can support so many rare plants, each adapted to a distinct set of ecological circumstances.
But how does a rare plant become an endangered plant? A species' survival is closely tied to changes in its habitat. For rare species there just isn't as much area that can be disturbed without directly affecting the plant. When conditions arise that threaten the continued existence of a rare species within its native habitat, it is considered endangered.
Endangerment can appear as a direct result of human activities such as development for housing or agriculture. Such is the case for vernal pools found in the San Diego and Central Valley areas.
Endangerment may also come from more indirect causes-for example, the introduction of invasive exotic weeds into native plant habitat or the lowering of an underground water table.
In most instances, endangerment is a combination of factors. The challenge is to design a way to allow for human use of this land of plenty while safeguarding its rarest natural treasures.