Woody Invasions of Urban Trails and the Changing Face of Urban Forests in the Great Plains, USA
Nemec, Kristine T., Allen, Craig R., Alai, Aaron, Clements, Greg, Kessler, Andrew C., Kinsell, Travis, Major, Annabel, Stephen, Bruce J., The American Midland Naturalist
Corridors such as roads and trails can facilitate invasions by non-native plant species. The open, disturbed habitat associated with corridors provides favorable growing conditions for many non-native plant species. Bike trails are a corridor system common to many urban areas that have not been studied for their potential role in plant invasions. We sampled five linear segments of urban forest along bike trails in Lincoln, Nebraska to assess the invasion of woody non-native species relative to corridors and to assess the composition of these urban forests. The most abundant plant species were generally native species, but five non-native species were also present: white mulberry (Morus alba), common buckthorn (Rhamnus cathartica), tree-of-heaven (Ailanthus altissima), honeysuckle (Lonicera spp.) and elm (Ulmus spp.). The distribution of two of the woody species sampled, common buckthorn and honeysuckle, significantly decreased with increasing distance from a source patch of vegetation (P = 0.031 and 0.030). These linear habitats are being invaded by non-native tree and shrub species, which may change the structure of these urban forest corridors. If non-native woody plant species become abundant in the future, they may homogenize the plant community and reduce native biodiversity in these areas.
Urban forests are comprised of the trees that naturally occur within remnant forests and the trees planted along streets and within yards and parks (McBride and Jacobs, 1976; McPherson et al., 1997). These patches of vegetation within a landscape dominated by concrete provide a variety of benefits to their residents and to wildlife. They remove air pollutants, store carbon, lower summer temperatures and allow people to interact with nature (McPherson et al., 1997; Bolund and Hunhammar, 1999; Chen and Jim, 2008). They also provide habitat for some birds that have adapted to urban settings (Savard et al., 2000), particularly seed eaters, omnivores, ground foragers and raptors (Chace and Walsh, 2006).
However, the properties of forests, including urban forests, can be affected by the invasion of non-native plant species (Martin, 1999; Vidra et al., 2006; Shustack et al., 2009). The invasion of ecosystems by non-native plants is one of the leading threats to biodiversity and can alter ecosystem structure and function (Vitousek et al., 1987; Braithwaite et al., 1989; Walker and Smith, 1997; Yurkonis et al., 2005). For example, non-native woody plants can increase decomposition rates, alter nitrogen cycling and increase soil moisture and pH within forests (Kourtev et al., 1999; Ehrenfeld et al., 2001; Ashton et al., 2005), which may provide a positive feedback loop by facilitating the establishment of quickly growing invasive species compared to slower growing native species (Dukes and Mooney, 1999; Gurevitch et al., 2008). Non-native trees and shrubs can decrease native plant species richness, abundance and density (Collier et al., 2002) and inhibit the growth of native saplings and forbs (Fagan and Peart, 2004; Miller and Gorchov, 2004; Galbraith-Kent and Handel, 2008).
Non-native vegetation can also impact forest fauna. The non-native shrubs European buckthorn (Rhamnus cathartica) and Japanese barberry (Berberis thunbergü) have been associated with high populations of non-native earthworms in woodlands (Kourtev et al., 1998; Heneghan et al., 2007), which can alter nutrient storage and availability, affecting soil food webs and understory plant communities (Gundale, 2002; Bohlen et al., 2004). Non-native shrubs can decrease nest success for birds, which may be attributed to structural differences between the non-native and native shrubs which may increase predation risk (Schmidt and Whelan, 1999; Borgmann and Rodewald, 2004).
Plant invasions may be facilitated by human activities that result in habitat fragmentation. Habitat fragmentation alters the microclimate along the boundary between fragments of vegetation and their surrounding habitat (Collinge, 1996) and provides more opportunities for invasive species to become established and invade fragments (Honnay et al. …