Quantifying the contributions of native and non-native trees to a city’s biodiversity and ecosystem services

Abstract

Urban trees are appreciated for their intrinsic value and their contributions to human well-being. Here, we analysed a database of 115′686 non-forest trees (1’025 species) to quantify the present contributions of native and non-native trees to biodiversity (taxonomic richness) in the metropolitan area of Geneva, Switzerland. Non-native trees made up 90 % of species and 40 % of individuals. A subset of these individuals with more detailed phenotypic information (N = 50’718 trees; 527 species) was used to quantify five regulating ecosystem services (micro-particle capture, carbon sequestration, water interception, microclimatic cooling, and support for pollinators), three cultural ecosystem services (natural heritage, recreational, and aesthetic value) and two disservices (allergies and biological invasiveness). Non-native and native trees generated roughly identical regulating services, on a per-tree basis, as these are linked primarily to tree morphology rather than to tree-origin. Non-native trees generated cultural ecosystem services that were greater than native trees, on a per-tree basis, with the exception of the notion of “natural heritage”. For example, 79 % (163/207) of trees independently identified as “remarkable” by the canton of Geneva were non-native. Our results illustrate that non-native trees represent a significant source of biodiversity and ecosystem services both in absolute terms and on a per-tree basis. Given the empirical importance of non-native trees in many cities, and the likelihood that their importance will increase with future climate change, we suggest that non-native trees be considered in conservation assessments and strategic planning both for intrinsic reasons and for their contributions to human well-being.