Lack of Species and Genetic Diversity in the Urban Forest

In today’s urban forest species richness and diversity is very lacking. It is not uncommon to see one species in urban forests consume over 40% of the available planting spaces within a city. The most common overused species within the Midwest are green ash, American elm, autumn blaze maple, honey locust, and northern white cedar. With the introduction of Dutch Elm’s Disease we witnessed the devastation that can result from a monoculture in the urban forest. Instead of changing our ways and increasing species diversity and richness we repeated the mistake by replacing our lost elm trees with green ash and honey locust. Now again with the threat of Emerald Ash Borer we are making the same mistakes over and many of the ash trees are being replaced with autumn blaze maples.

Monocultures within the urban forest make cities susceptible to wide spread economic and environmental losses. Each tree planted within the urban forest does not reach its full environmental and economic potential until it reaches maturity. Once mature a tree has extensive benefits to the urban environment such as erosion control, aesthetics, reduction in the heat island effect, increased health and welfare of the human population, and increases in commerce activity. Trees however take years to reach a mature level and provide these benefits making them a long term investment. In a monoculture this long term investment can be entirely lost if a single pest or pathogen comes through like Dutch Elm’s Disease. Monocultures are also financially dangerous to cities. If a pest or pathogen hits a city with a monoculture that city is looking at massive expenditures to replace those lost trees. In cities in Michigan and other Emerald Ash Borer infested regions upwards of 40% of the urban forest has had to be replaced. If the rules of species richness and diversity were followed within the urban forest the same pest would require less than 5% of the urban forest to be replaced.

The second step to having diversity within the urban forest is having genetic diversity. Whenever you walk into a nursery to buy a tree you see varieties like autumn blaze maple, green spire maple, or sunburst honey locust. The advantage to selecting varieties in a nursery is that you are guaranteed a specific set of characteristics. Take the autumn blaze maple for example; pyramidal shape when young, red leaves in fall, 35 to 45 feet tall, and every tree is exactly like that. However almost all of the autumn blaze maples are genetically identical. This means that if a pest or pathogen finds one tasty, they will find them all tasty. The other problem with this is that if you have twenty planting spaces and you fill four of them with autumn blaze maples you still have a monoculture within your planting spaces filled with maples. You need open pollinated sources of seed and plants in order to obtain genetic diversity within the urban forest or you need to plant four varieties of maples.