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Non-native trees have enormous value in urban areas

A team of environmental scientists from the University of Geneva (UNIGE) set out to investigate the effects of non-native trees planted in urban spaces. The study has revealed that the benefits of introducing trees can far outweigh the costs, regardless of their origin. 

In Geneva, 90 percent of tree species are non-native. The study shows that native and non-native trees make similar contributions to regional biodiversity and ecosystem services. 

Trees can drastically improve the quality of urban environments by reducing noise pollution, improving air quality, providing new habitats for various species, and mitigating the impact of heat islands. However, non-native trees can also create issues, such as threats to native biodiversity.

Study first author Dr. Martin Schlaepfer is a researcher in UNIGE’s Institute of Environmental Sciences.

“There’s an ongoing philosophical debate concerning this problem at the moment,” said Dr. Schlaepfer. “Should we promote native trees and ban – or at least put limits on – introduced species? Of all the species introduced into urban areas, only 5% are potentially problematic, such as the Tree of Heaven (Ailanthus altissima) located in the old town of Geneva. But what should we do with the remaining 95% of non-native tree species, and how should we value them?”

In collaboration with the City of Geneva, the UNIGE team analyzed data on all tree species found in urban regions across the entire state of Geneva.

After four years of detailed work, the researchers had recorded 911 different species and found that 90 percent of the tree species in Geneva are non-native. For each species, the environmental benefits and drawbacks were examined. The introduced tree species were found to be mainly beneficial, with just a few exceptions.

“Three invasive species were identified, and they are indeed potentially problematic when located in semi-natural sites outside the city. But in urban settings there is a low risk of propagation, and we document how they contribute to our well-being,” explained Dr. Schlaepfer. “Some non-native trees have been growing in the parks for several centuries, such as cedars and plane trees imported from North Africa and Asia in the 16th and 17th centuries for their aesthetic value, resistance to disease and perennial foliage. They are now part of our cultural heritage. 

“In addition, they have the ability to survive in an urban environment, meaning they can help attenuate urban heat-islands and reduce air-borne pollution. Caution must be employed when planting such non-native species, but they can have a clear beneficial role in certain contexts.”

The study is the first of its kind to compare the ecosystem services of both native and non-native species. The findings demonstrate that non-native tree species provide enormous value.

“Introduced trees are generally listed in the databases of other countries as potential threats, but when it comes to measuring the state of a nation’s biodiversity, they are generally dismissed or omitted,” said Dr. Schlaepfer.

“The climate is undergoing profound change, with predictions for our latitudes indicating that within 50 to 100 years – i.e., the life-span of a large tree – the climate in Geneva will be like that in southern Italy. That means we must be open to the idea of introducing species today that are able to persist both now and under future climatic conditions.”

The study is published in the journal Urban Forestry and Urban Greening.

By Chrissy Sexton, Staff Writer

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