Urban trees worldwide have low species and genetic diversity, posing high risks of tree loss as stresses from climate change increase

Popular trees that are known to grow well in a region are often the first ones people choose to plant in the landscapes that surround them. Historically, this has resulted in over-planting particular species, such as the American elm (Ulmus americana). This species had little genetic variation in its natural resistance to Dutch elm disease (Ophiostoma ulmi), leading to the death of most American elms. In many areas, the species of trees that are well suited to urban conditions are somewhat limited; those suitable as street trees are even more limited. Analyses of urban tree inventories worldwide show the problem of overplanting still exists in many cities, with the average abundance of the most common species being about 20% worldwide, while being over 40% in some cities. The current ability to clone trees with desirable characteristics means that many tree species planted today have even less genetic variation than those planted from seeds in previous decades. Climate change will bring new growing challenges for urban trees. Trees that grew well under environmental conditions of the past may no longer be suitable in the future. Coupling these factors places cities in danger of rapidly losing large numbers of urban trees, with grave environmental and human consequences, including increased urban heat island temperatures and more heat-related human deaths.