An evaluation of pest and disease vulnerability in the urban canopy of Washington DC
Urban forests are particularly at risk for biological stressors, such as insect pests and exotic diseases, because of diminished health caused by the abiotic stressors including compaction, nutrient deficiency, salinity, and pollution. Individual cities’ urban canopy can be evaluated using tools such as the Pest Vulnerability Matrix (PVM), which is based on individual species risks to infections and their relative overall abundance (McPherson & Kotow, 2013). Another approach is to evaluate biodiversity and species evenness using the Shannon Diversity Index. Urban forests can be insulated from pest and disease impact with a species composition of no more than 10% of the urban forest represented by the same species, 20% of the same genera, and 30% of the same family (Santamour, 2002). Our project incorporated spatial analysis to these urban forestry tools within the District of Columbia (DC). We explored PVM and diversity data across specific management units to create a list of species to avoid in future plantings and suggestions of species to plant in order to achieve a more even species composition. Relative Pest Vulnerability Matrices will prioritize specific areas for monitoring to avoid pest and disease outbreaks.
Preliminary results show that the overall Pest Vulnerability Matrix for D.C. was 11.88 and varied only slightly across specific Wards from 10.89 -12.90. It is interesting to note that there was not a linear relationship between pest vulnerability and biodiversity; some more diverse wards received high PVM scores based on the disproportionate percentage of “high risk” species. It is not only necessary to insulate with diversity, as is accomplished by Santamour’s 10-20-30 rule, but to also utilize low risk families, genera, and species. A narrative is being developed to inform the DC public regarding the health of their urban canopy. The data will be analyzed at the level of the Advisry Neighborhood Commission so that the bodies governing these smaller political districts can be educated as to what trees species should be planted (or avoided) in their specific area.