Lichen Trimlines in the Peace-Athabasca Delta: Variations in Flora, Form, and Disturbance Regime

Résumé

Lichen trimlines are characteristic of aquatic systems where lichen-covered rocks border fluctuating water bodies. This study examined water-origin saxicolous lichen trimlines on acidic metacrystalline bedrock outcrops in the Peace-Athabasca Delta, northern Alberta, Canada. Twenty-seven species of saxicolous lichens were found in the vicinity of the trimlines. Species richness above the trimline (26 species) was almost twice that found below the trimline (14 species). Colonization lag time, differences in susceptibility to disturbance, and site influences on lichen establishment and survival might be involved in the absence of many species below trimlines. In frequently inundated areas, rock surfaces are dominated by the amphibious lichens Staurothele fissa and S. drummondii. The dominant lichen colonizer below trimlines was Physcia caesia. Other important colonizers below trimlines included Phaeophyscia sciastra and Physcia dubia, and on rocks fertilized by bird feces, Xanthoria elegans. A curious feature of the saxicolous flora was the presence of many calciphiles. Variations in trimline form and height and constituent species are related to the hydrologic and disturbance regime. Trimlines in perched basins tend to be near current water level, horizontal, and distinct, and indicate a relatively stable, infrequently flooded environment. Trimlines at open-drainage sites tend to be high above current water, wavy, and indistinct, and indicate a strongly-pulsed environment with frequent disturbances. Those at restricted-drainage basins are variable in form and height and disturbance regime.