Understorey indicators of disturbance for riparian forests along an urban–rural gradient in Manitoba

Extensive agricultural and urban development has contributed to the decline of riparian forests across North America. An urban–rural gradient was used to identify species- and guild-level indicators of riparian forest degradation in southern Manitoba. Twenty-five sites were categorized according to urban, suburban, high-intensity rural, low-intensity rural, and relatively high quality reference land use. Generalists, which frequented all land use types, dominated (69%) the understorey community, whereas opportunistic (15%) and vulnerable (16%) species were relatively less common. Opportunistic species, which characterized city sites, tended to be exotic, woody and annual, and effective dispersers (i.e., endozoochores). In contrast, vulnerable species, which characterized non-city sites, tended to be native, perennial, and ineffective dispersers (i.e., barochores or anemochores). Indicators of disturbed forests were opportunistic and positively associated with disturbance measures including connectivity and cover of garbage, and negatively correlated with native and overall diversity. They included exotics Solanum dulcamara, Rhamnus cathartica, and Lonicera tartarica. In contrast, indicators of high-integrity forest were vulnerable, often excluded from urban sites and were negatively associated with disturbance measures and positively correlated with native and overall diversity. They included natives Rubus idaeus, Carex spp., and Galium triflorum. Our results suggest that opportunistic and vulnerable species, and their associated guilds, can be used as effective indicators of disturbance and forest integrity and to identify forest patches that warrant further protection or restoration.