Use new genotypes that are better adapted to future threats and conditions

It may be easier to introduce new genetic diversity into urban forests compared to natural forests, because it is already routine practice to introduce cultivars of a given species. With care, the introduction of new cultivars can be used to create a more resilient urban forest. In many cases, cultivars for a given species have already been developed for several different climate zones. For example, the sugar maple cultivar ‘Unity’ was developed for hardiness in Manitoba (USDA zone 3), and ‘Astis’ was bred to thrive in the U.S. Southwest (zone 5). As the climate changes, cultivars that were developed for drier and warmer regions may be best suited for new locations. Using cultivars to increase genetic diversity may be suitable only for developed urban sites, however, because these individuals may not reproduce in natural areas or otherwise may not thrive in a natural ecosystem.

Reduced gene flow of remnant native populations and the prevalence of genetically identical cultivars in urban areas contribute to a reduction in genetic diversity of the urban forest. Urban natural areas are heavily fragmented, which can cause reduced gene flow and lead to a decline in genetic diversity. Morphological uniformity is a priority in street and park trees. Most street and park trees are cultivars, or are grown from seeds that are sourced from a small number of parents to ensure that trees have predictable growth, survivorship, and tolerances. At the same time, morphological uniformity often comes at the expense of genetic diversity, and in changing conditions, a lack of genetic diversity may prove to be deleterious to long-term survivorship. For example, if all of the maple trees in an area are a single cultivar, they will probably react to climate change in a nearly identical manner. Increasing genetic diversity in the urban forest will ensure that some individuals are better equipped to withstand climate-induced stressors.