Maintain and restore diversity of native species

The reasoning and tactics for promoting native species diversity in urban natural areas are essentially the same as those for nonurban forests—that is, diverse communities may be less vulnerable to climate change impacts because risk is distributed among multiple species. The need for and difficulty of promoting native species diversity in urban systems may be greater, however, because of higher pressure from nonnative invasive species and more-extensive planting and promotion of nonnative species (McKinney 2002). Transitioning the urban forest to higher dominance by a diversity of native species would have multiple important positive effects on the adaptation potential of the urban ecosystem (Clark et al. 1997). Native species planted in urban locations can provide important habitat value for wildlife species; areas such as parks may be able to emulate a functioning ecosystem to some degree and support functioning food webs (Marzluff et al. 2001, Nowak and Dwyer 2007). These ecosystems may also be able to provide migration corridors through intensely fragmented urban landscapes (also see strategy 8) (Savard et al. 2000).

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).