Tamalpais lessingia holds on through summer on bare serpentine rock

Tamalpais lessingia (Lessingia micradenia var. micradenia) is a small daisy that grows nowhere on earth except a few serpentine outcrops around Mount Tamalpais.¹ It was in flower at Cascade Canyon Open Space Preserve above Fairfax at the end of August, and it has been logged through this season along Pine Mountain Fire Road toward Bolinas and on the Marin Municipal Water District lands between the two.¹ Its entire known range runs from Phoenix Lake on the north side of Tamalpais to French Ranch near Forest Knolls.²

It is an easy plant to walk past. It is a wiry annual, anywhere from a few centimeters to knee height, with spreading branches and often a hairy or woolly surface. The upper leaves are small and pointed, under two centimeters long. The lower leaves wither early, so by September the plant is mostly stem. The flower heads sit singly or in small clusters at the branch tips. Each head has no ray petals at all, just a few funnel-shaped, lobed disc florets in pale purple to nearly white, wrapped in bracts tipped with purple and sticky with glands. From a few steps away it reads as specks of pale pink scattered among greenish-gray rock and cured grass.

The rock is the reason it is here and not somewhere else. Serpentine soil weathers from the ultramafic rock exposed along the Tamalpais ridges, and its chemistry is hostile to most plants. It holds very little calcium relative to magnesium; most plants want the two roughly balanced, and serpentine topsoil commonly runs three parts magnesium to one part calcium. It carries elevated nickel, chromium and cobalt, and little nitrogen, phosphorus or potassium. It is also thin, gravelly and quick to dry. Plants that persist on it generally keep nickel out at the root or take up calcium selectively over magnesium. How this particular lessingia manages the chemistry has not been studied.

Its timing is unusual for a serpentine annual. Most of them flower early and set seed before the soil dries out. Tamalpais lessingia flowers from late May through September, in the weeks when the surrounding grassland has already gone brown.³ The likeliest explanation is the ground itself rather than any special drought physiology, though no study has measured it in this plant. Thin, rocky serpentine holds far less grass than deeper soil, so there is little around the lessingia drawing down moisture and nothing shading it. Whatever water lingers in the gravel and rock cracks through summer is largely its own.

That same gradient is where the pressure on it lies. Deeper serpentine soils hold more grass and more of it non-native; shallow, rocky serpentine holds more of the natives and endemics. Anything that deepens or enriches soil at the edge of an outcrop, such as fill graded along a road shoulder or added nutrients, moves the ground toward the grass-dominated side. In South Bay serpentine grasslands, nitrogen settling out of vehicle exhaust has been documented doing exactly that, fertilizing thin soils enough for annual grasses to crowd out the native forbs. The California Native Plant Society ranks this variety 1B.2, rare and moderately threatened.² It has no federal listing, unlike its serpentine neighbors Marin dwarf-flax and Tiburon jewelflower.⁴ The water district's own rare-plant inventory puts about half the watershed's rare species and 80 percent of its sensitive-species occurrences on serpentine ground, so any grading, grazing or vegetation work on those tracts lands on this plant's habitat by definition.

For the next few weeks it is still findable. Look where a fire road crosses bare greenish rock and the grass thins to almost nothing, on Pine Mountain Fire Road or the upper trails of Cascade Canyon, and check the wiry, near-leafless stems at knee height for pale heads no bigger than a fingernail.