May 27, 2017
I love walking my dog in the open space above Tiburon. The views are incredible and distracting. But if you only watch the cargo ships enter and leave the Golden Gate, you can easily miss one of the coolest sights in the world right at your feet.
The rare Tiburon black jewelflower, in bloom in May and June, is an odd member of the mustard family that is really a darkish maroon, almost black. This might explain its scientific name, Streptanthus glandulosus niger. I know: Streptanthus glandulosus sounds more like a disease than a flower. Believe it or not, that is not the only rare or endangered plant that lives in open spaces of Tiburon. You can also find Tiburon buckwheat, Tiburon paintbrush, Tiburon mariposa lily and Marin dwarf flax.
Rarities aren’t limited to Tiburon. Turns out our county has about 129 rare, endangered or threatened native plant species. Rarity in the natural world it seems is not that, well, rare. Thirty-five percent of the native plants in the United States are considered rare and these are only the plants we have found. Imagine all the ones yet to be discovered.
Marin is a treasure chest of rare plants, thanks to the California state rock, serpentinite. Marin County has major outcroppings on Mount Tamalpais and along the Tiburon Peninsula. Serpentinite is formed when the Earth’s mantle, lying under the sea, is forced upward where tectonic plates collide. The olivine and pyroxene in the upper mantle are squeezed under enormous pressure and changed into serpentinite, a metamorphic rock. Metamorphosis means change in Greek.
Serpentinite is smooth and jade-colored, like a serpent’s skin; hence its name. When serpentinite weathers and erodes, serpentine soils are formed. Ten percent of California’s native plants are restricted to growing in these serpentine soils, even though these soils only cover 1 percent of our state.
Serpentinite is pretty toxic as far as rock goes. There was a movement in 2010 to defrock the state rock because it can contain asbestos. It also contains toxic heavy metals such as chromium and nickel. Nickel is not great for plant growth. Serpentinite has high levels of magnesium, too, although this is an essential nutrient for plants, in high concentrations it interferes with the absorption of calcium, another essential nutrient for plants.
Serpentine soils are also low in calcium, phosphorus and potassium. Phosphorus and potassium are two of the three main macronutrients in plant growth, the P and K in fertilizer ratios. What does all of this mean? You aren’t going to be growing your tomatoes, or much of anything else, in serpentine soils.
Because serpentine soils are tough soils in which to grow plants you end up having little humus. Humus is the organic material in soil produced by the decomposition of leaves and plant material. No plants growing means no plants decomposing. Humus is an important source of nitrogen, the third important macronutrient in plant growth- the N in fertilizer ratios.
Most plant species cannot grow in serpentine soils at all. Some species can grow in serpentine soils because they have adapted, but they are often dwarfed. Some species can only grow in serpentine soils; these plants are called serpentine endemics. Because serpentine endemics like the Tiburon black jewelflower only grow on these uncommon soils, they are especially susceptible to extinction. Remember, serpentine soils only cover 1 percent of our state and that percentage shrinks every year.
Fortunately, the Tiburon black jewelflower is the focus of preservation efforts. In the 1990s, the town of Tiburon and the City of Belvedere purchased the area of its natural habitat to be preserved as open space. More recently, in a joint study by Mills College and the Marin County Open Space District, the Tiburon black jewelflower has been introduced to an area outside its normal range, a similar habitat area on Ring Mountain. The verdict is out on whether the relocation will be successful.