Home Sweet Serpentine
Tamalpais Manzanita, Mount Tamalpais State Park. Photo: randomtruth.
Serpentine, California’s state rock, is feeling some pressure—and not just because it’s a metamorphic rock! The California Legislature is considering a bill that would strip serpentine of its state rock status; geology blogger Brian Romans explained the details in this recent QUEST blog. Basically, proponents of the bill say that because asbestos is made from serpentine rock, and asbestos causes cancer, serpentine should not be the state rock. Never mind that serpentine does not cause cancer. In fact, many organisms thrive on serpentine soils. And that is what today’s post is about—the unique plants and animals that call serpentine soil home.
Serpentine soil is a tough environment: the soil is coarse, so water runs right through it, making it very dry. It is often dark in color, so it heats up in the sun. And its chemical makeup is challenging to plant life, to say the least. The soil has high concentrations of heavy metals, like nickel, iron, and chromium, and low concentrations of nutrients, like nitrogen and phosphorus. It is also really high in magnesium, which makes it hard for plants’ roots to take up those already-scarce nutrients. And it is low in calcium, which causes ion balance problems for plants.
With nutrients scarce, serpentine inhabitants tend to be small in stature—it’s hard to grow big without much food. And, with low water availability, serpentine plants are drought-tolerant. They often have tough little leaves, which don’t lose much water. Some examples are the Tamalpais manzanita (Arctostaphylos montana), and the Leather Oak (Quercus durata).
Plants on serpentine soils also have to deal with those heavy metals, which can interfere with metabolic processes. Some plants, like the Milkwort Jewelflower (Strepthanus polygaloides), have a really high tolerance for heavy metals. Milkwort Jelweflower is a nickel hyperaccumulator—it can take up lots of nickel from the soil, with no ill effects. In fact, some serpentine plants are used in bioremediation; people plant them in contaminated soil, where they pull the heavy metals out of the ground and sequester them in their tissues.
Serpentine soils are home to many endemic species—species that live in a particular habitat type, and nowhere else. Sometimes plants or animals are limited to one habitat because they can’t survive the physical conditions of other habitat types. But in the case of serpentine endemics, many can live in other habitats’ nutrient-rich soils, but are total weaklings when it comes to competition with other plants. They can’t live in other habitats simply because they are out-competed.
Serpentine soils are home to more than just plants—there are butterflies, too, like the beautiful California White (Pontia sisymbrii). Some, like a rare variant of the Edith’s checkerspot butterfly, Euphydryas editha luestherae, are serpentine endemics, because they lay their eggs exclusively on plants living on serpentine soils.
The Geoblogosphere is buzzing with commentary about California’s serpentine bill. If you feel passionate about keeping serpentine as the state rock, by all means write your state representative—but also visit some serpentine habitat! There are lots of places in the Bay Area where you can check out serpentine soils and their inhabitants. There are serpentine outcroppings on Mount Tamalpais, Mount Diablo (be sure to check out QUEST’s Mount Diablo State Park Exploration!), and in the Berkeley and Oakland hills.