Distributional and Morphological Differences between Native and Introduced Common Reed (Phragmites australis, Poaceae) in Western Canada

Phragmites australis (common reed) is a widespread perennial grass of wetland habitats, with cryptic native and introduced subspecies in North America. We determined the relative abundance of the subspecies and the distributions of plastid DNA haplotypes throughout British Columbia, Canada, at the northwestern distribution limit of common reed in North America. Of 203 specimens assigned to subspecies using molecular markers, we identified only 9 plants as the introduced ssp. australis; all remaining samples were the native ssp. americanus. The two subspecies co-occurred at only one locality. We identified four native haplotypes (one widespread in British Columbia and three others more localized) and two introduced haplotypes. Using plants of known haplotype, we assessed the utility of different morphological traits and trait combinations for distinguishing native and introduced subspecies in this geographic region. No single morphological trait was diagnostic, but principal components analysis and identification indices based on combinations of traits consistently separated the native and introduced subspecies in our sample. Two- or three-trait combinations of ligule length, lemma length and stem anthocyanic coloration gave the best separation. These indices could reduce the need for confirmation of the introduced subspecies using molecular tools, facilitating efforts to monitor and control this invasive plant.