California Eucalyptus a Charming Invader

SAN FRANCISCO —  There’s nothing like a stroll through a towering grove of eucalyptus trees on a foggy day. Breathing in their crisp, invigorating scent in a forest hushed by a carpet of bark and leaves seems to be one of those only-in-California experiences.

“They’re the trees of light,” says Alan Kaplan, an entomologist who leads nature tours through eucalyptus groves in the hills above Berkeley. “In the late afternoon, the sun streams through them and sparkles on the wax on the leaves.”

But the fast-growing bluegum eucalyptus, which has spread up and down California’s coast, is actually an Australian import. And therein lies a huge problem for native plants and animals as well as more recent arrivals, such as humans.

Freed from the insects that once kept them in check, bluegum eucalyptus can soar more than 200 feet here, multiplying rapidly in thick groves and shedding bark and leaves into deep piles that choke out other plants. That means less food and shelter for deer, owls and other animals.

Brought in by a San Francisco nursery in the 1850s, the eucalyptus turned out to be worthless as a get-rich-quick scheme. The wood twisted, cracked, rotted quickly and warped easily, making it useless as lumber or railroad ties. “A lot of people went broke,” says Jake Sigg, a California Native Plant Society member who advocated planting more eucalyptus trees as a gardener for the city of San Francisco.

About 200 species of eucalyptus have been introduced to California since then, and most make lovely additions to urban landscapes. Only the bluegum and a few others are invasive, spreading so thickly in places that they become fire hazards.

Eucalyptus trees contributed to the Oakland Hills fire that destroyed 3,000 homes and killed 16 people in 1991, as well as the 1996 Harmony Grove fire, which destroyed 54 homes and killed a man in San Diego County.

“The oil heats up and tends to burst out of the crown in a small explosion of burning embers,” said Mike Kelly. He leads the Special Weed Attack Team, a group of native plant enthusiasts who have been taking chain saws to eucalyptus trees and replanting native cottonwoods in San Diego’s Penasquitos Canyon Reserve.

Alien plants have invaded California since at least 1769, when a tiny ground cover known as storksbill hitchhiked with missionaries traveling up from Mexico. Soon it formed huge carpets, obliterating most native wildflowers on San Diego’s mesas.

More than 1,000 exotic plants have been introduced in the state during the last 230 years, for reasons ranging from crop and forage experiments to landscaping. Most have behaved well, or they failed and were not replanted. But eucalyptus is one of about 50 plants that have escaped cultivation and now run roughshod over wild areas.

Controlling invasive species is a billion-dollar prospect in California alone, but most government agencies can afford to do little more than arrange for volunteer weeders like Kelly to restrain their spread. At least a dozen eucalyptus-eradication projects are underway in California, from the coast of Mendocino County south to San Diego.

And just cutting the trees down won’t remove them--the stumps have to be either uprooted or coated with herbicide. “If you cut down one eucalyptus, it’s like a hydra--you’ll get three or four coming out of the trunk, and five or six years later they’ll be 30, 40, 50 feet tall,” said Kaplan, who advocates slow, careful replacement with native trees.

One place where eucalyptus trees were all but eliminated is Angel Island, a nature preserve in San Francisco Bay. The operation drew some protests at the time, but a mixed woodland forest now grows in their place.

“Eventually the eucalyptus would have destroyed every single native species on the island, so we were pleased to see them removed,” Sigg said.

Keeping eucalyptus trees in check is much harder in areas without such natural barriers. And Kaplan, for one, isn’t eager to see them go, since they now provide a habitat for a range of species. Salamanders and snakes thrive in the leaf litter. Hawks nest in the trees’ higher reaches. And some species of hummingbirds don’t migrate south in the winter anymore, now that eucalyptus nectar is available year-round.

“A community has developed around these naturalized citizens, and they deserve credit too,” said Kaplan, who claims that eucalyptus trees support more amphibians by weight than the mammals that could be supported by a mixed forest of native trees.

“At some point in our history, we all come from another place,” Kaplan said. “These trees have been here long enough to be recognized as citizens of the area.”

Sigg rejects this analogy as melodramatic and dangerous, since it might encourage people to surrender to exotic-plant invasions, doing nothing to stop the disappearance of native flora and fauna that took billions of years to evolve.

“Our way of life is tied into these ecosystem services that we’ve always taken for granted--clean air, clean water, pleasant surroundings,” Sigg said. “One day they may not be there.”