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Sources of variation in the symbiotic association between Alnus and Frankia in interior Alaska

Symbioses between plants and nitrogen (N) fixing bacteria are ecologically and economically important interactions with complex evolution and ecology. Theoretical and experimental studies suggest that host specificity and environmental variation are important determinants of both evolutionary and ecological patterns in such interactions, but detailed descriptions of these parameters in natural habitats are lacking for most Nfixing systems. The aim of this set of studies was to provide such information for the symbiosis between alder (Alnus spp.) plants and Frankia bacteria in interior Alaska. Major objectives were to determine whether: 1) different Alnus species (A. tenuifolia and A. viridis) associate with different Frankia genotypes in the field, 2) genetic composition and distribution of Frankia associated with the two hosts differ among successional habitats, 3) differences in Frankia are paralleled by differences in host physiology at plant (leaf N) or nodule (N-fixation rate) scales, and 4) occurrence of Frankia genotypes is correlated with specific soil variables. The two hosts were found to associate with Frankia representing largely different clades, even in sites in which the two hosts cooccurred. Genetic composition and spatial distribution of Frankia in A. tenuifolia nodules differed between successional habitats, but were largely consistent among replicate sites representing each habitat. Habitat-related differences were negligible in A. viridis. Leaf N differed among habitats for both hosts, but evidence for differences in Nfixation rate among Frankia genotypes was equivocal. Occurrence of the dominant genotype in early succession sites was strongly correlated with carbon : nitrogen ratio of