Use seeds, germplasm, and other genetic material from across a greater geographic range

Although seeds from local sources may be the best adapted for an area currently, they may be maladapted to the changing climate in coming decades. Alternatively, seeds that are native to areas with a climate similar to the projected climate of the target region may have higher survivorship than local seeds if other habitat factors (e.g., soils) are also suitable. In addition, the unique climate in an urban center may already necessitate a different set of genetic material than for more pristine natural areas. Trials can help ensure that seedlings from distant areas will thrive in a new environment before large-scale plantings are undertaken. Other risks associated with introducing seeds from distant sources include the potential for also introducing foreign pests and diseases, or the possibility that the introduced species may become invasive, hybridize with other local species, or cause outbreeding depression.

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.