Epiphyte diversity on Scottish aspen – a component of the extended phenotype
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Species interactions are recognised as an important evolutionary process, with foundation species in particular being of exceptional importance. Foundation species are those species exhibiting dynamic physical traits, under strong genetic control, that shape the natural processes of habitats and ecosystems. These traits lead to extended consequences for the associated organisms in their community. Therefore, intra-specific variation of a foundation species can have important evolutionary consequences for habitats, communities and entire ecosystems. One such foundation species is aspen (Populus tremula L.), which has important conservation value, particularly for the high diversity of associated species. In Scotland aspen exists in fragmented clonal patches, but has been found to contain a high diversity of associated organisms some of which have a UK Biodiversity Action Plan (BAP). One such group of organisms of high diversity and conservation value in Scotland are the epiphytic cryptogams (i.e. mosses, liverworts, lichens). To date more than 300 species have been recorded on aspen in Scotland, comprising approximately 40% of the epiphyte flora of Europe. The research presented here uses a combination of natural aspen system and two aspen common gardens to test the effects of aspen genetic diversity on physical traits potentially important for epiphyte diversity. The traits investigated were bark texture and bark phenolic chemistry. Bark texture in the wild clones was found vary significantly between clones and under strong genetic control (up to 40%). Bark phenolic chemistry also showed significant genotypic variation, but could not be correlated with patterns of epiphyte species richness and diversity. Nevertheless, epiphytes showed significant patterns related to aspen genotype, particularly along a gradient of bark texture. The results indicate that epiphyte communities are part of the ‘extended phenotype’ of native aspen populations in Scotland are very important for maintaining current levels of epiphyte diversity. A greater diversity and abundance of aspen genotypes in the landscape are essential for increasing epiphyte species richness and diversity, and for ecosystem health as a whole.