THE EVOLUTION OF CEANOTHUS
Ceanothus, aka California lilac, dapples our mountains and hills with a multitude of diversity. On the cusp of spring, the flowers burst with color and scream of place and existence—hearkening to the clicks and ticks of winter’s cues. Its tall stature billows over trails like an elven tunnel in chaparral landscapes and speckles the coastal sage scrub. I point to the periwinkle-colored flowers puffing out, and declare it an angel, a cloud, a vector of light. This illusion must be the long-flowered panicles that filter the light in a mesmerizing spring glow.
The diversity of the genus is incredible. We have close to 15 species in Ventura County, each occupying their own niche. Bigpod ceanothus (Ceanothus megacarpus), dusty and white, stands broad on the fingered foothills of the valley. The glossy greenbark ceanothus (Ceanothus spinosus) crawls closer to the valley floor. The hairy ceanothus (Ceanothus oliganthus) wanders into the woodland shade. Even if you do not understand where they belong, their characteristics will tell the story for you. I approach each looking at the thickness of the leaves and how that protects it from the sun. The depth of green gives me a window into how the chlorophyll arranges to talk to the light. All of this leads me to more questions. How did the genus get to this point? What was the mode of evolution?
We must travel back to the end of the Pliocene—when the weather shifted from hotter and wetter to cooler and drier and the mountain ranges lifted and formed at drastic rates. Ceanothus first occupied niches as a hard-leafed (sclerophyllous) understory plant that frequented dry rocky outcrops; however, a section (sect.) of ceanothus took advantage of a changing topography and climate. This new subgenus of Ceanothus, Ceanothus sect. Cerastes, rapidly hybridized and established newly adapted species for the varied ecological situations occurring. They moved into dry mountain tops, understory canopies, and 5
canyons. Individuals in this group are all over the valley—distinct with their opposite leaves. Another important distinguishing characteristic is that they are obligate seeders. Their seeds are readily available in the seed bank after a fire (lasting for decades), instead of recolonizing through basal sprouting. They tie this readiness back into their greater landscape, by partnering with a particular Frankia bacterium. They flood the soil with nitrogen, and prepare the ecosystem to shift to other habitat types.
However, though the sect. Cerastes branched off to fill the mosaic of the California Floristic Province, there is a group that stayed close to its primitive roots (quite literally). Ceanothus sect. Ceanothus (alternate leaves) does not hybridize readily, nor rely on seeding after disturbance. Instead, this group reflects its sclerophyll ancestry by focusing on resprouting after fire. This allows for quick vegetation immediately after blackened ground—nursing the fire-following annuals that relish in the canopy’s shade. I am continually curious to see how Ceanothus utilize these adaptations to respond to the next wave of a changing climate. Will the obligate seeders diminish due to invasive weed competition after fire? If more frequent fires occur, will it impair the ability of the sect. Ceanothus to resprout? How this genus will respond is unknown; however, it is clear that the diversity gives strength to a varying degree of change and disturbance—whether that be fire, climate change, or uplifting mountains. The plant populations within the valley have been evolving for millions of years, and we hope to bolster their health so they can continue evolving and thriving.
Sophie McLean, Nursery Manager & Native Plant Specialist