Particular species can play important roles in structuring ecological communities, but their effects can be context dependant. Recent research suggests that distinct genotypes of dominant plant species can also play important roles in structuring associated communities.   However, few studies have examined whether the effects of genotypic effects are context dependent. For example, it is not well known how genotypes of dominant plants expressing a range of susceptibility and resistance to attack by phytophagous arthropod species will be affected when grown in close proximity to each other. This study examined the genotype x environment interaction of experimentally manipulated plots of Solidago altissima, in which the environmental component was variation in the identities of genotypes growing in close proximity to the focal genotype. We quantified the abundance of free-feeding aphids (Uroleucon sp.) in a field experiment in which 21 distinct clonal families (2 replicates each) of S. altissima were randomly assigned to plots (n=63) of 1, 3, 6, and 12 genotypes. 
Results/Conclusions

Aphid abundance was similar among neighboring plots of S. altissima. The relationship between aphid abundance in a plot and abundance in neighboring plots was mediated by genotypic diversity. Plots with multiple genotypes of S. altissima supported higher abundances of aphids than did single genotype plots when aphid abundance on neighboring plots was high.  However, when aphid abundance on neighboring plots was low, this trend was reversed. These findings suggest that (1) the genetic identity of intraspecific neighboring plants are important factors in determining the expression of resistance traits on focal genotypes; and (2) that intraspecific genetic diversity may be an important factor in determining the strength and predictability of community level patterns and processes.