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Invasive Species Week: The Blue Gum Eucalyptus

Blue gum eucalyptus in Olema, near Point Reyes National Seashore | Photo: James Gaither/Flickr/Creative Commons License

Next week might be Shark Week, but the California Department of Fish and Wildllife has declared August 2-10 California Invasive Species Action Week, an event intended to boost public awareness of the problems some non-native species of plants, animals, and even microorganisms can cause when they're imported into the state.

We're celebrating, if that's the right word, by featuring five of California's most prevalent invasive species every day this week.

And to kick things off, we'll take a look at what's probably the best-known invasive exotic species in California: the blue gum eucalyptus tree, originally native to Tasmania.

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Blue gum eucalyptus, Eucalyptus globulus to the botanists, have been a part of the California landscape since the Gold Rush, long enough that legends have sprung up about how they first made it to the state. The usual story is that 19th Century Californians encouraged planting of the trees as a quick-growing source of quality lumber, then were disappointed to find out that Californian-grown eucs produce wood unsuitable for much besides rough fenceposts and firewood.

The stories have some factual basis: there was a speculative eucalyptus-planting rush in the first years of the 20th Century, with people planning uses from fine furniture to rot-resistant railroad ties. And the Californian-grown trees, which grew far more quickly than their Tasmanian counterparts, did turn out to make lousy timber.

But blue gums were planted here in huge numbers well before the timber speculation wave hit. Part of the reason for planting the trees was as windbreaks for farms and orchards, as blue gum seedlings can grow 25 feet per season for several years after planting, and don't need much in the way of coddling.

And part of the impetus for planting came from a sense among Californians that the state's native landscape needed visual improvement. Our rolling grassland hills, dotted with the occasional oak or stand of chaparral, seemed barren to Californian landscape aesthetes of the 19th Century. Blue gum and other eucalyptus species, along with other southern hemisphere imports such as Acacia and Grevillea, started lining the lanes and property lines of the settled part of coastal California in large numbers in the 1870s, according to Nathan Masters at L.A. as Subject.

The trees were also planted for their ability to suck up groundwater, a trait that was a liability in the long run. To farmers seeking to "reclaim" wetlands for plowing, though, or for communities worried about malaria mosquitoes, the thirsty blue gums seemed a godsend. The essential oil in the leaves was also considered a possible cure for malaria.

Blue gums started falling out of favor right about a century ago: plantings dropped sharply in 1914 when the speculative market in eucalyptus timber collapsed. Now, it's hard to find the tree available in nurseries.

But by the 1980s, according to the U.N.'s Food and Agriculture Organization, there were about 198,000 acres of blue gum growing in California. There are likely more than that now.

Blue gums' most obvious environmental impact involves the native plants they replace. Where they grow thickly, blue gums compete very effectively with other plants for water and sunlight, and their dropped bark and leaves contain chemicals that retard the growth of other plants. There are plenty of large stands of eucalyptus along the California coast with almost no other plant species to be found, and where the eucs grow thinly, they still seem to reduce native plants' vigor.

This is especially a problem in the foggy coastal hills where eucalyptus seems to be expanding its range most aggressively; the native plants they displace there once made up one of the state's most biodiverse landscapes, and are under pressure from other invasive plants such as broom and pampas grass as well.

The trees' impact on native animals is less certain. In 1997, the late Bay Area ornithologist Rich Stallcup reported that eucalyptus nectar congealed around the nostrils of hummingbirds who fed on the trees' flowers, later extending the range of species affected by the problem to include warblers, orioles, and a few other nectar-feeding birds. Stallcup's reports remain controversial among scientists.

Other birds seem to like to use eucs for shelter and nesting, though the trees' rot-resistant wood doesn't offer much in the way of housing for cavity nesters such as woodpeckers. Large birds in the heron family seem to have a strong preference for eucs as nesting sites, and raptors seem to like them pretty well, too. The Point Reyes Bird Observatory reports, though, that bird diversity in eucalyptus groves is about 70 percent lower than in oak woodlands or native riparian forests in the same places.

Some native animals like eucalyptus forests very much. Monarch butterflies readily use eucalyptus trees as overwintering sites, though at least one study has suggested that monarchs tend to cluster on native conifers during especially cold snaps even if they have eucs at hand. Slender salamanders do very well in the litter on eucalyptus forest floors, and both native and imported bees seem to like the trees' nectar.

One of the biggest ecological threats posed by blue gums is that the trees can radically change a landscape's natural fire cycles. The trees' shredded bark and oily dead leaves provide abundant fuel for wildfire, along with a way to carry that fire up into the treetops. That in turn increases the likelihood that wind will pick up and carry embers for miles. So does the tendency mature blue gums exhibit to explode when they catch fire. And though many Californian native plants respond to fire by resprouting, few match the blue gums' ability to do so. A fire in a eucalyptus grove merely strengthens the blue gums' hold on the land.

Thankfully, blue gums are not nearly as aggressive in conquering new territory as many other invasive plants. (We'll feature one of the very worst to wrap up this series on Friday.) Though they do reseed reasonably well in areas where there's a lot of moisture, as for example on hills within range of the marine layer, blue gums seem not to be expanding their range at all in drier places farther inland. A June 2014 draft review of the species by the California Invasive Plant Council found that along the coast, blue gums are still invading native riparian forests and coastal chaparral from Los Angeles to Arcata, but that plantings inland aren't spreading at all and may even be dwindling.

And even where they're growing along the coast, the trees seem not to colonize new areas where they haven't been planted deliberately. Since almost no one is planting blue gums deliberately any more, the future of eucalyptus invasions in California may be limited to edgewise spreading in places where they already grow.

That's reason for hope. Though the trees do cause problems along the coast, and though those problems will require massive and expensive responses, it looks as though we can actually change our ways when we find out that something we plant for pleasure and potential profit turns out to have a serious downside.

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Invasive Species Week: The Argentine Ant

About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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