Relative importance of host tree species and environmental gradients for epiphytic species composition, exemplified by pyrenomycetes s. lat. (Ascomycota) on Salix in central north Scandinavia

According to the continuum concept of vegetation, variation in species composition is primarily determined by complex environmental gradients. Species-gradient relationships of ground-dwelling, independent organisms are studied at scales ranging from centimetres to continents. In this study we use a balanced data set for pyrenomycetes on Salix to address if how the current species-gradient paradigm needs to be modified to apply to assemblages of organisms that are dependent on other organisms for their existence. The data from a transect across central-north Scandinavia included variation along climatic gradients in oceanicity (from oceanic to continental vegetation sections), and temperatures (from south boreal to alpine vegetation zones) and among five common and widely distributed Salix host species ( Salix caprea agg., S. glauca ssp. glauca , S. lapponum , S. myrsinifolia agg. and S. pentandra ). Ten individuals of each Salix host species were selected and carefully examined for pyrenomycetes within each combination of section and zone. Data for 28 species in the 28 combinations of section, zone and host were subjected to ordination and constrained ordination analyses. Host species was the most important source of variation in species composition, followed by zone and section which are the same major regional gradients that are important to plants. We use examples to discuss the contribution of local ecological and substrate gradients to the high variation explained by host species, concluding that host specificity per se occurs for these partly parasitic fungi. We therefore suggest that in order to account for variation in composition of species assemblages with strong degree of host dependence, general rules for species-gradient relationships need to be extended by inclusion of host specificity as separate factor.