Embracing well-adapted tree species with positive characteristics can be beneficial, but sometimes there can be too much of a good thing. A broad palette of tree species improves the resiliency of an urban forest and minimizes the effects of issues that affect certain tree species. It is recommended by urban forestry professionals that no single species make up more than 10% of the entire urban forest composition.

Effects of Inadequate Species Diversity

Pest & Disease Epidemics

Low species diversity can increase the impact of pest and disease epidemics in the urban forest. An example of this is the over-planting of American elm as street trees, which were decimated by Dutch elm disease in the 20th century. Due to the mass planting of a single species, the disease spread very quickly and was difficult to control. In addition, gaps left in the landscape by the trees that died produced quite a hole in the continuity of urban tree canopy.

A Mixture of Positive Features and Flaws

Even the most desirable of landscape trees can have some negative attributes as well, such as susceptibility to breakage in ice storms or cold hardiness that does not quite match the recent temperature extremes.  As with disease susceptibility, when a condition that conflicts with a tree's flaw arrives, it can hit those trees pretty hard.  An urban forest with a variety of trees possessing different characteristics and adaptability will result in less of the urban forest affected by different stressors.

Invasive Potential

Some tree species have a tendency to sprout up prolifically within natural areas.  Especially when dealing with nonnative species, it is important to consider management of opportunistic trees in order to prevent negative impacts on local ecosystems such as native grasslands. Some examples of exotic tree species that are commonly observed encroaching into natural areas include Bradford pear and Chinese pistache.

Planting Diverse Trees

By planting under-used, adapted species that are appropriate for a site, citizens can enhance a healthy and unique urban forest. To find a tree species, view Planting a Tree. Enhancing species diversity in Edmond's urban forest is one of the main goals of the Foster-A-Tree program.  You can contribute to this effort by signing up to receive a tree of your own.