The mighty valley oak
Local scientists study the species, which can reveal the health of an ecosystem
With the sun playing hide and seek with prestorm clouds and a light wind tossing thin branches, Christopher Ivey walked among rows of young valley oak trees.
This assemblage—at the Mendocino National Forest Genetic Resource and Conservation Center, off Skyway southeast of Chico—sprouted from acorns collected from across the state, then transplanted here and at a sister site in Placerville.
Ivey suddenly stopped at a sapling, his eye drawn to a distinction so small that, unlike the variation in leaf types, likely would escape detection from anyone but a biologist. He pointed to small dark pods nestled beneath the leaves and in branch junctions. They’re galls: growths induced by certain insects laying eggs on a tree or a plant.
For one species commonplace around Chico, he explained, “the gall falls off the tree when it’s mature and the larvae are still inside and wriggle around, and it causes the gall to jump up and down. Sometimes there are carpets of these galls that rustle … That Joni Mitchell song, ‘The Hissing of Summer Lawns,’ I’ve often wondered if that’s what she’s referring to.”
This particular sapling was the only one in the vicinity with galls. Another had evidence of caterpillar activity—a light-green pattern on one dark-green leaf, again catching only Ivey’s eye—that likewise proved distinct.
These tiny details may seem trace, but they’re significant to Ivey. An associate professor of plant and evolutionary biology at Chico State, he’s conducting an extensive study of California’s valley oak range with Jessica Wright, a geneticist with the U.S. Forest Service, and Victoria Sork, a UCLA professor in Ivey’s field.
The valley oak itself is significant, because of those aforementioned details. It’s considered a “keystone species”: an individual that plays a vital role in keeping an ecosystem intact. The tree provides shelter, food, even water for other plants and animals.
How and why valley oaks adapt to conditions altered by climate change constitute a key piece of environmental science.
As goes the valley oak, so goes the world around it.
“There’s a role it plays in monitoring of groundwater,” Ivey said, “and in the naturally savannah oak settings, it modifies the habitat locally.
“Right underneath its branches it provides shade; it changes water availability and temperature in a way that favors some species and disfavors others. Between the oaks there’s grassland; acorns are a huge resource for acorn woodpeckers, all kinds of mammals, and plenty of animals make their homes in valley oaks.”
Such as larvae in saplings under study: “These trees, which were planted out here just a couple years ago, will begin to serve as homes for these galling insects.”
The valley oak’s stature makes it not just a “keystone species” but also an “indicator species.” That is, to paraphrase Ivey, a recognizable type reflecting the health of an ecosystem conspicuously enough to get noticed (and studied).
This majestic tree has captured the attention of conservationists as well as biologists. For Jim Brobeck, water policy analyst for North State advocacy group AquAlliance, it’s a signal for all life that depends on underground water.
The valley oak has a taproot system that extends between 70 feet and 80 feet into the earth. [Drawing water from that depth during the day, when utilized for photosynthesis, the tree later releases some back into shallow earth through its second root system—thereby hydrating other plants.]
“It requires access to the water table,” Brobeck explained by phone. The Sacramento Valley from Red Bluff to Bakersfield “used to have hundreds of thousands of acres of valley oak; now, south of the Delta, the groundwater has been so overexploited that the only heritage valley oaks that exist, exist because of their shallow root system because they’re irrigated….
“In that way, it’s a strong indicator for shallow groundwater levels that interact with the groundwater-dependent ecosystem.”
That term, groundwater-dependent ecosystem, is a “buzzword” in requirements for the Sustainable Groundwater Management Act that will govern the use of underground water in the way surface water is currently regulated. Under the existing water management plan, Brobeck said, Butte County applied for funding (so far not granted) to monitor the shallowest portions of our groundwater basin for the ecosystem.
“As far as I know, nobody in California has done that [monitoring],” he continued, “even though I think it is the proverbial canary in a coal mine.”
Brobeck and Ivey agree that preserving valley oak trees is preferable to reaching a crisis-signal point. Thus, Ivey’s research not only aims to identify what causes problems, but also potential solutions.
“Valley oak is in trouble across the range,” Ivey said, even if faring well in Chico. “This is a project to figure out how to help it.”