| Summary: | 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.
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