By Mary K. Hanson
When talking about the various habitats under consideration for a National Conservation Area designation in the Berryessa Snow Mountain region, serpentine soil comes up a lot. For most people, though, it’s rather meaningless.
Most of us aren’t geologists or botanists, and if we’ve ever had an encounter with serpentine soil we probably didn’t even realize it. The truth is, though, if you live in the Coast Ranges of California just about anywhere between Santa Barbara County and the Oregon border (or in the Sierra foothills) you’re living in a region rich in serpentine outcroppings.
So what is serpentine soil? Simply speaking, if you think of the Earth as an onion made up of several different layers surrounding a molten core, we live on the very outer layer, on top of the soil that is a thin skin on the Earth’s crust. On our layer the soil is, for the most part, full of nutrients and organic matter that allow a lot of plants and trees to thrive.
Below us is another layer called the mantle. This layer is made of more dense, pressure resistant, 4 billion-year-old “ultramafic” minerals and rocks that are low in plant-nurturing calcium, potassium and other minerals, and high in things like magnesium, nickel and cobalt.
When, through tectonic action, veins or plugs or whole sheets of this mantle layer are thrust up to the surface, they come in contact with water, metamorphose and become “serpentinized” (converted into serpentine). And when these serpentinized outcroppings break down and mix with organic matter, they form what is called serpentine soil.
You’ve probably seen expanses of serpentine soil and didn’t realize it. Like scrubby islands in a sea of green, they look misplaced, almost “unearthly” — barren, rocky and sparsely vegetated by only occasional large trees and plants with few or very hardy leaves designed to reflect sunlight.
And you may ask yourself, why should we care about this place? It’s kind of ugly. But what’s really exciting about these serpentine expanses is that within their boundaries you can readily view examples of plant adaptation, natural selection and species differentiation at work.
Many of the plants and trees that can grow in serpentine soil are specialized and unique, not found anywhere else. In fact, about 280 of these serpentine species are listed as rare by the California Native Plant Society. Where the serpentine soils are, so are floral treasures not found in any other ecosystem … and they’re here right in your back yard.
This year is the 50th anniversary of the Wilderness Act, and to celebrate that you might try walking through some of the local wilderness areas, like Cedar Roughs, which is rich in serpentine soil and holds the largest stand of Sargent’s cypress trees in California.
Other serpentine areas include Walker Ridge, the public lands of the Bureau of Land Management Knoxville Unit (north of Lake Berryessa, part of which is designated an area of critical environmental concern because of the rich serpentine flora) and throughout the Putah and Cache Creek areas.
The bare monkey flower is an example of a plant species endemic to serpentine soil. With its pouty-lipped yellow flowers and feather-like leaves, it is found only in Lake and Napa counties. You also may find about 12 different species within the mustard family that grow in serpentine, including the most beautiful jewel flower, which grows in both Yolo and Solano counties. The best time to see these specialized plants in bloom are between late February and early June.
So, even though serpentine areas looked unusable or uninhabitable, they are not. To geologists, they offer a unique opportunity to see and analyze rocks and minerals from deep within the Earth’s mantle, and for the rest of us they are a cache of wholly unique, highly adaptable and rare plants just sitting out there waiting for us to view them on hikes and photo outings.
For more information about serpentine soils, check out “California Serpentines” by Arthur R. Kruckeberg and “An Introduction to the Serpentine Plant Community of the Putah-Cache Bioregion” by Kelly G. Lyons. There’s also “Serpentine: Evolution and Ecology of a Model System” by Susan Harrison and Nishanta Rajakaruna.
— Mary K. Hanson is an amateur naturalist and photographer. Tuleyome Tales is a monthly publication of Tuleyome, a nonprofit conservation organization based in Woodland. For more information about Tuleyome, go to www.tuleyome.org.