RAGWEEDS, AMBROSIA SPECIES, IN CANADA AND THEIR HISTORY IN POSTGLACIAL TIME

Abstract

Specific differences of Ambrosia artemisiifolia, A. trifida, and A. coronopifolia are described and their present Canadian distributions are reported in detail. Ambrosia pollens, identified in fossil assemblages as either A. artemisiifolia or A. trifida with a few in the size range of A. coronopifolia, were somewhat more abundant in late-glacial deposits than in younger postglacial sediments except the subrecent ones. It is only within the last 200 years that ragweeds have again become abundant in eastern Canada. The migration and spread of the three ragweed species in eastern and western Canada through postglacial time are discussed. There is evidence through pollen profiles that Ambrosia species existed further northward in Ontario than they do at the present time. In studying the presence and abundance of Ambrosia pollen in surface samples, the authors showed that ragweed pollen can be transported through the atmosphere at least 375 miles from any known source.