The Recent Spread and Potential Distribution of Phragmites australis subsp. australis in Canada

Résumé

To provide information on geographic occurrence, rate of spread, and potential distribution of European Common Reed, Phragmites australis subsp. australis, in Canada, we measured 1740 herbarium specimens from 21 collections across Canada, entered the information into a database, and mapped and analyzed these records. The European subspecies australis was first documented in Canada 100 years before it was recognized as an alien invader. It was not until the invading plants had entered a phase of rapid local increase after 1990 that they attracted sufficient attention that a comparison of the invasive and non-invasive plants was made. By 2001, two different races had been distinguished, and soon after they were separated as different subspecies. The first Canadian collection of the alien subsp. australis was made in southwestern Nova Scotia in 1910. By the 1920s, it occurred in southern Nova Scotia, along the St. Lawrence River near Quebec City and at Montreal. The first southwestern Ontario specimen was collected in 1948. Thus by 1950 subsp. australis was known from only four relatively small areas of Canada based on 22 collections. At this same time, the native race, subsp. americanus, had a widespread distribution in Canada represented by 325 collections. This strongly supported the comparable and limited distribution of subsp. australis at the time. By 1970, subsp. australis had spread locally but was still found only in southwestern Nova Scotia, in the St. Lawrence River valley, and in southwestern Ontario. By 1990, subsp. australis had become much more frequent in the St. Lawrence River valley and in southwestern Ontario, and it had extended westward into eastern Ontario. By 2010, it had spread throughout much of southern Ontario and southern Quebec, and it had a more extensive distribution in Atlantic Canada, but the biggest change was its spread into western Canada. It appeared in northern Ontario, northwestern Ontario, southern Manitoba, and interior southern British Columbia. The rate of spread is increasing and within a decade or two, based on the extent of appropriate plant hardiness zones currently occupied, it is expected to become abundant in the prairie provinces and across most of southern Canada.