by Andrea Williams, Vegetation Ecologist
This is installment seven of a 12-part series on grasses. Read the previous installment here.
Serpentine reedgrass (Calamagrostis ophitidis) is rare. We have tons of it.
Such seemingly incongruous statements happen quite a bit on Mt. Tam, through the magic of the “Matrix of Rarity.” It depends on the scale at which you look. The California Native Plant Society (CNPS), which reviews the distribution of and threats to California’s flora, keeps an inventory of plants and the degree of rarity they display. So statewide (and worldwide), there may be fewer than 100,000 individuals, but if all those individuals are in a three-square-mile area they may seem abundant in that spot.
|Abundant where found||Few plants per population|
|Broadly distributed||Common species, e.g. purple needlegrass (Stipa pulchra)||Often rare, e.g. semaphore grass (Pleuropogon)—next month’s topic!|
|Narrowly distributed||Often rare, e.g. serpentine reedgrass||Nearly always rare|
Almost half of our rare plants are restricted to serpentine soils; like wetlands and beaches, these habitat types are finite. Often, rare plants in these spots will be quite common and you may wonder why they’re considered rare at all. Mt. Tam manzanita is actually so common it’s the dominant plant over most of our serpentine soils. Serpentine reedgrass is pickier still, preferring to grow at the edge and in the interstices of serpentine chaparral.
Unlike Mt. Tam manzanita, though, serpentine reedgrass can be found outside Marin in Sonoma, Mendocino, Napa, and Lake counties, although we do have the bulk of it. This handsome perennial grass raises fluffy, open spikes one to three feet above clumps of deep green upright leaves. Some of the finest serpentine reedgrass grassland—a rare vegetation type—can be found along Pine Mountain Road opposite Azalea Hill.
Another factor feeding into a plant’s status is the threat to its populations. Serpentine reedgrass populations on Mt. Tamlapais are pretty stable: No one is bulldozing populations to build things, they don’t need fire to germinate, their habitat isn’t being taken over, no diseases are wiping them out. But in other counties that may not be the case. Like with the drought, we need to consider ourselves lucky that we are where we are and have what we have (90% of average water storage and 40 rare plant species), and continue to conserve.