Protecting the Urban Forest: Aliens, Biological Control, and Pests By Ken Fry, PhD T he urban forest is a highly disturbed, albeit managed, ecosystem. Quite often the trees in an urban forest are isolated, planted in sub-optimal conditions, or subject to minimal maintenance. These conditions are considered to be highly disturbed in ecological terms. Compounding the stresses arising from these conditions is the risk of introduction of alien invasive species as a result of trade occurring on a global scale. This article discusses alien invasive species, their routes of entry, actions taken to detect and mitigate their threat, and review methods to mitigate pest threats to trees in the landscape. 14 March 2014 CG_Mar2014.indd 14 2/26/14 2:00 PM