The ailanthus or tree of heaven (Ailanthus altissima) is a large tree, widely used as an ornamental plant in parks and gardens, not only because of its spectacular appearance, with hundreds of thousands of fruits that at the end of summer are dyed a beautiful orange color, but also because of its usefulness: it is very leafy, provides large areas of shade, and its roots, strong and dense, hold the substrate very well, preventing erosion or fixing slopes.
But all that glitters is not gold, this tree from China has a very dark side. It is an invasive species, present in the Catalog of Invasive Alien Species of the Ministry for the Ecological Transition and the Demographic Challenge and, since 2019, in the list of Species of Concern for the European Union .
What makes ailanthus invasive
Several traits make the Tree of Heaven an excellent invader. On the one hand, the fruits. Each adult specimen can produce between 350,000 and 500,000 fruits per year , most of them viable. The type of fruit is known as samara, the same type that ash or elm trees have. It is a dry fruit, with the seed arranged in the center, and flattened expansions that take on the shape of a wing, and that facilitate its dispersion by the wind .
Ailanthus takes root very well on the banks of rivers and streams and on slopes, along roads and paths. Places where there are frequent longitudinal winds —favored, in some cases, by the water current and the channeling of the winds through the basin, and in others, by the movement of vehicles— that facilitate the transport of these fruits over long distances, hundreds of meters or even kilometers from the parent tree.
In addition, the ailanthus produce a series of substances called allelopathic through their roots and dead leaves, which inhibit the growth of other plants, which makes it much easier for them to compete for space.
Ailanthus have a great ability to sprout from the stump or from the roots. When a trunk is cut, stump removed, and even chemically treated, it still requires constant maintenance for four or five years until its roots stop giving off shoots that emerge around the dry stump.
The consequences of their invasion
Invasive animals cause clearly visible, immediate and highly visible impacts. However, the impact caused by invasive plants is often barely noticeable. Many plants are slow in their actions and behaviors, we do not perceive their movements, and therefore, neither their impacts in real time, they only become evident when they have already happened.
The impacts caused by the tree of heaven, like those of any other invasive species, are classified into three categories: damage to native species , disturbance to ecosystems , and socioeconomic damage.
The aforementioned allelopathic substances of the tree of heaven generate an evident impact on the native flora , displacing it and even eliminating it. In addition, being so leafy, it enters successfully in the competition for the light resource, so even the plants that can withstand intoxication are condemned to live in its shade.
These behaviors are the cause of damage to ecosystems . Ailanthus causes the plant composition of the ecosystem in which it is installed to change, reduces the cover of shrubs and herbaceous plants, and biodiversity, favoring communities to become monospecific, that is, in which only one species remains. It monopolizes resources, thereby altering the biogeochemical cycles of the environment and food webs, and breaks the processes of ecological succession.
Of course, it is also a tree with significant socioeconomic impacts . Its roots, capable of entering cracks and breaking rocks, also raise sidewalks, crack roads or knock down walls. To the damage to infrastructure must be added the impact that its leaves, blown by the wind, cause in agriculture, by depositing allelopathic substances in agricultural soils and inhibiting crop growth. The toxicity of its leaves can affect native fauna and livestock farms, and its pollen can cause allergies in humans.
Due to the attractiveness of its flowers, bees frequently approach this tree; but, if the bees belong to honey farms, the production of honey can be spoiled , since the ailanthus nectar gives it a very unpleasant taste.
Is ailanthus still in our parks?
This is one of the most worrying questions surrounding the tree of heaven. Despite the fact that it is a species regulated by both Spanish and European regulations. Currently, however, ailanthus is still available for sale in multiple nurseries —something that, by the way, is illegal—, and is still present in many parks and public gardens throughout Spain , and they are frequently sources of expansion of the species to semi-natural and natural lands.
The same Royal Decree 630/2013, which regulates the Spanish Catalog of Invasive Alien Species, indicates, in its fifth transitory provision, that “in the case of those specimens of catalog species located in public parks or gardens (… .) the competent administrations will progressively eliminate , in cases where it is justified, these species».
The tree of heaven has been in this catalog since it came into force in 2013. It seems a more than reasonable time to have already removed it and replaced it with native trees.
Ballero, M. et al. 2003. Allergy to Ailanthus altissima (tree of heaven) pollen. Allergy, 58(6), 532-533. DOI: https://doi.org/10.1034/j.1398-9995.2003.00172.x
Bayón, A. et al. 2011. Ailanthus altissima (Mill.) Swingle (Simarubaceae) as potentially invasive. Ambiociencias, 0(7), 27-39. DOI: 10.18002/ambioc.v0i7.4915
Bayón, Á. et al. 2019. Horizon scanning to identify invasion risk of ornamental plants marketed in Spain. NeoBiota, 52, 47-86. DOI: 10.3897/neobiota.52.38113
BOE. 2013. Royal Decree 630/2013, of August 2, which regulates the Spanish Catalog of invasive alien species. BOE, 185(Sec. I.), 56764-56786.
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