The most important driver of Edgewood’s ecological diversity is its 160 acres of serpentine soils, about a third of the preserve. Derived from the weathering of the metamorphic rock serpentinite, this uncommon soil type supports unique grassland and chaparral communities.
Plants in these communities face a number of challenges. Serpentine soils have
- Extremely low levels of the nutrients most plants require (nitrogen, phosphorus, potassium, and calcium)
- Extremely high levels of magnesium, which interfere with a plant’s ability to use calcium and iron, as well as highly toxic trace elements (e.g. chromium, nickel, and cobalt)
- A thin, gravelly substrate with poor water retention
- Little build-up of organic matter because plant growth is inhibited, thus limiting the amount of decomposing material
Most plants, and the animals they support, will not survive on serpentine soils because they are starved of needed nutrients and water, and poisoned by toxins they cannot handle. The plants that do survive may be stunted.
Over millennia, certain native plants have evolved to live and even thrive in these harsh conditions. Serpentine habitats offer these plants a refuge from invasive, non-native plants not adapted to these soils. Many of Edgewood’s rare species grow on serpentine soils, and three–San Mateo thornmint (Acanthomintha duttonii), serpentine leptosiphon (Leptosiphon ambiguous), and Marin dwarf flax (Hesperolinon congestum)–are serpentine endemics, meaning they are strictly limited to serpentine soils.
The rock that breaks down to create these serpentine soils comes from beneath the ocean crust, deep in the earth’s mantle, and rises to the surface in places with lots of tectonic activity, like the Bay area. The name serpentine refers to the appearance of the rock, which can look like mottled snake skin. Serpentine soils and rocks cover only about 1% of California, but 12% of the state’s endemic plant species are restricted to these habitats. Because of the fundamental role it plays in California’s extraordinary biodiversity, “serpentine” is the state rock of California.
In recent decades, nitrogen pollution has been changing the chemical balance of Edgewood’s serpentine grasslands. The increased nitrogen acts as a fertilizer and allows non-native plants to take hold. That leaves less room and resources for the native plants and animals that call these grasslands home. You can read about the role nitrogen pollution plays in the story of the Bay checkerspot butterfly, as well as on-going efforts to restore Edgewood’s grasslands, here.
The serpentine grassland community is a mosaic of communities. Serpentine soils vary in their chemical composition. Other factors, such as the slope of a hill or the thinness of the soil cover, also play a role in what grows where. Click on the plant names below to find out more about some of the plants that grow on Edgewood’s serpentine soils.
To learn more about California’s serpentine soils and the flora and fauna they support, see
Alexander, E.B. 2010, Oct. & 2011, Jan. Serpentine soils and why they limit plant survival and growth. Fremontia 38/39: 28-31.
Safford, H. D. 2010, Oct. & 2011, Jan. Serpentine endemism of the California flora. Fremontia 38/39: 32-39.
U.S. Forest Service. Serpentine Soils and Plant Adaptations. United States Department of Agriculture.