Higher tree species richness and diversity in urban areas than in forests: Implications for host availability for invasive tree pests and pathogens

Urban and forest trees provide valuable ecosystem services. However, they are increasingly threatened by invasive forest pests and pathogens. Trees in urban areas are often the first potential hosts non-native tree-feeding insects and tree pathogens (“pests”) encounter after introduction in a novel region. If the trees encountered are suitable hosts, these pests can establish and become invasive – eventually also in surrounding forests. Here, we compared tree species and genus composition between urban areas and surrounding forests and examined the implications for host availability for forest pests and potential effects on invasibility. We compiled and standardised 26 urban tree inventories, containing ∼ 500.000 individual trees. We used multivariate analyses to compare urban tree composition with forest tree composition from forests surrounding each municipality (10 km radius), derived from the Swiss National Forest Inventory. With > 1300 different tree species, species richness of urban trees was 17 times higher than species richness in surrounding forests. Linear models and multivariate analyses revealed that host availability for forest quarantine pests is significantly higher in urban areas than in forests, with large differences in host suitability for different quarantine pests between urban and forest tree assemblages. This indicates that differences in species composition in urban and forest trees can result in increased host availability, possibly facilitating the establishment of quarantine forest pests.