A Struggle to Survive on Barren Serpentine Soil
Contributed by Dan Day
Reprinted by permission from the Northern California Geological Society Newsletter
Ultramafic rocks are scattered throughout the California Coast Range, the Trinity Mountains, and the Sierra Nevada foothills. That serpentine is the state rock proves it has caught the eye of California geologists. However, many are perhaps unaware that serpentinites have spawned a unique flora specially adapted to survive on their nutrient-poor soils.
The adaptive characteristics of one such species was explored in Cheryl Smith’s January 26, 2005, NCGS talk Geochemical Investigation of the Distribution Habitat of (McDonald’s Rock Cress) in the Six River National Forest, Del Norte County, California.
Cheryl, current President of the Peninsula Geological Society, did field work in remote Del Norte county on the California-Oregon border studying the geochemical characteristics of soils supporting isolated communities of this rare endangered plant, pictured to the left. (Arabis macdonaldiana)
Botanical and ecological data on this and other hardy plants surviving on ultramafic soils are voluminous, but to date, the actual adaptive relationships between the plants and their environment are vague. California is an excellent place to study these interrelationships because of its quite varied plant life—over 5,000 plant species grow in the Golden State, more than the combined total of the central and eastern United States and adjacent parts of Canada. Additionally, 30% of California’s flora occur nowhere else in the world. By comparison, only 13% of the flora in the Northeastern U.S. are endemic, and only 1% in the British Isles. One reason for California’s prolific flora is its remarkably varied habitats. The latter provide conditions for a plant’s successful survival and reproduction.
California has a multitude of climatic conditions as well as a wide variety of rock types to support its complex floral communities. Similar habitats have been grouped into landform provinces based on their comparable topographic and climatic conditions. Each province, however, often contains a diversity of unique habitats, in large part a result of California’s complex and active geological processes.
Landscape evolution and the accompanying cooler, drier climate, for instance, gradually transformed some of the Tertiary sub-tropical habitats in central and southern California into semi-arid and desert communities. Lush forests were restricted to the wetter areas along the temperate northern California coast. Subsequent uplift of the Sierra Nevada range provided wet, higher elevation habitats on its western slopes and parched deserts in the rain-shadow to the east. Glacial-induced climate fluctuations yielded even more microenvironments that survived in sheltered areas until today. Other important factors influencing a plant’s survival include its ability to interact with other plant species, compete with them for nutrients, protect itself from indigenous fauna, and successfully reproduce.
All of these geological changes drove evolutionary mechanisms to fill the new habitats, as existing species were forced to occupy restricted habitats called refugia. California’s tectonic activity and numerous microclimates have heavily influenced plant distribution in the state. Some restricted habitats are disappearing while others are emerging, but both support rare plant species. Isolated seasonal habitats likewise spawn unusual flora, often differing from one location to another.
Unique habitats often occur as “islands” surrounded by more common vegetation. Many of these isolated ecological communities exist because of the local geology. Complex intermixtures of rock types provide very distinctive soils that are home to rare plant life. Because they lack many key elements that support the usual floral species, and are enriched in harmful elements, serpentine soils are home to a variety of uniquely adapted plants. The soils are rich in heavy metals and barren of vital elements needed to support conventional plant life. They are shallow, low in calcium, high in magnesium, and do not hold water well.
Serpentine flora provide an exciting opportunity for botanists and ecologists to probe adaptive evolutionary mechanisms. The soils that develop on these ultramafic rocks contain some elements, like nickel and chrome, which are toxic to most plant species. The stresses induced by their extreme compositional characteristics have actually selected traits and mutations that allow certain hardy plants to adapt to serpentine soils. Some plants actually become tolerant to these toxic elements and are capable of assimilating large quantities without ill effects, a phenomenon known as hyperaccumulation. Mutation may play a role in this adaptive process.
Cheryl’s thesis study was conducted in a very remote part of the Six River National Forest in Del Norte County. Her field area was located on the Josephine ophiolite atop serpentine and ultramafic rocks. The area is isolated and inhabited by a very private rural population, wary of strangers. Cheryl needed to exercise caution as she hiked the backcountry with her trusty dog in search of Arabis macdonaldiana colonies. Serpentine chaparral interspersed with evergreen woodlands dominate the rugged landscape. The tiny magenta flowers hug the ground and are unobtrusive except in localized colonies where they form thick carpets.
Cheryl sampled the soils around the plants, being careful not to disturb them. The samples were used to determine the soil mineralogy and its elemental composition. Another element in high concentration at the plant sites is barium. Adaptation to the toxic influences of barium may be a key factor for flora that exist on serpentine soils. Toxins and growth inhibitors drive natural selection by favoring certain mutations. Some of these selective processes may involve changes in only a single gene. Cheryl’s studies, though not conclusive, have provided trace element data that can be used to further characterize the environmental effects surrounding Arabis macdonaldiana.
Audience discussion following the talk mentioned the pioneering work of California botanist Arthur Kruckeberg on serpentine flora. He summarized his studies in his 1984 publication California Serpentines: Flora, Vegetation, Geology, Soils, and Management Problems. This treatise, and additional research being conducted at the U.C. Davis McLaughlin Reserve in the California Coast Range north of the Napa Valley, have made significant contributions to understanding the mechanisms that control the state’s diverse vegetation. Kruckeberg echoes many of the reasons mentioned above that make serpentine soils so infertile: their high magnesium, nickel, and chromium contents, low levels of soluble calcium and nitrogen, and poor water retention. Included in the “serpentine” category are soils derived from partially serpentinized peridotite (an ultramafic rock), gabbro (the plutonic equivalent of basalt), and basalt greenstones (metabasalts of ophiolitic origin). All these soil derivatives share similar soil characteristics with the serpentinites and also support unusual plant life.
Kruckeberg described plant responses to serpentine soils as avoidance, indifference, and endemism. Indigenous taxa that cannot survive on serpentine substrates are the avoiders; the flora that can endure both serpentine and nonserpentine soils are indifferent; and the endemic species are restricted to serpentine soils. It is the latter (endemic) species that have caught the eye of evolutionary biologists. Theories regarding the origin of the endemics are twofold.
One champions the paleoendemic hypothesis, which propose that ancestral species occupied several habitats until climate changes caused extinction of the nonserpentine populations. The other is the neoendemic theory, which suggests the “insular” taxa with extremely limited ranges evolved from ancestors living on adjacent nonserpentine soils. Botanists have shown that the endemics will grow on nonserpentine soils if carefully nurtured, and that they will flourish there if cultivated alone. This would imply that competition with other species on the nonserpentine substrates forced them to occupy the more harsh conditions of the serpentine soils. The degree of plant endemism is also variable, from 100% serpentine restriction to only partial restriction, depending on the local geology.
Reduced restriction is exhibited by “indicator” taxa, which are serpentine-restricted in only part of their ranges. Kruckeberg estimated an approximately equal count of serpentine endemic and serpentine indicator species, totaling over 425 taxa. He also noted that the Northern Coast Range serpentinites are particularly rich in plant life. Continuing serpentine flora research is being conducted at the McLaughlin Reserve, and is methodically revealing the survival strategies of these unusual plants.
The NCGS gratefully acknowledges Cheryl Smith for sharing her research on the major and trace element geochemistry of serpentine soils and its potential impact on the endangered plant Arabis macdonaldiana (McDonald’s rock cress). The botanical research surrounding this and other endemic serpentine soil inhabitants is making major contributions to evolutionary biology. However, the soil mineralogy and elemental chemistry, as pointed out by Cheryl, needs further clarification to identify its specific role in the survival of these hardy plants.
Note: The biological commentary on serpentine floral species and their evolutionary development was taken from the McLaughlin Reserve website, and from a short article called Why Rare Species? authored by Susan Cochrane and posted on the Ceres website.