Diverse forests shut down non-native tree invasions

Human activities have left a lasting mark on the distribution of plant species for centuries. The transplantation of plants into regions beyond their indigenous territories (whether intentionally or unintentionally) has been a defining characteristic of this human influence. 

On a global scale, approximately 10 percent of non-native species become full-blown invaders that ultimately wreak havoc on ecological systems. Led by ETH Zurich, an international team of researchers has investigated the susceptibilities of various regions to non-native tree invasions. 

The results, published in the journal Nature, provide brand new insights into the human and ecological factors that amplify the issue of tree invasions worldwide.

Focus of the study

Human activity is one of the biggest contributors to tree invasions, particularly in regions surrounding maritime ports. These ports, being the hubs for the movement of massive volumes of commodities, inadvertently introduce plants and seeds from diverse geographical origins. 

However, it’s not just proximity to human activity that determines the gravity of tree invasions. The study sheds light on the shield provided by native biodiversity. 

Forests with abundant indigenous species create an intricate web of ecological niches. This makes it very difficult for alien tree species to establish themselves.

Some invaders have the right traits

The inherent characteristics of the invasive tree species also play an important role in gaining a foothold. Regions known for their harsh climate extremes tend to favor invaders bearing functional similarities to indigenous species. 

By contrast, regions with more temperate conditions offer a fertile ground for non-native species –  but only if they exhibit functional differences from native trees. 

Tree diversity can halt invasions 

This divergence helps them to avoid intense competition for resources, including sunlight, space, nutrients, or water. Ultimately, the richness of native forest diversity emerges as a formidable line of defense against tree invasions. 

“We found that native biodiversity can limit the severity or intensity of non-native tree species invasions worldwide,” said study lead author Camille Delavaux. “This means that the extent of invasion can be mitigated by promoting greater native tree diversity.”

 Global conservation efforts

“By identifying regions that are most vulnerable to invasion, this analysis is useful for designing effective strategies to protect global biodiversity,” noted Professor Thomas Crowther.

The global biodiversity blueprint, adopted during the COP 15 summit in Montreal in 2022, pinpoints the reduction of invasive species as a chief objective. 

Professor Crowther emphasizes the potential of such research to shape policymaking: “This global understanding of non-native tree distributions can help countries to prioritize decision-making in efforts to halt and reverse the loss of biodiversity.”

More about non-native tree invasions 

Tree invasions occur when non-native species have been introduced to new environments and have become established. In their new habitats, these trees can spread rapidly, out-competing native species, altering habitats, and affecting ecosystem services.

Characteristics of invasive trees

• Rapid growth and reproduction.
• Ability to thrive in a variety of soil types and conditions.
• Resistance to pests and diseases in the new environment.
• Effective seed dispersal mechanisms.

Examples of invasive trees

• Australian Pine (Casuarina equisetifolia): Native to Australia and Southeast Asia but has invaded coastal areas in Florida and the Caribbean.
• Tree of Heaven (Ailanthus altissima): Native to China but has spread widely in North America and Europe.
• Eucalyptus: While they are native to Australia, they have spread rapidly in places like California, and can pose fire risks.

Environmental impact

Displacement of native species

Invasive trees can outcompete native trees for resources, leading to a decline in native biodiversity.

Change in soil chemistry

Some invasive trees release chemicals into the soil that hinder the growth of other plants, a phenomenon called allelopathy.

Increased fire risk

Some invasive trees, like the eucalyptus, are more flammable and can increase the risk and intensity of wildfires.

Public awareness

Educating the public about the risks associated with invasive species can prevent unintentional introductions. Gardeners, for instance, might opt for native species or non-invasive alternatives if they’re aware of the risks.

While trees are generally seen as beneficial, when they’re introduced to regions where they didn’t evolve, they can become problematic. Efforts are ongoing worldwide to manage and mitigate the effects of invasive tree species.

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