USA — Biologists now count invasive species as a major threat to biological diversity second only to direct habitat loss and fragmentation. Why do they worry when new species enter an ecosystem? More than 90 percent of introduced plants in California have overcome barriers to survival and reproduction in their new home without harming native species. But a fraction display invasive traits, displacing native species and reshaping the ecological landscape.
Tasmanian blue gum eucalyptus (Eucalyptus globulus), a symbol of California for some, never knew California soil until the 1850s, when seeds from Australia were planted, first as ornamentals, then mostly for timber and fuel. The California Invasive Plant Council (CAL-IPC) classifies blue gum eucalyptus as a moderate invasive because the trees need certain conditions to thrive. For the most part, theyre not a problem in the drier regions of Southern California or the Central Valley. But along the coast, where summer fog brings buckets of moisture, its a different story.
Blue gum invades neighboring plant communities if adequate moisture is available for propagation, state resource ecologist David Boyd noted in a report for CAL-IPC. Once established, the trees can alter local soil moisture, light availability, fire patterns, nitrogen mineralization rates and soil chemistry.
Introduced species can disrupt ecological relationships among species that co-evolved over millennia, which is why many groups work to remove eucalyptus and restore coast live oaks. Californias native oak woodlands still sustain more biodiversity than any other terrestrial landscape even though more than a century of intensive agricultural, rangeland and urban development has claimed some 5 million acres of woodlands. (While settlers cleared the land of oaks, entrepreneurs planted eucalyptus trees by the millions.)
Historic fire risk Thats why many ecologists welcome a plan to remove tens of thousands of eucalyptus and other non-native trees from the East Bay Hills to reduce fire risk. UC Berkeley, together with the City of Oakland and the East Bay Regional Park District, applied for up to $5.6 million in grants to remove the non-nativesprimarily eucalyptus, Monterey pine and acaciaunder the Federal Emergency Management Agencys Pre-Disaster Mitigation and Hazard Mitigation Grant programs. The total project would cover just under 1,000 acres and includes plans to encourage regrowth of native oak and bay trees.
Fifteen major fires roared through 9,000 acres of the East Bay Hills between 1923 and 1992, incinerating some 4,000 homes and killing 26 people. The Oakland Tunnel fire, considered the worst in California history, caused an estimated $1.5 billion in damage, destroyed more than 3,000 homes and killed 25 people. Following the Oakland fire, disaster experts urged large landowners in the East Bay Hills to work together to manage vegetation to prevent another catastrophic wildfire, says Tom Klatt, who manages environmental projects for UC Berkeley and serves on the UC Fire Mitigation Committee.
Blue gum eucalyptus is one of the most fire-intensive plants, says Klatt. Trees not only put a lot of fuel on the ground as they shed bark, leaves and twigs, but in intense fires, volatile compounds in foliage cause explosive burning. Once bark catches fire, it gets blown ahead of the flame front and drops burning embers by the tens of thousands per acre in the urban community.
A 1923 fire started at Inspiration Point ran through the eucalyptus trees until it hit the ridgeline at Grizzly Peak, then came down to University and Shattuck before the wind finally changed direction, Klatt says. It took out 568 homes on the north side of the Berkeley campus in two hours.
Despite the fire risk, the plan remains contentious. Some residents worry about the use of pesticides, some feel eucalyptus flammability is overstated and others who consider the trees cultural icons view the plans as an attack on a species thats been here so long we should consider it native. (For the record, the California Native Plant Society defines native as any species that predated European contact.) Predicting how an introduced species will behave is complicated by the fact that ecological effects are difficult to observeand may only appear when its too late to control.
Ecological impacts of eucalyptus Evidence of the trees impacts on East Bay ecosystems is relatively scarce. A 2002 study of the Berkeley hills found similar numbers and diversity of species in eucalyptus and native woodlands, but the species themselves were different. Monarchs use groves in Point Pinole as resting spots and several bird species, including herons and egrets, nest in eucalyptus in and near the tree-removal project areas, though how their use affects their reproductive success isnt clear. (Klatt says that though he hasnt seen nests in the UCB project areas, the law requires that they take steps to protect nesting birds and any species under state and federal protection.)
More evidence comes from the Central Coast. At a 2004 workshop on the blue gums impact on the ecology of coastal ecosystems, researchers reported conflicting effects. Eucalyptus stands can provide habitat for birds near cities and water bodies, and for overwintering monarch butterflies. But the trees change the composition of insect and bird communities as they invade: the loss of native trees that grow along rivers could spell trouble for neotropical migratory songbirds and for species that nest in tree cavities. And when eucalyptus leaves enter streams, aquatic macroinvertebrate communities change, altering the food chain, likely because the chemical content of eucalyptus leaves differs from native foliage.
By the time the eucalyptus trees were planted in the East Bay, typically in 12 foot by 12 foot plots, most native woodlands and perennial native grasslands had already been converted to annual European grasslands, says forest ecologist Joe McBride, professor of environmental science, policy and management at the University of California at Berkeley. And certainly by now a number of species are using those trees but they were here before the eucalyptus was planted, using oak woodlands, riparian woodlands and redwood forests in the East Bay. They just spread to eucalyptus and Monterey pines when the trees grew big enough. These populations arent going to disappear if eucalyptus is removed.
But removal has proven difficult. After two previous removal efforts in the 1970s and again in the 1980s, the trees have grown back, Klatt says. Successful eradication requires at least 10 years of maintenance and drizzling about 2 ounces of diluted herbicide directly to the cut stump immediately after felling a tree, he explains. If you do it within the first three minutes, we see 95 percent to 98 percent success with a single treatment. But if the trees resprout, more applications will be needed.
The plan aims to selectively cut eucalyptus while leaving bay, oaks and other native trees in the understory. The more understory we preserve, the faster it recovers, says Klatt. The plan also calls for retaining all the cut wood as chips for erosion control and moisture retention, and to encourage native regrowth, aided by birds and squirrels that plant acorns in chip beds.
McBride hasnt seen evidence of eucalyptus invasive tendencies in the East Bay Hills but worries about its combustible nature. We imported this plant from Australia but we didnt import the normal fungus that decays the litter in Australia, he says. Accumulations of bark and leaf litter under eucalyptus stands have measured up to 100 tons per acre, compared to about 3 tons per acre for coast live oaks. Its an enormous increase.
Selected for flammability? So how does the blue gum act in its native environment? For David Bowman, a forest ecologist at the University of Tasmania in Australia, the question isnt whether the trees are native or non-nativeits whether theyre dangerous. Looking at the eucalyptus forest outside my window in Tasmania, I see a gigantic fire hazard.
At very high temperatures, eucalypt species release a flammable gas that mixes with air to send fireballs exploding out in front of the fire. With eucalyptus, you see these ember attacks, with huge bursts of sparks shooting out of the forests, Bowman says. Its just an extraordinary idea for a plant.
Though its difficult to prove, Bowman suspects the trees evolved to be uber flammable. Sixty million years ago eucalyptus species hit on a way to recover from intense fire, he explains, using specialized structures hidden deep within their bark that allow rapid recovery through new branches, instead of re-sprouting from the roots like other trees. They have this adaptive advantage of not having to rebuild their trunk. Whether their oil-rich foliage is also an adaptation, we dont know.
If you arent familiar with the idea of a plant designed to burn in its life cycle, you can get fooled by its beauty and nice smell, Bowman says. But on a really hot day, those things are going to burn like torches and shower our suburbs with sparks. And on an extremely hot day, theyre going to shoot out gas balls.
With tiny pinhead seeds that germinate only in disturbed soils, the trees really arent good invaders, Bowman sayswith one exception. Fire opens up the woody capsules that hold the seeds, which love growing on freshly burned soil. Give a hillside a really good torching and the eucalyptus will absolutely dominate. Theyll grow intensively in the first few years of life and outcompete everything.
The evolutionary dimensions of fire ecology are controversial, Bowman allows. But if eucalyptus are these evolutionary freak plants that massively increase fire risk, he says, it raises a troubling question: Are these intense fires a consequence of climate change or the interaction of climate and biology? If its the latter, then what the hell have humans done? Weve spread a dangerous plant all over the world.