Serpentine soils

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Pepperwood and the Santa Rosa Junior College partner to offer a two-semester Bio 85 – Natural History of Pepperwood course that certifies participants as UC California Naturalists. Students have the option to submit an article for the Pepperwood Field Notes Blog as part of the course. The following is a student-submitted article.


Serpentine soils and their limitations for plant growth

By Elaine Hards, Fall 2016 Bio 85 participant

If you hike the trail above the Bechtel House at Pepperwood Preserve you may notice that the soil has a rusty-red tinge to it. You may also notice that there is a rock outcrop on that hill which is basically grayish in color. You might not connect the two, but they are indeed connected. The rock is serpentinite, and the soil is serpentine. You can find serpentine soils in many places in California and at Pepperwood. Besides above the Bechtel House, there are also serpentine soils at the entrance to Pepperwood from Franz Valley Road and by the Dwight Center.

What is serpentinite? 

The rock serpentinite forms under low heat and high pressure along fault lines. It formed along our coast where the existing sedimentary rocks underwent a metamorphic process that did not melt the rocks but, under high pressure and under water, changed its mineral structure. Serpentinite displays many different colors from whitish to grayish green, to blue and even black. The rock feels waxy or soapy depending on the talc content. At Shell Beach on the Sonoma County coast—another location to find serpentinite—it is blue, and when wet forms clay.

What is serpentine soil?

Soils form under many conditions, but there are five main factors in their formation. These are climate, topography, organisms, time, and parent material. In the case of serpentine soils, the parent material serpentinite is what makes them so distinct. Serpentinite is basically magnesium silicate, so serpentine soils are very high in magnesium. In comparison, calcium, a mineral necessary for plant growth, is in very low quantity in serpentine soils. Plants need calcium to make new plant cells, but cannot distinguish between calcium and magnesium so they take in magnesium in place of calcium. This causes a calcium deficiency and magnesium toxicity in the plant.

A grassland near the entrance to Pepperwood that contains serpentine soils.

Serpentine soils are also very low in nitrogen and deficient in molybdenum, which are nutrients necessary for plant growth. This challenging combination of minerals and nutrients makes plant growth difficult. Serpentine soils also erode easily, are very thin, drain poorly, and can be very dry for long periods of time. Even so, some plants thrive in these conditions. Plants that would normally be outcompeted on nutrient rich, faster-draining soils, have adapted to grow on serpentine soils over time.

So what plants live in serpentine soil?

Serpentine at Pepperwood occurs at a few locations in the grasslands as well as the entrance of the preserve and near the Dwight Center where there is serpentine chaparral. Serpentine chaparral at Pepperwood is dominated by shrubs including chamise (Adenostoma fasciculatum), toyon (Heteromeles arbutifolia) and especially leather oak (Quercus durata var. durata) which is affiliated with serpentine soils. There are also several flowering plants that grow in our serpentine chaparral including rosinweed (Calycadenia pauciflora), Mt. St. Helena morning glory (Calystegia collina var. oxyphylla) and the charming common wooly sunflower (Eriophyllum lanatum var. achillaeoides).

Pepperwood’s serpentine grasslands host a rich wildflower display each spring with carpets of goldfields (Lasthenia gracilis), delicate spinster’s blue eyed marys (Collinsia sparsiflora), blue pollen producing bird’s-eye gilia (Gilia tricolor subsp. tricolor), and the tiny poppy called cream cups (Platystemon californicus).


References

Best, Catherine, John Thomas Howell. Walter and Irja Knight and Mary Wells. A Flora of Sonoma County. California Native Plant Society. 1996.

Ornuff, Robert, Phyllis, and Todd Keeler-Wolf. Introduction of California Plant Life. University of California Press. Berkeley. 2003.

Pepperwood Vascular Flora

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