Substrate surfaces provide an essential habitat template for sessile organisms. The orientation and type of substrate on which sessile organisms grow are an important determinant of community composition and influence its resistance and resilience to perturbation. Saxicolous, or rock inhabiting, lichens are important pioneer species of bare rock and dominate the aquatic-terrestrial transition zones in lakeshore habitats. To investigate how substrate type, slope and aspect influenced lichen communities, we surveyed the rocky shores of glacial lakes within the Boundary Waters Canoe Area Wilderness (BWCAW) of northern Minnesota. We quantified lichen distribution and abundance from sixteen shoreline sites on six lakes. At each site, a 1 X 2 meter plot was placed perpendicular to the high water mark (HWM) and divided into 32, 0.25m2 quadrats. Eight quadrats from each plot were randomly selected and measured for species abundance and total percent cover as a proportion of a quadrat. Lichens were identified to the lowest taxonomic level as possible. Shoreline aspect, slope, distance to vegetation, and distance to treeline were measured, and rock samples were collected to determine substrate composition. The effects of these factors on lichen species richness and percent cover were examined using backward selection linear models.

Results/ConclusionsA structural equation model (SEM) was generated to evaluate relationships between environmental variables and their effect on lichen species richness and percent cover.  Slope and percent cover had the two strongest positive effects on lichen richness. Lichen richness increased with slope until the slope reached a 60o incline, and then began to decrease. Lichen richness also increased with percent cover. West-facing aspect had a negative effect on lichen richness and total percent cover, with an average of 2 fewer species and the lowest percent cover of all aspects.  Prevailing western winds of the BWCAW is hypothesized to be the cause of this relationship. Percent cover was also negatively affected by substrate type. Variables associated with high levels of disturbance and stress, such as steep slope and a large distance to vegetation, had strong negative effects on lichen richness. These observations are not consistent with previous studies in vascular plant communities and suggest that lichens are adapted to survive in areas of high disturbance.  Results of the SEM illustrate many indirect effects on lichen richness that are related to disturbance variables and resulted in lowered species richness. Further research should aim to quantify wave, temperature, and light intensity to verify our findings.