The phytogeography and ecology of the mosses within the San Juan Islands, Washington State

Floristic work on bryophytes in the state of Washington has been confined mostly to the mainland with little information available for the San Juan Islands. After four years of field work and an extensive search of historical records from herbaria throughout the region, the San Juan Islands prove to contain a diverse moss flora within a small geographic area; this flora consists of 224 species and varieties, 33 families and 97 genera. Four species Drepanocladus crassicostatus, Orthotrichum hallii, Tortula papillosa and Tortula laevipila var. meridionalis are reported new for the State of Washington. Tortula laevipila var. meridonalis is new for the United States and represents the second North American location. Detailed ecological observations were made for each collection and distributions for each species have been mapped. Keys are presented to both genus and species. The bulk of this flora is composed of circumboreal species that are derived from a once widespread Arcto-Tertiary flora. During the Pleistocene these islands were completely glaciated and the present flora represents, therefore, species that have migrated back into the region predominantly from southern refugial sites probably during the Hypsithermal Interval described by Deevey and Flint (1957). A cooling trend about 2000 yr. B.P. probably caused the southern element species to retreat southward throughout the region with fragments persisting only in those areas where favorable conditions also remained. The San Juan and adjacent islands can be interpreted as a "modern" refugium for southern mediterranean type climate species.Science, Faculty ofBotany, Department ofGraduat