Does past contact reduce the degree of mutualism in the Alnus rubra-Frankia symbiosis?

Although most vascular plants have symbiotic relationships with soil microb es, and there is an extensive theoretical literature on the evolution of mu tualism, there has been little experimental examination of the evolution of mutualism between plants and their microbial symbionts. We inoculated red alder (Alnus rubra Bong.) seedlings from three high- and three low-elevatio n populations with crushed nodule suspensions containing the nitrogen fixin g bacterium Frankia from either the parent trees (familiar strains) or the other plant population sampled within the parent watershed (unfamiliar stra ins). The inoculated seedlings were planted on three high- and three low-el evation sites. Growth was monitored over the second and third year followin g planting, after which the whole plants were harvested. The proportion of nitrogen derived from fixation was estimated from the ratio of stable nitro gen isotopes in the harvested leaves. On low-elevation sites, which had hig h soil nitrogen, plants with familiar Frankia strains were half the size an d derived less fixed nitrogen from their symbionts compared with plants ino culated with unfamiliar Frankia strains. On high-elevation sites, which had low soil nitrogen, the type of inoculum had little effect on plant perform ance, although plants with familiar inoculum were consistently larger than plants with, unfamiliar inoculum. These results suggest that the degree of mutualism in this symbiosis depends on environmental conditions and may dec rease with time.