Promote diverse age structure

A diverse age structure can be beneficial because trees are most vulnerable to specific stressors at different ages. For example, droughts are typically more damaging to seedlings than to mature trees, whereas older individuals may be more susceptible to damage from wind events. In many cases, urban forests are dominated by trees that persisted on the landscape as it became urbanized, and many of these trees are reaching the end of their lifespan (Fahey et al. 2012). Active management may be necessary to promote regeneration and development of younger age classes. Furthermore, in developed urban areas, managers often focus on individual trees, which are removed upon death or damage and replanted as soon as resources permit). Age class diversification may consequently be less straightforward than in natural forests. However, a diverse age structure in these locations could increase the habitat value of the urban forest and spread out tree losses from natural mortality (Clark et al. 1997, Millar et al. 2007). In developed urban sites, some planting and tree removal practices could help develop a more diverse tree age structure both within and among management units.

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 (Dreistadt et al. 1990, McKinney 2002). Urban forests have been decimated because of a lack of species diversity in the face of pest introductions (Poland and McCullough 2006, Santamour 2004). Widespread acknowledgement of this problem has led to guidelines focused on diversification of the urban forest (Santamour 2004). 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 (Whitlow and Bassuk 1987). 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 (Gill et al. 2007, Kirshen et al. 2008). 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.