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.
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.