The Manzanitas of El Morro

When it comes to Manzanitas (Arctostaphylos spp.), there is a lot going on in San Luis Obispo County. And this becomes apparent in the case of two Manzanitas growing in areas around the Morro Bay Estuary. 

The Morro Bay Estuary is situated along California's central coastline and lies roughly 70 miles north of Point Conception. To the east of the estuary are the Los Osos and Chorro Valleys. These are ruderal valleys lined by agricultural fields and cattle ranches, but they remain beautiful in their own right. And they are home to two rare and endemic species of Manzanita. 

Coastal sage scrub intermixes with non-native grasses. Much of the valley has undergone a type conversion.

Today, the open space corridors that line the Chorro and Los Osos valleys are home to Arctostaphylos osoensis (Oso Manzanita) and Arctostaphylos morroensis (Morro Manzanita). These plants grow nowhere else in the world and they are restricted to areas surrounding the Morro Bay Estuary.

Morro Manzanita is generally more prevalent than Oso Manzanita. Morro Manzanita primarily inhabits the stabilized sand dunes at the south end of the Morro Bay Estuary, with much of the population occurring within Montana De Oro State Park. Many of the upland dune complexes in the area date back to the Pleistocene.

Arctostaphylos morroensis (Morro Manzanita) in the Elfin Forest Preserve

Arctostaphylos morroensis (Morro Manzanita) in bloom

During the last glacial period of the Pleistocene, the sea level along the central coast of California was 400 feet lower than where it is today. The coastline was far to the west of its present line. Since that time, the climate has warmed, sea ice has melted, and the sea level has risen to its present line. As the sea rose, long shore currents off the coastline deposited sand along the western stretch of Morro Bay (as well as the greater central coast). Also during that time, the Los Osos and El Chorro creeks were depositing sediment along the beaches. As sand and sediment accumulated along the coast, the combined forces of wind, tides and surf created large piles of sand along the coastline. As these piles grew, persistent wind and tides transferred the sand inland. Slowly, a dune complex began to form and vegetation eventually moved in.

Initially, low growing pioneer plant species established themselves on the dunes. As the pioneer species further stabilized the shifting sands, vegetative succession took place and allowed for other larger shrubs and perennial species to take root. Morro Manzanita eventually became one of those larger shrub species. 

Then there is Oso Manzanita. Oso Manzanita is by far more restricted than Morro Manzanita. While Morro Manzanita grows throughout the upland dune complexes surrounding the towns of Los Osos and Morro Bay, Oso Manzanita basically hugs the slopes and rock outcroppings surrounding Cabrillo Peak. Oso Manzanita occurs in only a handful of occurrences. Yes, like 6 or 7 stands. 

Arctostaphylos osoensis (Oso Manzanita)

One of few locales where Arctostaphylos osoensis (Oso Manzanita) grows - tucked in a rock outcropping on Cabrillo Peak

Oso Manzanita seems to hold a close bond with two peaks that divide the Chorro and Los Osos valleys. Its 6 or 7 stands are in areas around Cabrillo Peak and, to a lesser extent, Hollister Peak.  Primarily, Oso Manzanita seems to prefer the south and west facing rock outcroppings of Cabrillo Peak.

Cabrillo Peak is one of the former volcanic peaks that divides the Chorro and Los Osos valleys. The peak, like the others in its chain, is basically a big chunk of volcanic rock that formed long ago (~22 – 26 mya). This chunk of volcanic rock is thought to be the result of when a dike ran through the underlying geologic plates and allowed molten igneous rock to well upward, where it then cooled near the earth’s surface. Over time, erosion stripped away the earth’s outer surfaces and exposed the mass of igneous rock that we today call Cabrillo Peak. 

The primary habitats of Arctostaphylos osoensis (Oso Manzanita) and Arctostaphylos morroensis (Morro Manzanita). 

Given the limited locations of Oso Manzanita, it may seem as if Oso Manzanita has a preference for dacite rock outcroppings. And to some extent that could be true, since there are many rare plants that take to geologic anomalies. But, Oso Manzanita can also be found growing in the Jurassic and Cretaceous era sandstone deposits of the valley. These are the sandstone deposits that generally lie at the toe of those dacite rock outcroppings. It's old sandstone that formed long before the Pleistocene.   

If there is likely one dominant influence driving the occurrences of Oso Manzanita, it would be safe to place a bet on microclimate. Oso Manzanita is entirely restricted to the western end of the Los Osos Valley. It seems to lie no further east than that ever persistent summer fog bank. And it likes to hug an elevation line that is squarely in line with the inversion layer - the line at which swaths of moist fog suck into the valleys. 

The fact that Oso Manzanita grows under coastal conditions is enough to limit its prospects for distribution elsewhere. Its proximity to the coastline presumes it needs a relatively benign microclimate (by comparison to other inland parts of California). But the prevalence of fog adds layers of influence to that need. Even amidst a relatively benign microclimate, it still needs sips of water throughout the long dry summer.

As summer fog moves inland through the valleys, the areas around Cabrillo Peak become shrouded in fog. That fog provides a cool and moist reprieve during California's long dry summer months. Fog tamps down the intensity of sunlight and keeps evapotranspiration rates low. And when fog drip occurs, the build up of moisture provides a noticeable uptick in available soil moisture. This uptick can be a very impactful influence during long dry stretches when rainfall is essentially absent. 

Like other manzanitas, Oso Manzanita still prefers its well drained soils and direct southern sun. But is appears that Oso Manzanita also wants to bathe in moist summer fog. It needs periodic drinks. It's tough but maybe not so extreme.   

It remains to be seen how climate change and a waning fog pattern will affect the future of Oso Manzanita. 

Arctostaphylos morroensis

Arctostaphylos osoensis

Arctostaphylos morroensis   vs.   Arctostaphylos osoensis

Additional Reading

Manzanitas of San Luis Obispo County by Michael Kauffmann (CONIFER COUNTRY BLOG)

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