Since eucalyptus trees arrived in the United States in the mid-19th century, they spread and grew to encompass huge tracts of land, forcing out competing trees, fuelling wild fires and destroying coastal scrubland habitats for dozens of different native species. A movement is afoot, even among environmentalists, to halt the march of eucalyptus trees and restore biodiversity to some native areas.
Eucalyptus trees are adaptable to very dry conditions in locations such as Australia and California where wildfires are common. The eucalyptus has a peelable bark and highly flammable leaves that fall off the tree and accumulate on the forest floor. This provides ample tinder piles of dried leaves and bark to feed fires. A flammable oil produced by some types of eucalyptus trees, such as the Blue Gum, helps fuel an intense fire. Blue Gums can explode, spreading fire, when ignited.
The Audubon Society has called the towering eucalyptus tree "America's largest weed." Sturdy and fragrant, the eucalyptus is an exotic invasive, a non-native species that was transported to America more than 150 years ago. Loggers cleared and sold off California's native redwoods, replanting with imported eucalyptus trees. The eucalyptus quickly filled in and eventually dominated woods, forests, valleys and coastal scrubland. Its spread continued through decades, thanks to its aggressive nature, adaptability and virtual lack of predators.
Eucalyptus trees are big drinkers, especially when young. They're spread their roots to suck up groundwater and find nearby creeks, often emptying the creeks and pushing dirt into the dry creek bed with their massive, invading root system. This depletes the water supply and exacerbates the fire hazards in areas already prone to wildfires. The threat of diminished water sources caused the Rwandan government to halt plantings and begin uprooting eucalyptus stands in marshlands and near waterways in 2006. The dryer earth in eucalyptus forests is susceptible to severe erosion and uprooting trees, creating a hazard of falling trees as the giant eucalyptus' topple down hills and mountains into roads and population centres.
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