Pollen and Macro-Fossil Assemblages in Disturbed Urban Wetlands on South Vancouver Island Reveal Recent Invasion of Reed Canarygrass (Phalaris arundinacea) and Guide Restoration

Paleoecological analyses and historical information were used to characterize pre-disturbance conditions in Swan Lake wetland of suburban Victoria, Vancouver Island, British Columbia, to provide a reference for restoration and management. Highly invasive reed canarygrass (Phalaris arundinacea) dominates Swan Lake wetlands and inhibits restoration. Grass pollen presumably produced by Phalaris predominates only in the top 5 cm (south site) to 35 cm (north site) of sediment cores. Below these levels assemblages are variously dominated by taxa including Salix, Alnus, Lysichiton, Cyperaceae, and Rosaceae. Pollen grains of agricultural disturbance indicators, such as plantain, liguliflorate Asteraceae, and cereals occur to depths of 35 cm. The results strongly suggest that Phalaris communities arose in historical times following agricultural disturbance and have no pre-European equivalent. Pollen assemblages below the Phalaris zone, corroborated by historical documentary references, show diverse original wetland types. Disturbance and crop species pollen indicators may be useful indicators of intensity and depth of disturbance. Pre-agricultural plant communities are a guide for restoration, provided that ecologically limiting factors are managed.