Tree of heaven: a successful species in a deforested world
The tree of heaven is invading all continents except Antarctica. The experts say they cause much damage to native ecosystems, and even call them trees of hell. But maybe it's just nature's way to laugh at our deforestation rage? Let's reflect about invasive species and how we have actually rolled out the red carpet for them.
Text and photos: Kathelijne Bonne.
In May, I thought it was a shame that a lovely almond tree was dying of drought on a street corner full of wild flowers, in my village near Madrid, Spain. By the start of summer all the vegetation was cut down, including the tree, as part of wildfire protection measures. When I returned in September, a dense bunch of trees of heaven (Ailanthus altissima) had grown in the same spot, the largest already 2.5 meters tall and despite the extreme heat and drought of last summer, they're heavily laden with healthy, beautifully shaped compound leaves. I was also in southern Italy in summer, noticing the invasion of the same species, small and large, everywhere on poorly maintained roadsides, street edges, and between pavers.
Better an invasive tree than bare tarmac, right?
Let's ask ourselves a few questions. Does the tree of heaven grow so well because it encounters little resistance in our impoverished ecosystems? Or looking at it another way: Shouldn't we actually be glad that there is at least one tree, and a beautiful one at that (but a rather smelly one), that doesn't fall victim to our logging mania and hunger for forest products?
I am not a forestry specialist, so it is not my intention to post a technical report on the state of this tree species or how to deal with it. I rather engage in the tree of heaven polemics to bring it up as a metaphor, and to provide food for thought.
On the ruins of capitalism
Because I have read a remarkable book that may have an impact on how we see nature: "The Mushroom at the End of the World," by American-Chinese Anna Lowenhaupt Tsing. She describes the ecology and sociology revolving around the matsutake mushroom, a delicacy in Japan. A whole tangle of human wanderings and marginality surrounds the search for - and consumption of - this fungus. The matsutake is found throughout the northern hemisphere, but it only grows where it wants to grow; you can't grow it in a farm or nursery. You have to go browsing in a certain kind of forest.
It prefers 'new' post-apocalyptic forests, where once primeval forests were completely cut down, burned or where atomic bombs have exploded, and where now unwanted exotics and less valuable (for logging) species reign.
Fond of polluted places
The metaphor Anna Lowenhaupt Tsing uses, "the matsutake shows that life is possible on the ruins of capitalism," sounds a bit grim, but it is not. The bottom line is that where humanity has depleted nature, places have emerged that are worthless by capitalist standards, but that can sustain new life, albeit life that we may not like. Let's now return to the tree of heaven.
Just as the matsutake, it loves disturbed and polluted places and is eager to colonize them. In this respect, it's a true pioneer. It has excellent resistance to drought and is one of the most pollution-resistant plants in the world (it fixes mercury from the soil in its biomass and doesn't even blink; it tolerates ultraviolet radiation reasonably well). The tree of heaven comes from China and was imported to Europe and the New World as an ornamental tree in the eighteenth century, when chinoiserie was in fashion. Meanwhile, this tree has become a plague throughout the world. It is listed as an "unwanted organism" in several countries.
Just one weakness, like Achilles
The tree of heaven reproduces at the speed of light, via seeds and shoots. It emits allelochemicals in the soil that interfere with other plants' development, gaining a competitive advantage. But like most pioneer species and daredevils, or heroes like Achilles, the tree of heaven is adventurous, lives fast, ... and dies young. Often it does not reach fifty years, despite its imposing appearance.
How does nature withstand such invasive force? According to Peter Wohlleben, forester and author of the astonishing The Hidden Life of Trees, natural interactions ensure that a healthy native ecosystem will keep invasive species in check. The local ecosystem will always recover from the blitz of a seemingly unconquerable exotic species, though recovery may take decades or centuries. So, in the wild, the tree of heaven should naturally tone down. But as you will know, we are far from a healthy natural integrated ecosystem. Invasive species are given free rein.
Still, every species has a weakness.
If there is one thing the tree of heaven does not like, it is shade and a lack of light. Hence, it germinates or sprouts mostly in sunlit clearings where other trees have fallen, or where logging is ongoing. Since bare spots are expanding and connecting, the tree of heaven simply follows the command of its genes: Go and Multiply! And so it does.
Unless we detect more weaknesses. And here we stare in the face of a fungus again. Some specialists have discovered a soil fungus that is detrimental to the health of the tree of heaven. It is lethal. But how wise would it be to release such lethal fungus in places that are not its natural habitat? Didn't we just learn a lesson?
Forest specialists certainly know better than I do how to address invasive species. It is desirable that we keep biodiversity as high as possible and that we do not end up in a tree of heaven monoculture. But forest experts must then be heard by policy makers, their expertise taken seriously and their nature restoration plans implemented.
Forest clearing is not only an issue in the Amazon Rainforest and in developing countries in the Tropics, it also happens in so-called 'civilized' parts of the world like Europe. As you read this, virgin forests are being cut down, moreover in Eastern Europe, not to mention Ukraine, to satisfy our hunger for furniture, pellets, biomass and other forest products - delivered with conscience-soothing responsible-forestry labels. Wherever the forest clearing takes place, it creates opportunities for advancing armies of unwanted organisms, which will colonize the new battlefields (but cannot make up quickly for the loss of ecosystem services carried out by the old forest).
One personal note: Despite the fact that we're in the eye of a tree of heaven storm by our own actions, I am glad that tree exists, a hopeful sign that nature is stronger than us. If we give nature a chance to recover, we can leave it to other tree species to teach the tree of heaven a lesson, and put it back in its place, as just one good tree amongst many.
And if we can't do that, I personally would rather live in a world full of trees of heaven than no trees at all.
More articles on Environment.
- Anne Lowenhaupt Tsing, 2015, The Mushroom at the end of the world.
- Peter Wohlleben, 2016, The Hidden Life of Trees.
- Wikipedia: Ailanthus altissima. https://en.wikipedia.org/wiki/Ailanthus_altissima
Some keywords: tree of heaven, tree of heaven invasive species, tree of heaven china, tree of heaven colonizer, tree of heaven deforestation, the mushroom at the end of the world, hidden life of trees, anna lowenhaupt tsing, nature philosophy
Article written by Kathelijne Bonne, geologist and soil scientist. I also write on Good Climate News.
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