Reaching heights of more than 100 feet, the main kind of eucalyptus you’re likely to see here is Tasmanian blue gum, eucalyptus globulus. They feature sickle-shaped leaves hanging from high branches, and deciduous bark that is forever peeling from their shaggy trunks. Some people experience the smell of eucalyptus as medicinal; others say the trees just smell like California.

So how did eucalyptus trees get here?

“They came here as envelopes of seeds on boats coming to California in the 1850s,” explains Jared Farmer, author of "Trees In Paradise: A California History."

During the Gold Rush, Australians were among the throngs flocking to a place where wood was in short supply.

The predominantly Australian eucalyptus genus includes more than 700 species, ranging from tall trees to shrubs. Hundreds have been tried out in California, but only red gum and blue gum reproduce on their own here.

“This was the era of wood power,” Farmer says. “Wood was used for almost everything. For energy, of course, but also for building every city, for moving things around, all the things where today we use concrete and plastic and steel.”

Besides the practical need to plant more trees, settlers who were used to dense forests also felt that the lack of trees in California’s grassy, marshy, scrubby landscape made it feel incomplete. So within a few years, nurseries in San Francisco were selling young eucalyptus grown from seed.

The trees grew remarkably quickly here, even in poor soil.

“In an average rainfall year here in California, these trees probably put on 4 to 6 feet in height and maybe, in their early growth years, a half-inch to an inch in diameter,” says Joe McBride, a UC Berkeley professor emeritus of landscape architecture and environmental planning.

Beyond the drive to change the landscape and provide firewood, Californians also planted eucalyptus (mainly blue gum) to serve as windbreaks.

In fact, that was the original purpose of what’s now the largest, densest stand of blue gum eucalyptus in the world, on campus at Berkeley, says McBride. It was planted around 140 years ago to provide a windbreak for an old cinder running track -- to keep its fine ashen gravel from blowing into athletes’ faces.

The trees’ success in California owed to a lack of enemies here. Because they were grown from seed, they hadn’t brought along any of the pests or pathogens (or koalas) they contend with back in Australia.

An early 20th century boom

Within a few decades of its arrival, many Californians grew disenchanted with eucalyptus. Blue gum proved terrible for woodworking — the wood often split and cracked, making it a poor choice for railroad ties. The trees also proved thirsty enough to drain nearby wells.

“If you go back to California farm journals of the 1870s, '80s, '90s, there’s just report after report of disappointment, like ‘these trees are no good,’ ” says Farmer, the historian.

But things changed in the early 20th century when U.S. Forest Service officials grew concerned about a looming timber famine. They feared forests in the eastern United States had been overexploited and wouldn’t grow back, and predicted the supply of hardwood would dwindle over the next 15 years.

Investors saw an opportunity: California had a tree capable of growing to full size within that time frame. If hardwood was about to be scarce, they reasoned, such trees could be in high demand and yield sizable returns within a few short years. (These people, Farmer says, were not reading blue gum’s lousy reviews in old farm reports. “And even if they did read them, maybe they wouldn’t care because they just wanted to make a buck; they were just flipping land.”)

This played out as a speculative frenzy — a bubble. Boosters began selling plantations dense with eucalyptus — hundreds of trees per acre. Farmer writes in his book that claims were made like: “Forests Grown While You Wait,” and “Absolute Security and Absolute Certainty.” In just a few years, millions of blue gums were planted from Southern California up to Mendocino.

The anticipated timber famine never came to pass. Forests further east proved more resilient than expected, and the need was offset by concrete, steel and imports, like mahogany. Ultimately, the thousands of acres of eucalyptus planted around California were not even worth cutting down. Much of what you see today is a century-old abandoned crop.

What’s fire got to do with it?

Eucalyptus trees have lovers and haters in California. A big part of the debate over whether the trees should be allowed to persist here traces back to the East Bay firestorm of 1991, which left 25 people dead and thousands homeless. Vast swaths of eucalyptus burned.

“People at the time, I don’t think, associated that with a planted plantation; it was just a eucalyptus forest,” says CalPoly botanist Jenn Yost. “And then when the fire came through — I mean that fire came through so fast and so hot and so many people lost their homes that it was a natural reaction to hate blue gums at that point.”

Because the trees shed so much bark, critics argue they worsen the fire hazard and should be cut down. Defenders point out California’s native plants also have a tendency to burn. Both say the science is on their side, but so far no landmark study has shut down the dispute.

That ongoing dispute is also politically entrenched. A few years ago, federal funding to cut down trees in the East Bay hills was rescinded, after protesters got naked and hugged the eucalyptus trees on campus at Cal.

Are they here to stay?

Blue gums can’t reproduce on their own just anywhere in California; Yost says they need year-round moisture. They’re able to regenerate in places like California’s coastal fog belt, but elsewhere “there are some plantations that don’t reproduce at all. When you go there, the trees are all in their rows, there’s few saplings anywhere to be seen, and those trees are just getting older.”

Not all non-native plants capable of reproducing on their own do it enough to have an ecological impact, Yost says.

“As soon as it starts outcompeting native species or fundamentally changing the environment so that native species can’t grow there, we would consider that an invasive species," she says.

Blue gum is classified as a “moderate” invasive, putting it a tier below such uncharismatic weeds as yellow star-thistle and medusahead. McBride, the retired Berkeley professor, says “although there’s been marginal expansion of some eucalyptus stands, it’s really not well adapted for long-distance dispersal. It hasn’t really spread very much on its own.”

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With an estimated 40,000 of eucalyptus planted across the state, the trees aren’t easy to get rid of. Slicing down a large blue gum near a building can require a crane, at an expense of thousands of dollars. And keeping them from resprouting can also be its own chore.

Long term, as the climate changes over the coming decades, it's possible the aging eucalyptus groves that don’t get enough water to reproduce will begin to die.

Then again, if the state becomes hotter and drier, it may become the type of place where some Australian species are able to thrive.