The Ailanthus Tree: Once Adored, Now Dreaded
The tree that appears above these lines surely sounds familiar to you because you have come across it many times, either in your city or while walking on vacation through any municipality on the Mediterranean coast. A clue in case you don’t recognize it at first: its seeds have a kind of wings, which helps them spread further and better.
The Invasion of Ailanthus
It is no coincidence: the ailanthus (Ailanthus altissima) arrived from Southeast Asia two centuries ago, prized for its stature and rapid growth in diverse soil types. However, today it is prohibited by law due to its classification as an invasive species that disrupts local flora and strains sewage systems.
Characteristics of the Ailanthus
Ailanthus can reach heights of up to 30 meters. It has gray bark and uniquely structured leaves, consisting of many elongated leaflets arranged in a row. Its seed pods resemble winged fruits, hanging in clusters. One unmistakable identifying feature is the foul smell emitted when a leaf is torn and rubbed.
Ecological Impact
Spread Across Spain
Since its introduction, ailanthus has spread throughout much of the Iberian Peninsula, particularly in Catalonia and the Valencian Community. This tree colonizes ditches, slopes, degraded banks, and abandoned gardens; wherever there is a free patch of land, the ailanthus takes root. A single mature tree can generate up to 350,000 winged seeds annually, aided by wind for dispersion.
Threat to Biodiversity
According to official sources from MITECO, ailanthus alters forest ecosystems and modifies soil chemistry, making it difficult for other species to thrive. This species competes aggressively for space and light, particularly near rich vegetation areas, releasing toxic substances that inhibit the growth of surrounding plants. Studies show that ailanthus presence can lead to a 35-40% reduction in vegetation compared to unaffected areas.
Infrastructure Challenges
The impact of ailanthus extends beyond ecology; it poses significant risks to urban infrastructure. The Ministry for the Ecological Transition indicates that its extensive and reticulated roots can extend up to 15 meters from the tree’s base. These roots exploit cracks and crevices in buildings and infrastructure, leading to the destruction of pipes, sewers, foundations, and other structures.
Legislative Response
The ailanthus, or “tree of heaven,” originates from China. Though cultivated in Europe as an ornamental tree since the 18th century, it has become naturalized in Spain since the early 19th century. Regulations to combat its spread are now stringent: possession, transport, and trade of ailanthus are prohibited under the Spanish Catalog of Invasive Exotic Species. Fines for violations range from 100 to 3,000 euros for minor infractions, with serious cases reaching up to 2 million euros.
Managing Existing Ailanthus
There are provisions in place for existing trees planted before the regulations’ implementation. While private gardens, urban parks, and botanical gardens are exempt, public administrations are responsible for gradually removing ailanthus from public spaces.
Control and Eradication Efforts
Efforts to eradicate ailanthus are complex and costly. MITECO advises against simply cutting down trees, as this can stimulate new growth. A combination of mechanical removal of young seedlings and targeted chemical treatments is deemed most effective. Meanwhile, ongoing research into biological control methods is promising, including the use of the mite Aculus taihangensis, which has shown effectiveness as a control agent.
As communities work to address the ailanthus issue, understanding its ecological impact and rigorous management will be essential for preserving Spain’s biodiversity.