Retain biological legacies

In urban areas, pre-urban legacy trees often provide much of the ecosystem services and functional value (e.g., carbon storage, shading, and habitat) of the urban forest. Preservation of these features will be essential to adapting the urban forest to future climates, because old trees may have superior genetics and they may play a valuable role in helping species persist on the landscape. Tactics for urban areas may differ from those used in rural forests managed for timber because there is rarely pressure to harvest or salvage in urban areas. However, legacy retention is often difficult or impossible because of development pressure. Despite the challenge, the retention of legacy trees can be helpful to promote the habitat value that these features provide.

Promoting species and structural diversity is as important in urban forests as in nonurbanized forest landscapes, if not more so. Urban areas are highly susceptible to introduction of nonnative pests and pathogens and often exhibit high occurrence of invasive plant species. Urban forests have been decimated because of a lack of species diversity in the face of pest introductions. Widespread acknowledgement of this problem has led to guidelines focused on diversification of the urban forest. However, urban areas contain difficult sites, and only a limited set of tree species may be able to tolerate the conditions of many of these sites. Species and structural diversity are especially important as a climate adaptation strategy because urban habitats (both natural areas and urban land uses) are likely to be stressed in the future in many ways, some of which will be unforeseeable. A diverse set of species, carefully selected to match the urban environment, will be more likely to maintain adequate forest cover and ecosystem services under a changing and increasingly variable climate.