Background/Question/Methods   It is established that genetic variation in foundation species affects organisms dependent on them for energy, however there are few examples of genetic variation affecting associated primary producers.  We hypothesize that genetic variation in a cottonwood hybrid system (P. Fremontii by P. angustifolia) affects associated lichens sensitive to variation in tree phenotypes.  To test this, we measure cover of a dominant bark lichen species, Xanthomendoza fulva, aas a and factors that may influence lichen, including photosynthetic active radiation (PAR), bark roughness, and bark condensed tannin (CT) and nitrogen concentrations (N), on different cross types (tree classes) and replicated genotypes within each cross type.

Results/Conclusions

X. fulva cover varies among genotypes within cross types, but not among cross types.  Within the backcross/P. angustifolia cross type broad-sense heritability estimates indicate that 31% of the variation in X. fulva cover is attributable to differences among genotypes.  PAR, rough bark cover, and bark CT and N all vary among cross types and/or genotypes within cross types.  Broad-sense heritability estimates indicate that 38 and 32% of the variation in rough bark cover and CT, respectively, within the backcross/P. angustifolia cross type is due to differences among genotypes.  We use structural equation modeling (SEM) to test if closely related trees have similar X. fulva cover, PAR levels, and bark characteristics relative to distantly related trees, and to examine relationships among these variables.  SEM indicates that the genetically-linked variables, in conjunction with genetic relatedness among trees, explain 54% of the variation in X. fulva cover across both the hybrid system and among backcrosss/P. angustifolia genotypes only, and that the influence of tree relatedness does not change between the two models.  SEM also shows that bark roughness and N are characters most important to X. fulva, however in the SEM analyses neither trait provides the major link between lichen and tree relatedness.  This is the first study to link genetic variation in a foundation species to variation in the abundance of lichen, demonstrating that the effects of genetic variation in a foundation species can extend to associated primary producers.