Tracking long-term changes in the arable weed flora of Canada.
The weed flora of arable fields in Canada is largely the result of the accidental or intentional introduction of many alien species as documented by weed surveys from the 1900s to the 2000s. The objectives of this paper are to review the various methodologies that have been used in Canada to survey for cropland weeds since the early 1900s and to examine these data for long-term changes in the number, frequency, abundance, and distribution of alien and native weed species. The first contemporary survey was initiated in 1973 and this methodology, with minor modifications, has been used for surveys in nine Canadian provinces from the 1970s to the 2000s. Alien weed species accounted for 52% of the weeds recorded in these surveys. Earlier weed surveys in the Prairie Provinces from the 1900s to the 1960s were used for retrospective analysis of the general trends in weed populations. About two-thirds of the 36 most abundant species in the 2000s were present or considered bad weeds in the early 1900s. The percentage of alien weed species has increased from 43% to nearly 70% during this time period. Alien weed species accounted for 93 to 96% of the total relative abundance index in the contemporary surveys in the prairie region; however, the density of alien weed species has declined significantly since the 1970s from approximately 100 to 30 plants m-2. While the abundance of many weed species, such as cow cockle, is greatly reduced, weed species such as cleavers, kochia, and barnyard grass have expanded in range and/or abundance based on distribution maps constructed from survey data. The relative abundance index and density of native weed species are insignificant in comparison. While the methodology used in the contemporary weed surveys discussed in this paper are not designed to detect the early stages of weed invasions, they are effective for the long-term monitoring of shifts in the arable alien and native weed species during the later stages of invasion.