Background/Question/Methods - An important approach for understanding community structure and ecosystem processes is to first understand the ecology and genetics of foundation species that are by definition, community and ecosystem drivers.  Although foundation species probably occur in all taxonomic groups and interact with one another, they are likely to be a small subset of the whole community.  We addressed the hypothesis that plant genotype would play an important organizing role in microbial community structure and that community phenotype represents a heritable plant trait.  Studies were conduced in common gardens using replicated clones of individual tree genotypes of a foundation riparian tree, Populus angustifolia.  Results/Conclusions - We found that tree genotype matters in defining a much larger community of organisms from microbes to vertebrates.  Specifically, we found that: 1. Different tree genotypes of Populus angustifolia support different soil microbial communities, different twig endophyte communities, different lichen communities, and different leaf pathogen communities.  2. These microbial community patterns are similar to that observed with canopy arthropods suggesting similar responses of very different organisms to plant genotype.  3.  Significant broad-sense heritability of these diverse communities was detected; i.e., the community phenotype can be considered a heritable plant trait.  4. These communities are likely to have important feedbacks on the fitness of the individual trees genotypes they are associated with.  For example, cottonwood seedlings planted in the soils of the maternal genotype enjoyed higher survival than seedlings planted in non-maternal soils.  5. Network theory that has been applied to arthropods on individual tree genotypes may also be productive when applied to soil microbes.  6. Similar patterns have been found with the communities of pinyon pine and eucalypts suggesting that there may be broad generality to these findings.  A key issue is to understand how climate change might interact with the genotypes of foundation plant species to alter community structure, community stability and ecosystem processes.