A study realised by environmental scientists from the University of Geneva (UNIGE) delves into the significance of non-native trees in urban environments, shedding light on their contributions to biodiversity, ecosystem services, and human well-being.
“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?”
Martin A. Schlaepfer, co- author of the study.
Urban environments worldwide face growing challenges in maintaining biodiversity and ecosystem services amidst rapid development and climate change. In recent years, a paradigm shift has emerged, challenging the traditional perception of non-native trees as ecological disruptors.
Trees stand as silent guardians in our urban landscapes, providing an array of invaluable ecosystem services. From mitigating air pollution to offering sanctuary for wildlife, their contributions to urban landscapes and human well-being are undeniable. However, alongside their benefits, trees also pose challenges, particularly when they fall under the ‘invasive’ or ‘alien’ labels, which immediately prompt questions about their impact on biodiversity and ecosystem services.
Environmental scientists from the University of Geneva (UNIGE), in collaboration with the Botanical Gardens and Conservatory of the City of Geneva, embarked on an analysis of the region’s tree diversity. Their findings reveal a surprising revelation: the majority of tree species in Geneva are non-native, with only a small fraction posing potential threats, such as the Tree of Heaven.
The research highlights the adaptability of non-native species to urban settings, emphasizing their role in regulating ecosystem services (micro-particle capture, carbon sequestration, water interception, microclimatic cooling, and support for pollinators) as well as their cultural ecosystem services (natural heritage, recreational, and aesthetic value). Non-native and native trees were found to generate roughly identical regulating services, on a per-tree basis, as these are linked primarily to tree morphology rather than to tree-origin.
“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,”
Martin A. Schlaepfer, co- author of the study.
The study’s results illustrate how non-native trees represent a significant source of biodiversity and how they provide ecosystem services. Given the empirical importance of non-native trees in many cities, and the likelihood that their importance will increase with future climate change, the research suggests that non-native trees should be considered in conservation assessments and strategic planning both for intrinsic reasons and for their contributions to human well-being.
As cities grapple with the challenges posed by climate change and urbanization, understanding the role of non-native trees is becoming increasingly pertinent in shaping sustainable urban environments for future generations. Rather than systematically addressing invasive plants through a negative prism, studies such as the one conducted in Geneva can provide valuable insights into the importance of embracing non-native species in our urban landscapes.
Martin A. Schlaepfer, Benjamin P. Guinaudeau, Pascal Martin, Nicolas Wyler, “Quantifying the contributions of native and non-native trees to a city’s biodiversity and ecosystem services”, Urban Forestry & Urban Greening, Volume 56, 2020, 126861, ISSN 1618-8667, https://doi.org/10.1016/j.ufug.2020.126861.