Pest Vulnerability Matrix (PVM): A graphic model for assessing the interaction between tree species diversity and urban forest susceptibility to insects and diseases

The limited tree diversity in urban areas increases the likelihood of mass mortality from outbreaks of insects and disease. Although information is available on pest susceptibility of individual tree species, it is difficult to determine from such data the vulnerability of a multi-species assemblage, such as an urban forest, to insects and diseases, or to assess the effects of either changing the tree species composition or the arrival of new pests. Our model, the Pest Vulnerability Matrix (PVM), enables municipal arborists and urban foresters to evaluate the overall vulnerability of their urban forest, to display this information and communicate it to others, and to evaluate the potential effects of emerging pests and diseases. PVM is a Microsoft Excel spreadsheet that provides a means for rapid graphic display of the interaction between tree species diversity and the susceptibility of the urban forest to pests by displaying each tree–pest interaction as a colored cell in a table. PVM calculates the percentage of trees affected by each insect or disease, and enables the urban forester to quickly identify (1) the most important pests and (2) the most vulnerable tree species. The model is designed to be flexible and easily modified by the user, and includes several newly emerging pests to allow the exploration of future “worst-case” scenarios. Two case studies of Northern California cities are presented demonstrating two potential applications of PVM. We conclude with a brief overview of the diversity–stability debate in the context of urban forests.