Why are there so many acorns in Sonoma County this year?

Across Sonoma County, there’s a mass event taking place at twilight Sunday: hundreds of thousands of eager creatures will be out collecting treats — not the sugary variety, but an unusual bounty of oak acorns.

According to reports, many of Sonoma County’s massive oak trees are exceptionally loaded this year. Large numbers of the conical shiny brown nuts have begun falling on roadways, sidewalks, decks and woodlands. And that’s drawing a wide assortment of hungry wildlife, from black bears to wood ducks.

The massive acorn bonanza is not simply a local event. Mysteriously, observers say, oak trees all over the region, some more than a hundred miles apart, are all producing a bumper crop at exactly the same time. It’s a synchronized natural phenomenon called “masting.” And science has yet to figure out why and how it occurs.

Paul Weber has watched it happening as a member of the Sonoma County Regional Parks’ Natural Resources Division. Weber has boots-on-the-ground experience with oak woodlands, having restored and managed vegetation throughout parks across the county.

“I’m seeing a large acorn crop this year in the parks,” he said, “from Sonoma Mountain over to Ragle Ranch park, especially black and live oaks.” Those falling acorns are a good source of new trees, Weber said. Parks staff are collecting them to restore forests and replace aging, failing legacy trees.

Brent Reed, restoration projects manager at the Laguna de Santa Rosa Foundation, is also seeing a larger crop of acorns.

“This looks like a very good year for the valley oaks,” he said. Like many residents, he’s also finding acorns accumulating in yards and smashed under tires in Santa Rosa parking lots.

Various theories

So what is masting, and why do oaks do it? There are a number of theories, Reed said, but it’s still something we don’t fully understand.

We do know masting happens in many oak species, and in some other plants, in seemingly unpredictable patterns. Trees may go years with little or modest acorn production, and then hundreds or thousands of trees will suddenly produce a boom harvest all together, as if on cue, across wide sections of the landscape.

Back in the late ’70s, the oaks’ mysterious behavior caught the attention of Walt Koenig, now a senior animal behavioral scientist with Cornell University and emeritus zoologist at UC Berkeley.

Koenig was researching the peculiar antics of acorn woodpeckers — they breed in mixed-sex clans, collectively store acorns in precisely drilled logs and go to war to defend their territory. He decided to also investigate what might be causing the 100-year-old oaks they live in to act in an apparently synchronized way. At the time, few people were asking why.

“Actually, oaks were easier to chase; they don’t move as much,” Koenig joked. Forty years ago, he said, he began a project to annually count acorns on individual trees. Eventually he was counting acorns on oaks scattered over hundreds of square miles of central California.

Valley Oaks, Koenig learned, exhibited synchronized mast years over almost their entire geographic range, hundreds of miles apart. But there were no obvious explanations for how they pulled off such a feat.

Various theories have been proposed. Perhaps masting is a last-ditch response to stress or threats like drought or fire. Or is it a strategy to overwhelm acorn-eating animals with many acorns once so more survive to sprout? Or are the trees shifting their resources after years of growth to a single year focused on seed production?

The theories are still being hotly debated, Koenig said, and there’s likely a very complicated combination of factors at play. But he sees strong evidence pointing to the influence of weather.

“We know acorn production is correlated with weather,” he said. “And in particular, after a warm, dry April, they do very well.” April is typically the month when acorn flowers are pollinated.

There are tantalizing hints that hormones or other chemical communication may be involved. There’s also evidence to support the “predator-satiation” theory, Koenig said: the idea that more acorns will become seedlings if the animals that eat them are overwhelmed by sheer numbers of acorns all at one time.

Food chain

A surprisingly large range of California wildlife depends on oak acorns as food, explained Reed of the Laguna de Santa Rosa Foundation. Not just dozens of species of birds, but rodents, foxes, deer, bears and insects eat acorns, too.

In the Laguna de Santa Rosa, the giant spreading stands of valley and other oak species are essential to the entire ecosystem, because many other animals prey on the species that eat acorns.

Even today, oaks remain at the center of a vast food network, a network which for thousands of years also has included human communities.

“Acorns are important to all tribes, locally and throughout the Bay Area, not only as a food source but culturally. Our identity is tied to acorns,” said Peter Nelson, a Coast Miwok and citizen of the Federated Indians of Graton Rancheria.

Nelson, an assistant professor at UC Berkeley, is working on a joint project with the Audubon Canyon Ranch in Sonoma County to learn more about how managed fire affects acorn pests, the timing of acorn and oak life cycle events and acorn gathering.

The study will compare acorns in oak woodlands on Bouverie Preserve in Glen Ellen with woodlands on Sonoma Land Trust’s Glen Oak Ranch that have not been prescriptively burned.

The project is motivated by native peoples’ ancient practice of using patchworks of purposeful burning to manage the landscape. Careful fire management improves useful resources and food but also reduces fuel loads that can lead to catastrophic firestorms, Nelson said. Recent wildfire frequency and increasing intensity is causing many to re-evaluate and study such native practices, which over thousands of years shaped the state’s landscape.

And oaks still are central to the state’s ecosystems. “Oaks are the most extensive old-growth forest in North America,” Koenig said. “Some, like blue oaks, grow slowly and can live 300 to 400 years.

“One mature valley oak can produce tens of thousands of acorns in a year,” he said. “I don’t think it’s a stretch to suggest that oak trees in North America produce more acorns than every other kind of nut grown here, cultivated or wild, combined.”

“Oaks are nodes,” Reed said, “with radiating associations to hundreds of other species of wildlife. They provide viable habitat for other life; they’re a keystone species.”

According to a 2007 study, at various times of year, California’s oak ecosystems provide food and habitat for roughly 2,000 plant species, 5,000 insect and arachnid species, 80 amphibian and reptile species, 160 bird species and 80 mammal species; for these animals, they are like a cross between towering condominiums and open-air food markets.

Drought, fire, human construction and hundreds of years of grazing and lumbering practices have reduced California’s once-vast oak woodlands dramatically, according to California Oaks, a project of the California Wildlife Foundation focused on preserving the state’s oak forest ecosystem.

Nov. 5 has been designated California Native Oak Day, said Maggie Hart, outreach manager for the Laguna de Santa Rosa Foundation. California Oaks provides information to the public on everything from how to plant and care for the 20 species of California native oaks to broader efforts to ensure they survive the next century, she said.

Several oak restoration and protection projects are underway at local organizations, including Sonoma Regional Parks, the Laguna de Santa Rosa Foundation, Audubon Canyon Ranch and others; public support is welcome.

In an era of Door Dash and stocked grocery shelves, it’s hard to imagine waiting for meals to drop from trees, as many critters, and even native Californians who collect them, still do. And after a year of wildfires, record drought and swamping rains, it’s nice to have a little good news from nature: the oaks are masting.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker, with local nature stories at Contact him at

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