1997 Volume 82 Pages 143-155
Based on the western North American distributions of 71 taxa and chemotypes of Cladina and Cladonia occurring at temperate and boreal latitudes, the Coast Mountains of British Columbia are shown to form a major phytogeographic barrier, dividing 21 oceanic taxa to the west from 24 continental taxa to the east. Maximum floristic diversity in these genera occurs between 52°N and 56°N, in a region occupied by glacial ice until roughly 10,000 years ago. Following deglaciation, many Cladoniaceae must have colonized this region from south of the Cordilleran Icesheet, presumably deriving from regions that no longer support them. South of 52°N, modem-day rates of decline average between three and five taxa per degree of latitude, and appear to be correlated with a southward trend from summer-moist climatic conditions to summer-dry. By contrast, the present southern limits of Cladina stellaris, Cladonia macroceras and C. stricta are believed to reflect historical, as opposed to strictly ecological, factors. Such species may still be extending their ranges southward.