Species interactions are recognised as important components of ecological and evolutionary processes, with foundation species having a particularly strong influence on the distribution and diversity of dependent species. If foundation species exhibit variable phenotypic traits, under strong genetic control, this trait variation can lead to extended consequences for dependent organisms in their community. Thus intra-specific variation of a foundation species could have important consequences for habitats, communities and entire ecosystems.

One such foundation species is aspen (Populus tremula L.), which in Scotland exists in fragmented clonal patches. Aspen has important conservation value because it is associated with a high diversity of rare organisms, many of which have UK Biodiversity Action Plans (BAPs). One such group are the epiphytic cryptogams (i.e. mosses, liverworts, lichens). To date more than 300 species have been recorded on aspen in Scotland, representing approximately 40% of the epiphyte flora of Europe. The research presented here uses a combination of a natural aspen system and two aspen ‘common garden’ trials to test whether genetic diversity in aspen affects the composition of dependent epiphytic communities. In addition it investigates whether variation in two phenotypic traits of aspen, bark texture and bark phenolic chemistry, can account for the variation in epiphyte communities present on different aspen clones.

In the wild population of aspen, the proportion of smooth bark and the concentration of a number of bark phenolic compounds were found to vary significantly between clones, and variation was under strong genetic control. Epiphyte community composition also varied among aspen clones and was significantly affected by the proportion of smooth bark possessed by clones. In the common garden experiment epiphyte community composition was again affected by genetic differences between clones, but the pattern could not be related to variation in either bark type or bark phenolic chemistry. The results indicate that epiphyte communities are part of the ‘extended phenotype’ of native aspen populations in Scotland. A diversity of aspen genotypes in the landscape is essential for maintaining and possibly enhancing current levels of epiphyte diversity with benefits for ecosystem health as a whole.