Composition and cover of epiphtyic lichens on Pseudtosuga (sic) menziesii and Populus tremuloides in southwestern Idaho

Abstract

Species composition and percent cover of epiphytic lichens were determined on trunks of Pseudotsuga menziesii and Populus tremuloides at two study sites in Southwest Idaho: Marshall Lake Trail and Mores Mountain. In this study, we tested whether bark pH and directional position on the tree trunk influenced the distribution of lichens on these two tree species. Forty trees were examined using a 5X10 cm quadrat to determine lichen cover in the four cardinal directions. Five lichen species were found on P. menziesii at the Marshall Lake Trail study site and six at the Mores Mountain study site. Three different lichen species were found on P. tremuloides at Marshall Lake Trail site: where as no lichens were detected on P. tremuloides at the Mores Mountain site. For both tree species, lichens were most abundant on the south aspect of the tree trunks. This result differs from other studies, and may be due to optimal lichen growth conditions occurring on the south aspect of tree trunks during cooler and moister winter season, at these arid study sites.Introduction. Typically, the bark of deciduous trees is more basic than the bark of conifers. Lichens can produce secondary chemicals that change the chemistry and texture of their substrate, making bark a more suitable habitat for further colonization by lichens and expansion of lichen thalli (Rogers 1988). Bark pH influences availability of minerals and organic substances, changes in diffusion rates and toxicity of the substrate (Brodo, 1973). Hardwood gaps in forests can greatly boost lichen diversity (Neitlich and McCune 1997). The hardwood species in this study, Populus tremuloides, is one of the most geographically wide spread deciduous trees in North America (Jonescu 1970) and stands of this tree species are distributed throughout Idaho. Stands of P. tremula in Europe are scattered throughout conifer forests, often support a variety of threatened lichen species and increasing the biodiversity of the lichen flora in these forests (Gustafsson and Eriksson 1995).This study investigated the composition and cover of lichens on two tree species: Populus tremuloides Michx. and Pseudotsuga menziesii (Mirbel) Franco. We hypothesized that differences in lichen composition may be related to differences in bark pH of the two trees, and that differences in cover should be related to the aspect of the trunk for both species. Varying lichen occurrences have been found to reflect differences in moisture levels, temperature, light and stability of conditions (Eversman 1982). Many studies show an increase in lichen abundance on the north and east sides of trees due to cooler and moister microclimates (Eversman 1982, Gough 1975, Jonescu 1970, Peard 1988. In some climates, the conditions associated with the north side of a tree trunk allows lichen species more time to photosynthesize and obtaining maximal growth, before daily desiccation during the summer (Eversman 1982). Morphology of the bark may also play a role in the colonization of lichen species. For instance, P. tremuloides ty...