Kelly G. Lyons
I once pictured myself working in a mild mountain field site, recording data under a nodding Douglas fir, with my hair lightly windblown. It is perhaps because of my naiveté as a blossoming field ecologist that I haven't always appreciated serpentine habitats. Serpentine vegetation is dwarfish so if you want to see it you often have to squat. There is very little shade and the rocky, exposed soil can get so hot you can feel it through the soles of your shoes. Sometimes when I hike these landscapes they feel so remote that I imagine they are best suited for disposing of a corpse.
It was not until I found a good guide who showed me how to see serpentine in a more enlightened way that I paused long enough to learn its secrets. On serpentine, you can find unspoiled habitat, beautiful "mini" flora, bubbling springs, swimming holes, bobcat tracks and solitude. I now look forward to exploring these landscapes every season. Here I offer a brief introduction to serpentine in the Putah-Cache bioregion but, ultimately, nothing can replace first hand exploration to tease the subtle sublimity from serpentine's harshness.
Serpentine is rare on the surface of the earth but common in California and the Putah-Cache bioregion. About 13% of the geologic substrates in the bioregion are serpentine. Frequent seismic activity and proximity to the ocean account, in part, for their abundance. Serpentine habitat is easy to recognize because of its distinctive green-gray, shiny rocks, which are often mottled black and streaked white, and sparse, tough vegetation. Over time, weathering oxidizes the iron in the rocks, rendering the soil burnt-orange.
During their time below the earth's surface, serpentine rocks are under high pressure. Once exposed, the rocks break down into layered, unstable talus. If you examine serpentine talus you find that each edge is very different. Where it is smooth and dense it has been placed under great pressure, and you can feel the direction in which the rocks slid past one another by rubbing your fingers along the smooth surfaces; they feel scaly.
Serpentine outcrops have been referred to as barrens because they are often sparsely vegetated and serve few of our agricultural needs. Inhabitants of these habitats experience drought, nutrient stress, heavy metal exposure and exposure to high light. Serpentine is severe because of the underlying substrate. Soil derived from serpentinitic rock presents so many hardships to plant life that Hans Jenny coined the term serpentine syndrome to refer to them collectively. The "syndrome" influences the overall biomass (amount of living material) on serpentine, which is low compared to other plant communities. Nonetheless, many species, equipped with unique physiologies to withstand the stress, thrive on these soils.
Serpentine, like most igneous rocks, contains iron and magnesium mixed with silica, but iron and magnesium are unusually high in these substrates. Soil scientists call this condition ultramafic ("ma" stands for magnesium and "f" for ferrum, or iron). Serpentine soils are also high in heavy metals such as chromium, cobalt and nickel. While serpentine outcrops differ in their concentrations of these metals, many sites contain levels that are toxic to many plant species. Serpentine is also rich in silica fibers known as asbestos although these minerals appear to pose more of a problem for humans than plants.
Another challenge serpentinic soil poses is a lack of nutrients. Calcium is unusually scarce and, for these soils, is reported in terms of its ratio to magnesium. Many scientists believe this ratio is a key factor in determining plant survivorship. Both elements are positive ions that compete for the same uptake sites in plant roots. When the concentration of magnesium is high, these uptake sites become saturated, making it harder for plants to obtain calcium. Low levels of calcium exacerbate this condition. Other important nutrients lacking in serpentine include potassium and phosphorous, which are both rapidly lost through weathering and leaching. Nitrogen is also scarce due to limited plant growth.
Finally, serpentine soils are thin. This means there is less substrate on which nutrients and water can be held and made available to plants. Deep serpentine soil occurs only in valleys, in alluvial soil, where rains wash small particles downward. Vegetation in these valleys is denser, but the soil is still ultramafic. The assumption that valley soils are richer because they support more biomass has misled those who tried to use them for grazing or agriculture. Successful ranching on serpentine is shortlived because removal of the vegetation results in rapid loss of cover and the eventual loss of water and minerals. Most farmers don't bother.
Serpentine species are generally smaller than their non-serpentine counterparts and, as with most plant species, can be lumped into two categories: tolerators and avoiders. They either put up with the dry conditions or avoid it. The tolerators are the dominant vegetation. These are primarily low-growing shrubs and occasional large trees with tough leaves that look silvery or flat gray due to hairs designed to reflect light. The avoiders germinate from seed or sprout from underground storage structures and complete their lifecycle while water is available. Many avoiders require close examination to be appreciated. Squatting works best. In closer proximity you can see patterns on the tiny petals, veins in the leaves, and miniature fruits.
Endemism occurs when a species is found exclusively in one designated location and no other. These locations can be defined naturally, by environmental conditions, or culturally, by lines of a map. For example, a species could be a serpentine endemic, found only on serpentine, or endemic only to Lake County. In some cases both occur simultaneously.
Serpentine habitats are geologic islands in a sea of other soil types. When these rocks were exposed, new species dispersed on them from the surrounding substrates. Eventually, those that could colonize and survive on serpentine evolved on a separate trajectory from their non-serpentine relatives. In many cases, the new species survived on a patch of serpentine because they were poor competitors on other substrates. As a result of this island effect, serpentine substrates house a large number of species that are found only on serpentine and have highly restricted ranges.
Approximately 282 serpentine species are listed by the California Native Plant Society as rare. Some of these are threatened due to anthropogenic causes, but many are rare simply because they are unusually restricted in their distribution. And, while they are not found in many locations, they may be locally abundant. Unlike rare species that have faced an unnatural contraction in their numbers, serpentine endemics are rare by nature. This suggests that for many serpentine species conservation is straightforward. Set the land aside, and as long as it remains undisturbed the plants will persist as they have for thousands of years.
Bare monkey flower (Mimulus nudatus) is an example of a highly restricted, serpentine endemic. This small seep plant with large pouting yellow flowers is found only in Lake and Napa counties. In rainy years, when serpentine seeps are well saturated and the growing season is prolonged, its populations are large. It may be confused with common yellow monkey flower (M. guttatus), which also grows on serpentine. But bare monkey flower has fewer leaves and that are shaped like thin feathers rather than spades.
Large and widespread serpentine endemics can act as flag species. Because they are loyal to the substrate, you know you are on serpentine when you see them. The most abundant and conveniently large flag species in our bioregion is leather oak (Quercus durata var. durata). This is a stumpy, dense shrub with small, dark green, cupped leaves that may be spiny on the margins. The acorns are oval with rounded tips. The common name is easy to remember once you have come in contact with a specimen because the leaves are tough and leathery.
Other common endemic shrubs include musk brush (Ceanothus jepsonii var. albiflorus) and white leaf manzanita (Arctostaphylos vicida ssp. pulchella). Musk brush is a California lilac that is restricted to the Inner Coast Range. It has gray stems with spiny leaves and tight clusters of tiny white flowers. Manzanita species have reddish bark, elliptic or oval leaves and small red berries that, as the Spanish derived common name implies, look like little apples. White leaf manzanita is an erect shrub with generally gray leaves and sticky stems and flowers.
The cypresses that grow on serpentine in the Putah-Cache Creek Bioregion are also distinctive and easily recognized. McNab cypress (Cupressus macnabiana) is a widespread species that is relatively loyal to serpentine soils in our watershed. You can tell it is a cypress because it has minute, awl-shaped leaves and round woody cones with shield-shaped scales, similar to an interior redwood. Sargent cypress (Cupressus sargentii) is another woody species commonly found on serpentine in the bioregion. It can be distinguished from McNab cypress because the resin glands (small dots) on the tiny scale leaves are located at the base, instead of the middle, of the leaf.
Some genera are particularly well represented on serpentine. For example, half of the 24 Californian species of the genus Streptanthus are associated with serpentine soils. Although easily overlooked due to their spindly growth habit and cryptic (camouflaging) colors, these herbaceous plants are distinctive. In early spring, the plant displays a basal rosette, a whorl of leaves growing near the ground. When ready, the apex expands producing a series of small flowers along a tall stem. The wavy petals protrude, sometimes ever so slightly, from an urn-shaped whorl of chartreuse and/or purple sepals.
Art Shapiro of UC Davis discovered a rare form of mimicry in the serpentine species Streptanthus breweri. These plants form small protrusions on their leaves that resemble the eggs of pierid butterflies. This is an effective defense mechanism because it deters butterflies from laying their eggs where they believe their herbivorous larval offspring would have to compete with other butterfly larvae for resources.
Not all species growing on serpentine are restricted to these soils. Taxa with the ability to grow on and off serpentine are called bodenvags, a German word meaning indifferent. This indifference allows them to be more common and widely distributed. In the Putah-Cache bioregion our most prominent bodenvag is Pinus sabiniana, also known as gray, foothill and, my personal favorite, ghost pine. This is a tall, branching pine with gray needles. The tree looks light and airy and appears to hover in the landscape, especially at dusk.
Two common chaparral shrubs of the rose family, chamise (Adenostema fasciculatum) and toyon (Heteromeles arbutifolia) are easily recognized bodenvags. In late spring, chamise's small, white flowers blanket the bush. This pervasive chaparral species is often found in dense stands on south-facing slopes. Its short needle-like leaves aid in water conservation allowing it to tolerate dry conditions. Toyon's white flowers emerge later in the season. It has deep green, elliptical (eye-shaped) leaves with serrated margins. It is also called Christmas berry because it dons bright red fruits in fall and winter.
Other notable shrubs are flannel bush (Fremontodendron californica ssp. napense) and silk tassel (Garrya congdonii). Flannel bush, a member of the cacao or chocolate family, has salmon yellow flowers that stand out in dramatic contrast to the dark vegetation. "Flannel" describes the leaves, which have star-shaped hairs that can irritate your skin if you handle the leaves. Silk tassel might be mistaken for an oak, but if you are lucky enough to catch it in bloom, you will find that the female flowers resemble a drooping stack of bells and the fruits balls on a string.
Herbaceous bodenvags easily recognized from the speeding car include the herb Globe gilia (Gilia capitata) and the grass big squirreltail (Elymus multisetus). Globe gilia has round clusters of small dusty-purple flowers at the tips of tall stems. The inflorescence (cluster of flowers) of big squirreltail is bushy with an airy feel. If you try to grab the flowering stalk of a dried late-season specimen the fruits will separate, becoming an amorphous clump.
The best way to learn about serpentine is to explore it on your own. You will find the highest density of flowers from late February through June, although all months offer something interesting. In August and September the late-season sunflowers and tarweeds are beautiful, but it can be unbearably hot.
The following list provides areas for hiking, swimming, mountain biking, hunting, ORV use and botanizing. Dogs are prohibited on the UC Reserve but they are allowed on BLM and California Department of Fish and Game (CDFG) property. Pay close attention to the signs and avoid trespassing on Homestake property. Also, watch for researcher's flagging so that you, your children or your dog doesn't trample someone's research. Hunters actively use the public lands so you might want to take precautions and wear some orange. Finally, be sure to bring plenty of drinking water.
Guenoc Winery and Vineyard: They have a wine-tasting room, a picnic area hedged by leather oak, and a nice view of the surrounding lands.
Indian Valley Reservoir (BLM): From Hwy 20 go north on Bear Valley Rd. (Although not serpentine, Bear Valley is a great place to view wildflowers in the spring.) Follow Bear Valley Rd. past the Wilbur Hot Springs turn-off, to Brim or Bartlett Springs Rd. and turn left. This undeveloped road will take you onto a large serpentine intrusion. If you continue west you will reach the north end of Indian Valley Reservoir. I like this area because it is so remote and the reservoir water is pleasantly warm for swimming. You may access the south end of Indian Valley Reservoir from Hwy 20, west of the Bear Valley Rd. turn-off.
Knoxville Public Lands, McLaughlin Mine and UC Natural Reserve (BLM, Homestake and UC System): Follow Knoxville-Berryessa Rd. along the lake and north along the Eticuera and Knoxville creeks. This is a somewhat rough road with low water crossings. The road takes you through several stands of calmingly beautiful blue-oak woodland. At 19 miles past the Knoxville-Berryessa/Pope Canyon junction, you come to the Knoxville-Devilhead Rd. Take a left onto Knoxville Public Lands. The road is bumpy and, when not well maintained (usually in winter), best navigated with a high-clearance vehicle. It leads to a parking lot where you can leave your car and hike.
The Cedar Roughs (BLM and CDFG): From the Knoxville-Berryessa/Pope Canyon junction turn left (west) on Pope Canyon Rd. Follow it for 2.2 miles and park on the south side of the road near a brown and yellow gate. Follow Pope Creek upstream for approximately 1/4 mile. Look for trail markers. At the marker, cross the creek and climb the steep hill to an abandoned jeep trail. This road will take you onto the Cedar Roughs. The Cedar Roughs are remote and rugged. Watch for ticks and rattlesnakes.
If you wish to learn more about serpentine flora and do more serious botanizing on your own the two publications below are very useful. Both may be purchased through the California Native Plant Society (www.cnps.org).
Hickman, J.C. 1993. The Jepson Manual of Higher Plants of California. University of California Press, Berkeley, CA.
Kruckeberg, A.R. 1984. California serpentines: Flora, vegetation, geology, soils and management problems. University of California Press, Berkeley, CA.
Upper Putah Creek
The structure and design of the Putah and Cache website is copyright © 2001 University of California.
The material on this page is copyright © 2001 Kelly G. Lyons.