Why Diversity Is So Important

A generation ago, many communities were devastated when chestnut blight and Dutch elm disease destroyed millions of stately street trees. Today, the emerald ash borer has killed more than 40 million ash trees in southeastern Michigan alone, with tens of millions more lost in Ohio, Illinois, Indiana, Pennsylvania, Maryland, West Virginia, Missouri, Wisconsin and Virginia. This growing infestation threatens to decimate ash trees throughout North America.

Healthy and resilient urban forests are managed for diversity and a high percentage of native trees. Native species are well adapted to the local climate and soils and require minimal water, fertilizer or mulching. They also help restore a fragmented ecosystem by offering habitat for migratory birds and other wildlife.

Urban forests are often more diverse than surrounding native landscapes, because they contain a mix of tree species that existed prior to the development of the city and the introduction of exotic species. Although increased tree diversity can minimize the overall impact of destruction by a species-specific pest, some exotic species can also out-compete and displace native trees. Invasive, non-native trees such as Norway maple reproduce quickly and can take over open space areas, creating a monoculture canopy that is once again vulnerable to a species-specific disease.