Many Californians are unaware that there exists a “state rock”. It is the lovely, slick-green rock, serpentine, which covers some 2,200 square miles throughout the state. Because of the unique chemical composition of serpentine, it often supports plant species and communities found nowhere else. While non-native plants dominate most of the grasslands in California, serpentine areas often have examples of native plant communities nearly uninvaded by weeds and other aliens. Serpentine is found in several areas throughout the globe, but the North Coast Ranges of California support one of the world’s richest serpentine floras.
The University of California is fortunate to have one of its 33 Natural Reserves, the Donald and Sylvia McLaughlin Reserve, located at the junction of Napa, Yolo, and Lake Counties in the North Coast Ranges, an area rich in serpentine specialist plants, or endemics. The McLaughlin Reserve is one of the newest and largest (7,050 acres) reserves in the system and the only one with significant expanses of serpentine. It occupies the site of the Homestake Mining Company’s McLaughlin Gold Mine.
In 1985, as part of the permitting process for the mine, Homestake promised to create an environmental field station on the site after mining had ceased. Coincidentally, in 1986, a group of UC Davis biologists recommended that the reserve system be expanded to include a serpentine reserve. In 1993, an agreement between the UC Regents and Homestake created the reserve, which now has a small residential facility for researchers and a growing legacy of first-rate research on serpentine species and plant communities.
The reserve contains many plant communities, including riparian woodlands, grasslands, blue oak woodland, and many kinds of chaparral or shrub communities. Many of these communities are common throughout California—blue oak woodland and chamise chaparral are both among the most widespread habitat types in the state. Consequently, we will focus primarily on the unique serpentine habitats in this article.
Serpentine-derived soils present a number of challenges to plants. They tend to be low in calcium, nitrogen, and phosphorus; high in toxic heavy metals such as magnesium, chromium, and nickel; and they have low water-holding capacity. Some plants cannot tolerate serpentine soils—both blue and live oaks are included in this group. Others appear indifferent, growing both on and off serpentine—toyon, California bay, and gray pine are in this group. But the most botanically interesting group is the serpentine endemic species, those plants found only on this soil type.
Serpentine endemism has long fascinated evolutionary biologists. Two evolutionary pathways are thought to have produced serpentine endemics. So-called paleoendemics are widespread species such as leather oak (Quercus durata) and McNab’s cypress (Cupressus macnabiana), found on serpentines throughout California. These are believed to have descended from ancestral species that grew in many different habitats, until climate change caused the non-serpentine populations to become extinct. In contrast, neoendemics, or “insular” taxa, are species with ranges often as small as a single county or less (e.g. several Ceanothus and Arctostaphylos spp.), which are believed to have arisen from ancestors on nearby non-serpentine soils. Experiments show that serpentine endemics often grow better on non-serpentine soil when grown alone, suggesting that competition with other species is the reason for their restriction to serpentine. Perhaps not surprisingly, then, the degree of endemism varies from species found 100% on serpentine to ones that are sometimes found on other rocky substrates such as volcanic outcrops.
Serpentine habitats on the McLaughlin reserve encompass a variety of plant habits, ranging from small trees and stunted shrubs to carpets of diminutive, colorful annuals in the meadows. There are also riparian areas on serpentine substrates and rare serpentine seeps with a showy array of specialized species.
The McLaughlin Reserve serpentine supports two kinds of chaparral communities—mixed serpentine chaparral and cypress chaparral. The first is dominated by a number of endemic species of familiar genera including leather oak (Quercus durata), white-leafed manzanita (Arctostaphylos viscida), and musk brush (Ceanothus jepsonii var. albiflorus). The silk-tassel bush (Garrya condonii) and fremontia (Fremontodendron californicum), both frequently found on serpentine, are common as well. Additionally there are a number of unusual understory herbs such as oniongrass (Melica spp.), squirreltail grass (Elymus elymoides), jewelflowers (Streptanthus spp.), morning glory (Calystegia collina), dwarf wild flax (Hesperolinon spp.), and sickle-leafed onion (Allium falcifolium).
Cypress chaparral includes both Sargents (Cupressus sargentii) and the endemic McNabs (Cupressus macnabiana) cypresses. Sargents cypress occurs on soils with especially low calcium levels but high water-holding capacity, primarily in serpentine riparian areas; but extensive stands may also occur on hillsides. McNabs cypress may be found within mixed serpentine chaparral or may form nearly pure stands. The two species can be distinguished by the short, wide stature and light green leaves of McNabs cypress, in contrast to the darker, taller, and more treelike Sargents cypress.
In the spring, the meadows and grasslands at the McLaughlin Reserve are a riot of color. The thin, infertile soils formed from serpentine and related rocks in these areas tend to be less dominated by Mediterranean grasses than most of the California grasslands, allowing the masses of tiny annual plants to be conspicuous. Native perennial bunchgrasses, which have been replaced by exotic annuals in many areas of the state, also thrive in the serpentine meadows at McLaughlin. Luxuriant stands of purple needlegrass (Nassella pulchra), squirreltail, bluegrass (Poa secunda), and oniongrass complement the spectacular floral displays.
The most specialized, unusual habitats on the reserve are the serpentine seeps, or springs, with their highly alkaline water and their diverse array of showy, localized endemics. Because the seeps flow well into the summer months, many of these species continue to bloom much later than the surrounding vegetation. Serpentine endemics in this habitat include the serpentine sunflower (Helianthus exilis), Cleveland’s butterweed (Senecio clevelandi), swamp larkspur (Delphinium uliginosum), Cleveland’s milkvetch (Astragalus clevelandi), and bare monkeyflower (Mimulus nudatus).
Researchers from throughout California have begun to work at the McLaughlin Reserve and many University classes visit to study the unique geology and habitats. The reserve is not open to the public, but there are occasional interpretive tours. For more information see http://nrs.ucop.edu.
Boucher, Virginia L. and Susan Harrison
edited by Adina Merenlender and Emily Heaton