Unexpected connections between residential urban forest diversity and vulnerability to two invasive beetles

Invasive pests pose a threat to the key environmental and social benefits provided by urban forests, and diverse tree planting is a primary management strategy for reducing pest vulnerability. For example, past urban forest losses to Dutch elm disease (DED) prompted municipal foresters to emphasize diversification, but it is unclear whether residential properties developed after the peak DED outbreak are actually more diverse than older properties. To address this issue, we inventoried all public and private trees on 150 residential properties in the Twin Cities Metropolitan Area, Minnesota, USA, and compared genus diversity on pre- and post-Dutch elm properties. We then quantified vulnerability to two current invasive pest threats, emerald ash borer (EAB) (Agrilus planipennis) and Asian longhorned beetle (ALB) (Anoplophora glabripennis), to evaluate whether higher diversity corresponds with lower pest vulnerability. We assessed vulnerability based on two fundamental urban forest metrics–frequency and size of vulnerable trees. Surprisingly, properties developed after the peak DED outbreak were less diverse than older properties. At the same time, less diverse post-Dutch elm properties exhibited low ALB vulnerability and modest EAB vulnerability, while more diverse older sites were highly susceptible to ALB. The importance of pest host specificity in characterizing urban forest vulnerability was underscored by low EAB vulnerability and high ALB vulnerability on our oldest study sites. This research highlights an apparent disconnect between the theoretical notion that higher diversity should reduce invasive pest vulnerability, and our empirical data indicating that genus diversity does not necessarily correspond with pest vulnerability.