Impacts of land use on the riparian forest along the Assiniboine River

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Moffatt, Shaun F. H.
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Extensive landscape modification from surrounding land use has led to the decline of riparian forests across North America. An urban - rural gradient was used to assess the impact of land use on riparian forests along the Assiniboine River. Above ground flora and seed bank were examined and species- and guild-level indicators of disturbance identified. Twenty-five sites were categorized according to land use, and included urban, suburban, high intensity rural, low intensity rural, and relatively high quality reference forests. Changes in herbaceous, shrub, and tree species composition and diversity were related to landscape level measures of disturbance that included the proportion of surrounding land use, forest patch size, connectivity, and area:perimeter ratio. Urban forests were highly fragmented and the most adversely affected by surrounding land use. They were small, isolated, lacked interior, and characterized by relatively dry and alkaline soils. They had the lowest native and overall understorey and seed bank species diversity, highest proportion of exotic species, and the lowest seed density. Indicators of disturbance, typically opportunistic species, were significantly more common in these urban forests and included Solanum dulcamara, Rhamnus cathartica, and Lonicera tartarica. Suburban forests were less disturbed, but had been recently subjected to extensive development-related clearing and fragmentation. Although reference sites were relatively large and exhibited greater connectivity, there was little difference in species composition among low and high intensity rural and reference sites. Indicators of high integrity forest, typically vulnerable species, were significantly more frequent in these non-city land use types, and included Rubus idaeus, Carex spp., and Galium triflorum. Generalists dominated (69%) the understorey community, whereas opportunistic (15%) and vulnerable (16%) species were relatively less common. Opportunistic species tended to be exotic, woody and annual, and effective dispersers (i.e. endozoochores). In contrast, vulnerable species tended to be native, perennial, and ineffective dispersers (i.e. barichores or anemochores). In total 197 taxa were identified in the above ground flora, compared to 90 taxa in the seed bank. Of the latter, the three most frequent species, Poa pratensis, Sonchus arvensis, and Cirsium arvense, were all exotic. These results suggest that landscape measures of disturbance, and related changes in environment, may be confidently used to assess the impacts of land use along urban-rural gradients. Changes in seed bank of this important riparian forest system suggest that the current decline will continue especially if future forest regeneration is solely dependent upon the seed bank. Opportunistic and vulnerable species, and their associated guilds can be used as effective indicators of disturbance and forest integrity and be used to select and monitor forests for further protection or active management.