Heaven Can Wait: Why the Tree of Heaven Threatens Native Australian Ecosystems

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It's no secret that Australia's native wildlife is under threat from hundreds of invasive species. The cane toads and rabbits are poster children for this infestation, but it's less well known that a large number of plant species have also invaded the country, putting fragile native ecosystems at grave risk. Some of these species are trees, trees that breed and spread quickly, sapping resources away from native plants and encroaching on their territory.One invasive species stands out as a particularly aggressive, almost malevolent invader—the tree of heaven.

Why Is the Tree of Heaven so Dangerous?

Native to mainland China and Taiwan, the  tree of heaven is hardly divine. Indeed, the tree is often referred to as 'tree of hell', and for good reason, as the damage these trees can do is extraordinary in scope. There are six main reasons why this unassuming, attractive deciduous tree is such a serious threat to native wildlife:

• Rapid breeding - Female trees can produce over 30,000 seeds per kilogram of tree. On a fully grown tree, this means seeds number well into the millions. Like sycamore seeds, they are encased in papery, winged sheaths that catch the wind as they fall, meaning seeds can dispersed over enormous distances.
• Rapid growth - The tree of heaven cannot thrive in low-light conditions, and therefore grows quickly to avoid being shaded by older, larger trees. In its first few years of life it can grow as much as two metres per year.
• Prolific suckering - Suckers are shoots that grow out of the root system of a tree, and can potentially grow into new trees themselves. These trees will be genetically identical to the host tree, and will often stay connected by the same roots. Trees of heaven are especially prone to spreading in this manner, and dense thickets of cloned trees can arise in just a few years, choking the life out of other plants in the area.
• Hardiness - Trees of heaven can grow in extremely acidic or salty soils, and withstand large amounts of air and water pollution. Its extensive root system makes it resistant to drought and low levels of groundwater - the roots are also strong and aggressive enough to penetrate underground obstacles, and can sometimes break into sewer pipes and subterranean water tanks.
• Pungent perfume - The tree of heaven has a host of unflattering nicknames, including 'stink tree', 'ghetto palm' and others unsuitable for a piece that may be read by children. Even in its native China, where the tree has a long history of cultural importance, it is known as the 'foul-smelling tree'. This distinctive and powerful odour, likened by some to rotting peanuts, is designed to attract pollinating insects. It does this so effectively that less malodorous plants in the area are ignored by insects.
• Chemical warfare - Perhaps the tree's most sinister quality. The tree produces a unique chemical, alianthone, and laces the surrounding soil and groundwater with it, as well as its own bark and roots. This chemical inhibits the growth of other plants and destroys seeds and sprouts, eradicating the competition. Native plants have no defence against this.

How Do I Get Rid of Them?

Removing tress of heaven is no minor undertaking - interlinked thickets of trees with widespread roots mean that uprooting them by the stumps has little effect, particularly with their ability to sprout anew from fragments of root that aren't removed. More potent methods of control are therefore required:

• Herbicides - Many herbicides are ineffective against this hardy tree, but triclopyr, designed to kill invasive broadleaved trees, has been found to be reasonably effective. Triclopyr will not damage grasses or conifers - however, other deciduous trees, particularly eucalyptus, are vulnerable, so take stock of the surrounding wildlife before you proceed. It is also mildly toxic to fish and ducks, rendering it unsuitable for use on riverside trees. Triclopyr should be applied to the base of the tree, or directly into the stump of a cut tree, and should be mixed with oil to increase penetration and water displacement. Repeat applications will usually be necessary.
• Preventative pulling - If you're lucky enough to identify these trees while they're still young, yanking them out now is by far the most effective way to deal with them. Trees of heaven cannot produce replacement suckers until they have developed a robust taproot, which takes about a year to form—catch them before this and the root will die once severed from the trunk.
• Death by degrees - If you are willing to be patient and want to minimise risks to surrounding wildlife, a singular tree or a small stand can be tackled by cutting them down—a lot. This process may take a few years, but eventually the tree's ability to resprout becomes exhausted and the wood begins to break down and decay. 
• Burning - Some tree removalists have reported success by drilling deep into a stump and torching it from within with kerosene or diesel - however, the potential environmental damage, not to mention shady legality of such an act, means it cannot be recommended.
• Biological warfare - Some fungal pathogens, such as verticillium dahliae, are effective heaven-killers, but the risks of fungal infection spreading mean that this method should only be conducted by trained professionals, under close supervision.

20 April 2015