THE ECOLOGICAL SIGNIFICANCE OF ARBUSCULAR MYCORRHIZAL FUNGAL EFFECTS ON CLONAL REPRODUCTION IN PLANTS

The population ecology of clonal plants depends on the number and distribution of ramets formed during growth. Variation in clonal reproduction has previously been explained by variation in effects of abiotic resource heterogeneity and by plant genotypic variation. Different co-occurring species of the mutualistic arbuscular mycorrhizal fungi (AMF) have been shown to differentially alter growth traits of Prunella vulgaris, which we hypothesize would lead to changes in clonal reproduction. Two experiments were carried out to test whether different co-occurring mycorrhizal fungi significantly influence clonal reproduction of P. vulgaris, whether this effect also occurs when P. vulgaris is growing in an artificial plant community, and how the effects compare with plant genotype effects on clonal growth of P. vulgaris. In the first experiment, the number of ramets of P. vulgaris, growing in a plant community of simulated calcareous grassland, was significantly affected by inoculation with different mycorrhizal fungi. The number of ramets produced by P. vulgaris differed by a factor of up to 1.8 with different mycorrhizal fungi. The fungal effects on the number of new ramets were independent of their effects on the biomass of P. vulgaris. In a second experiment, 17 different genotypes of P. vulgaris were inoculated with different mycorrhizal fungi. There were significant main effects of genotypes and mycorrhizal fungi on clonal reproduction of P. vulgaris. The effect of different mycorrhizal fungi contributed more than the effect of plant genotype to variation in size and ramet production. However, mean stolon length and spacer length, which determine the spatial arrangement of ramets, were only significantly affected by plant genotype. There were no mycorrhizal fungal × plant genotype interactions on clonal growth of P. vulgaris indicating that there is no obvious evidence that selection pressures would favor further coevolution between P. vulgaris and mycorrhizal fungal species. In natural communities plants can be colonized by several different AMF at the same time. The effect of the mixed AMF treatment on the growth and clonal reproduction of P. vulgaris could not be predicted from the responses of the plants to the single AMF. To what extent, however, the patterns of colonization by different AMF differ among plants in a natural community is unknown. Since the effects of AMF on growth and clonal reproduction occur on a population of P. vulgaris in a microcosm plant community and because the effects are also as great as those caused by plant genotypic variation, we conclude that the effects are strong enough to potentially affect population size and variation of clonal plants in communities.