The Klamath-Siskiyou Ecoregion, hereafter KSE, is an oblong area that extends from Roseburg in southwestern Oregon to the Yolla Bully range in northwestern California. The varied geology, location, and microclimates of the KSE accelerated plant evolution and migration but slowed extinction. At least 3,000 types of plants and all the major forest types in western North America occur here. More than 200 of these plants are KSE endemics, the name for a species found only in a particular area. Oregon Caves National Monument, as a small but important zone of transition in the KSE, illustrates this floral diversity by boasting almost one plant species per acre.¹

Geologic controls

Ocean basins were spread apart or squeezed for more than half a billion years while molten magma crystallized into rocks as different as basalt and granite. Erosion and metamorphism created another range of strata as well: sandstone, marble, pebbly conglomerate, glacial silt, and "baked" muds. Faults uplifted and split rock masses apart, changing what once were islands and ocean basins into a complex rock mosaic. This fragmentation of habitat favors small populations on each type of soil or rock, a situation in which mutations that give rise to new species are not diluted out of existence by interbreeding with large populations.

A stand of Port Orford-Cedar near Oregon Caves.

Peridotites are rocks with potent quantities of minerals like iron and magnesium that change to metal-rich serpentine when hot water is added. Since most life is not adapted to metals normally found deep in the earth, these metals disrupt photosynthesis and inhibit microbes. The most toxic may be nickel, chromium, and cobalt though plant distribution in the KSE seems to be governed by the occurrence of magnesium. Little soil forms because clays need aluminum, an element lacking in serpentine. This and the toxicity of metals in serpentine will not allow clay, organics, or soil clumps to hold water, The cycle snowballs and becomes a situation where thin soils are often dry, hot, and nutrient-poor. This provides open habitat, increases population turnover, and thus encourages the evolution of new species. Because their populations are smaller, species that are rare usually evolve faster than common and/or widespread plants. Of the 200 endemic plant types in the KSE, 141 are either rare or uncommon, a very high ratio for endemics.

The low productivity of serpentine soils limits the dispersal of endemics to new areas because of fewer pollen grains, seeds, or tall plants. Seeds in serpentine tend to be larger because of being in such stressful habitat, so as to give seedlings a head start on life. This characteristic may also limit dispersal, thus increasing the number of endemics.

The most common response for most plants on serpentine is to keep nickel out of their cells. Even some mariposa lillies and wild buckwheats living on non-serpentine soils tolerate high amounts of normally toxic metals, so they appear better prepared for evolving new species on serpentine soils, Some KSE plants have found other ways to avoid serpentine toxicity. In an endemic pennycress mustard and jewel flower, the plant stores nickel within its cell tissue. Another jewel flower (Streptanthus tortuosus) has developed a race of serpentine-tolerant plants and so may be on its way to becoming a new species.

Rapid evolution is also indicated by the fact that roughly two-thirds of KSE endemics are varieties or subspecies that likely are on their way to becoming full species. The crowding of habitats in the KSE results in many hybrids, some of which have given rise to new species.

An avenue for plant migration

Imperial Lewisia. Drawing by Heather McDonald.

The KSE is unusual in that it has more serpentine than any other ecoregion. The serpentine masses and size of the KSE helps plant migrants find suitable habitats more easily but are big enough to keep extinction rates low. Serpentine in the Illinois Valley, for example, is fragmented and possesses different chemistries—an ideal situation for rapidly evolving small populations. The effects of fire or other disturbances may be so long lasting that plant populations are separated sufficiently and can evolve into new species. By the same token, disturbances in the KSE are not so large and competition among plants is not intense enough for extinction rates to increase. Varied rainfall amounts, frequent burns, and areas that serve as barriers (riparian zones, serpentine, cliffs, north slopes) tend to limit fires to patches of moderate size and intensity. Consequently, no one successional stage dominates with its restricted number of species.

Another reason for the relatively high species diversity in the KSE is because it contains the only mountains linking coastal ranges in California and Oregon with the Cascade-Sierra cordillera. Plants more easily cross over east-west oriented mountains, unlike north-south ranges where plants must migrate along lines of longitude if they cannot cross high elevations. Migration can promote speciation because it produces numerous small and isolated populations near the range limit of a species, a situation common in the KSE. Proximity to the endemic-rich Cascades, Sierra, and the coast ranges of northern California has increased plant diversity as the KSE shares over 200 endemics with these physiographic regions. At least half of those plants probably originated in the KSE.

Vollmers Tiger Lilly. Drawing by Heather McDonald.

Extinction is low among shrubs and trees generally, furnishing an important reason why they comprise many of the paleoendemics, or "living fossils." If you live a long time, you have more chance of reproducing at least once successfully. Even serpentine herbs tend to be long-lived, a trait indicative of harsh environments, and one the likely increases the number of endemics. During the great climate changes over the last few million years, the closely packed habitats of the KSE allowed plants to grow in adjacent habitats that increased the chances for survival when the climatic regimes shifted. Some habitats shrank considerably, but paleoendemics in them continued to thrive. Port Orford-cedar and Brewer (or weeping) spruce are examples of paleoendemics that once had more extensive ranges.

Another type of endemic commonly found in the KSE is the edaphic endemic or geoendemics—those species mostly restricted to one soil type or topographic situation. Neoendemics (plants with no nearby relatives) in the KSE also appear to be more common near the north end of their range at high elevations, perhaps because they are also glacial relicts that found suitable cool and open habitats to colonize. Among the endemic plants in the KSE, 80 types are found only on serpentine, while seven are confined to granite, four on marble, and three on volcanic rock.

Other factors promoting diversity

A lack of nutrients and water (up to a point) encourage greater diversity because plants then spend more of their energy surviving such conditions rather than competing with other plants and causing them to become extinct. The leaching of soil nutrients through high temperatures and rainfall lowers the productivity of soils and may increase the diversity of herb. Since the KSE is characterized by low rainfall during the growing season for herbs, habitat diversity is heightened because there are marked differences in slope and aspect that control evapotranspiration and the water retention capacity of soils.

More nutrients and water allow certain plants to dominate and thus reduce plant diversity, the so-called paradox of enrichment. The KSE is an area of climatic extremes, with annual rainfall amounts ranging from 100 inches near the ocean to 15 inches further inland. The differences in rainfall gives rise to a patchy distribution of plants, with the many subspecies and varieties of certain plants indicate rapid speciation—especially in their adaptation to dry soils of serpentine, marble, and granite. For example, dwarf ocean spray, myrtle, buckthorn, and tanoak stay small in stature even if grown in gardens with lots of water. Drought adaptations in endemic plant species include large tubers (as in lillies and toothworts), woodiness (as in pussytoes and pincushion), and waxy, hairy or divided leaves. Storing carbon dioxide at night so that water is not lost through leaf pores by day has favored many endemic sedums and lewisias.

The varied habitats and climate change over thousands or millions of years resulted in 50 or more disjunct species, plants whose brothers or population centers are hundreds of miles distant. Mutations are favored in such situations because of their small populations and the need for new adaptations to survive in a less than ideal habitat. The KSE also hosts at least 100 plant species at the edge of their range, where speciation most likely occurs due to isolated populations undergoing rapid change. Being at the right location between northern and southern plant communities, the KSE is situated so as to have a high number of disjunct species as well as plants at their geographic limit.

As a refuge for plants that once ranged from Japan to Georgia, the KSE provides rare habitat in the western United States. Many of the plant relicts are members of old families: heathers, orchids, honeysuckles, birthworts, and lillies. Plants such as fairybells, woodland stars, dogwoods, rhododendrons, redwoods, trilliums, gaultheria, and coralroots have their greatest diversity of species in the northwest and southeast United States, as well as in eastern Asia. Paleoendemics evolved once tectonic forces and climate changes cut the connections to other landmasses. Trees such as Port Orford-cedar and Baker cypress, for example, have "twin" species in Asia. Likewise, cousins of plants in the KSE such as vanilla leaf, tanoak, Oregon grape, redwood, and skunk cabbage grow in eastern Asia.

Conclusion

California Lady Slipper. Drawing by Heather McDonald.

The KSE enjoys the best of diversity among plants; it contains older flat areas where the lack of major disturbances has allowed paleoendemics to survive, but also provides newer habitats like cliffs and cirque lakes where new species can evolve due to isolation and a lack of competition. Northern Florida may possess more paleoendemics and Hawaii has greater numbers of neoendemics than the KSE. Parts of Nevada and Arizona display more edaphic endemics, but the distinctiveness of the KSE lies in its mix of all three types of endemic plants—more profuse than anywhere north of Mexico. In few other places will the location, size, varying ages and geodiversity of mountains with their varied climates combine to produce so many relicts, disjuncts, endemics, varieties, hybrids, and plants near their geographic limit. The Illinois Valley is a botanist's delight each spring, while Oregon Caves constitutes a representative slice of the fascinating floral diversity found throughout the KSE.

Note

¹The monument list contains some 400 plant species in only 480 acres, whereas Crater Lake National Park boasts fewer than 700 in 183,220 acres.

John Roth became fascinated with the plants of southwestern Oregon upon arriving at Oregon Caves National Monument in 1988.