THE DISJUNCT MOSS ELEMENT OF THE GULF OF ST. LAWRENCE REGION: GLACIAL AND POSTGLACIAL DISPERSAL AND MIGRATIONAL HISTORIES

Abstract

  The Gulf of St. Lawrence has a moss flora of 522 species. Of these, 217 (42%) are disjunct to this region from western North America, eastern Asia, or Europe. The Gulf of St. Lawrence and eastern North American distributions of the disjuncts were analysed and their possible migrational and dispersal histories during and after the Last Glaciation (Wisconsin) were examined. Based on eastern North American distribution patterns, the disjuncts fell into 22 sub-elements supporting five migrational/dispersal histories or combinations of these: (1) migration from the south, (2) migration from the north, (3) migration from the west, (4) survival in refugia, and (5) introduction by man.

  The largest groups of disjuncts have eastern North American distributions supporting either postglacial migration from the south, or survival of mosses in Wisconsin ice-free areas of the Gulf of St. Lawrence. About 23% of the disjuncts have complex distributions which suggests they support two possible histories. These may have migrated to the Gulf from the west and/or north, or from the west and/or survived glaciation in Gulf ice-free areas. The eastern North American distributions of some species in these groups suggest that survival in ice-free areas best explains their presence in the Gulf of St. Lawrence. Human introductions to the Gulf and migration from the north are relatively unimportant migrational/dispersal histories for disjuncts in the region. No mosses supported the hypothesis of long distance dispersal to the Gulf of St. Lawrence.

  Within the Gulf of St. Lawrence, species having migrated from the south generally show widespread distributions or occur primarily in the southern portions of the study area. Mosses introduced to the Gulf by man occur in eastern Newfoundland and in central Nova Scotia. The remaining species groups show generally similar patterns; they are restricted to the northern half of the study area, or show disjunctions between western Newfoundland, Gaspé and, occasionally, Cape Breton.

  Important habitats for disjunct mosses are late snowbeds and limestone barrens. The disjunct mosses in late snowbeds are primarily montane species, whereas the disjunct mosses in the limestone barren habitat are predominantly of arctic affinity. The disjunct mosses in the snowbed habitat provide some of the strongest evidence for survival of mosses in Gulf ice-free areas.

  The importance of disjunctions to the phytogeography of the Gulf of St. Lawrence varies with geographical scale. Continental disjunctions are best explained as resulting from climatic and geological changes occurring since the Tertiary whereas disjunctions at the eastern North American scale provide the best evidence for solving problems relating to Wisconsin dispersal and migrational histories in the Gulf. Moss disjunctions within the Gulf are explained adequately by climatic and ecological factors operating during the Holocene.