The Fog Factor

It’s summer in San Francisco, and a tide of white has washed ashore from the Pacific. The windmills at Ocean Beach were the first to go under; then the glass walls of the Financial District lost their glint. Across the bay, residents as far away as the Berkeley hills are bracing for a four-day immersion. Meanwhile, inland Sacramento is stewing in triple-digit temperatures.

Chronic bouts of summer fog are a hallmark of California’s Mediterranean climate. The Golden State is one of only five places on Earth that experience this pattern of hot, dry summers and mild, rainy winters. The others are southern Australia, Chile, South Africa, and the Mediterranean basin itself. Located on the southwestern edges of continents, at middle latitudes swept by the trade winds, their weather is moderated by the cold waters along adjacent oceans.

When warm, tropical air masses encounter California’s chilly currents, their moisture condenses into droplets of fog. High temperatures in the Central Valley pull these damp, woolly blankets far inland. Meanwhile, an area of high atmospheric pressure in the middle of the Pacific deflects most storms to the north, keeping summer months bone dry. The moderating effect of the ocean keeps coastal temperatures well above freezing. This temperate climate regime holds sway over roughly half the state.

Mediterranean biomes are a magnet for both people and plants. Though California represents 4.4 percent of the nation’s area, it houses 12 percent of its population. Likewise, Mediterranean areas around the world host 20 percent of Earth’s known vascular plant species on 2.2 percent of its landmass. Species diversity in the Golden State is just as extreme. California supports about 4,300 different species of plants — a third of all species of flora found in the entire United States.

A Galapagos of the California Coast

Biologist Michael Vasey measuring conductance on manzanitas. This patch of maritime chaparral is located at a Pajaro Hills reserve managed by the Elkhorn Slough Foundation. Photo by Michal Shuldman, UC Berkeley.
Biologist Michael Vasey measuring conductance on manzanitas. This patch of maritime chaparral is located at a Pajaro Hills reserve managed by the Elkhorn Slough Foundation. Photo by Michal Shuldman, UC Berkeley.

Nowhere is that botanical richness more evident than in the habitat known as maritime chaparral. Found within a few tens of miles of the breakers, maritime chaparral is dominated by woody shrubs, such as Arctostaphylos (manzanita) and Ceanothus (California lilac). Like inland chaparral, it is punctuated with such xeric species as coast live oak, toyon, sage, and poison oak. But as an incubator of diversity, maritime chaparral has more in common with tropical rainforest. Of the 95 species and subspecies of manzanita found in California, over half are local endemics that occur along the immediate coast.

Maritime chaparral is a relative newcomer to California’s habitat list. It was not recognized as a distinct plant community until 1978. Since 1986, however, it has been protected as an Environmentally Sensitive Habitat Area and receives the same stringent protections against development as wetlands. Even so, maritime chaparral has not been well studied and is still poorly understood.

The NRS’s Fort Ord Natural Reserve shelters one of the few well-protected stands of maritime chaparral left in the state. Sandwiched between the airport and city streets of the Monterey Bay town of Marina, the site is about the size of Treasure Island in San Francisco Bay. But less acreage hasn’t kept this petite reserve from revealing the relationship between the cooling touch of fog and a hotspot of biological diversity.

The Diversity-Climate Connection

One Fort Ord research convert is biologist Mike Vasey. A long-time biology and environmental studies instructor at San Francisco State University, Vasey has returned to school as a doctoral candidate in environmental studies in Karen Holl’s lab at UC Santa Cruz. He received a 2006-07 Mildred E. Mathias Graduate Student Research Grant to investigate why evolution shifts into hyperdrive in maritime chaparral. And he has a strong hunch about the answer: “It’s the fog,” he says.

The damp and chill of fog, Vasey suspects, softens the stress of California’s rain-free summers. Photosynthesis requires water, and plants can only continue to grow if moisture is available. Other theorists who have examined global patterns of diversity support Vasey’s view. As surely as loam sprouts seedlings, they argue, a favorable climate encourages speciation.”In places where conditions are rigorous, really cold or really dry, only the hearty survive. But along the coast, you can try this or try that, and you’re more likely to succeed and survive,” Vasey explains.

At each site, Vasey assessed how hard local manzanitas needed to work to pull water from the soil in both wet and dry seasons. His three years of sampling established that coastal plants are consistently better hydrated than their interior counterparts. He corroborated those findings with such measurements as soil moisture, temperature, humidity, and leaf condensation.

An approximation of the summer marine layer gradient in the Monterey Bay Area, averaged over six years, using daily reflectance data collected via satellite. Core research sites of M. Vasey illustrated by stars. Image modified by M. Vasey from Park Williams, UC Santa Barbara.

Vasey is now in the midst of analyzing how fog affects chaparral diversity. He has already sampled the plant mix and habitat variables at 88 plots from Sonoma to Santa Barbara. He will relate the distribution of these plots to the extent of fog exposure at each site. At that point, he plans to obtain cloud-ceiling data from nearby airports to determine the floor elevation of fog banks and use overhead satellite images to identify the range and height of each fog bank. Then, using GIS, he will superimpose this data onto digital elevation maps of each plot. The maps may ultimately establish a means of analyzing regional fog patterns.

So far, species diversity and the clammy presence of fog appear closely related. “Wherever you have a break in the mountains and the fog can push inland, like through the Golden Gate, in Bodega Bay, Monterey Bay, and Morro Bay, there are clusters of totally different species,” Vasey says.”These chaparral stands are like the Galapagos of the coast. Each little archipelago has its own set of endemic species. They are all islands of diversity within the broader system.”

At least some fog moisture, Vasey suspects, must be making its way into the plants. One route could be fog drip. Microscopic droplets condensing on leaves should merge and eventually fall to the soil where they can supplement the groundwater available to roots. Alternatively, the plants may be absorbing fog directly through leaf pores. Redwood needles certainly practice such foliar uptake, as proven by UC Berkeley professor of integrative biology Todd Dawson and colleagues. Vasey plans a similar study with manzanitas.

The idea is that the moisture in rain, fog, and groundwater contains different ratios of heavy and light water isotopes. Together with UC Berkeley doctoral student Michal Shuldman, Vasey plans to analyze these isotope ratios in manzanita tissues to identify the sources of plant moisture. — KMW

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