Climate predicts disjunct arctic-alpine plant species richness and abundance along Lake Superior shorelines better than local habitat

Climate refugia can serve as remnant habitat or stepping stones for species dispersal under climate warming. Lake Superior serves as a model system for understanding lake-mediated cooling, as its cool water temperatures and wave action have maintained shoreline habitats suitable for disjunct populations of arctic-alpine plants since deglaciation. We use structural equation modeling (SEM) to evaluate hypothesized causal pathways linking climate, site conditions, and bedrock type to abundance and richness of arctic-alpine plants. We surveyed macroclimate variables and local site conditions for 43 shoreline sites across Lake Superior’s north shore, and found that mean July temperature directly and indirectly affected disjunct species richness by maintaining low shoreline temperatures and shortening the growing season. Species richness of disjuncts was directly affected by shore width and indirectly affected by elevation above lake level. Competitive species did not significantly affect disjunct species richness or abundance at bedrock shoreline sites; however, disjunct species are competitively excluded from adjacent forested habitats. Currently, competition with encroaching boreal species is not limiting in bedrock communities; however, ongoing warming of Lake Superior’s waters may reduce cooling-mediated refugia, allowing for colonization of boreal species and a decline in disjunct plants from Lake Superior’s north shore.