The Janzen-Connell hypothesis and plant-soil feedback theory both highlight the importance of host-specific pathogens in impacting tree seedling recruitment and promoting tree species diversity in tropical forests. Existing studies suggest that tropical pathogens specialize on tree species or at higher taxonomic levels, but agricultural and model plant systems suggest that microbial impacts also depend on plant genotype within a single species. Whether microbes that specialize on tree genotypes exist in tropical forests and impact tree diversity is not known, but if present, genotype-specific microbes could promote tree genetic diversity within populations via the plant-soil feedback mechanism. To test for evidence of genotype-specific microbes and evaluate soil microbial impacts on seedlings within a tree population in a naturally-occurring tropical forest, we conducted a plant-soil feedback greenhouse experiment in Panama with the canopy tree Virola suranimensis. We grew seeds from 11 maternal sources in one V. suranimensis population for eight months in experimental treatments containing the soil microbial community from beneath either 1) the seedling’s own maternal tree, 2) one of 10 less-related, non-maternal trees in the population, or 3) a heterospecific tree. At the end of the experiment we quantified seedling growth, survival, disease, and colonization by mutualistic arbuscular mycorrhizal fungi.
Results/Conclusions
Using linear mixed-effects modelling, we found that V. suranimensis seedling biomass and colonization by arbuscular mycorrhizal fungi were both decreased when seedlings were grown in the soil microbial community from beneath their maternal tree vs. a soil microbial community from beneath a less-related, non-maternal tree in the population. In addition, plant-soil feedback analysis of seedling growth yielded a negative system value for plant-soil feedback within the V. suranimensis population. Our experimental results provide new evidence that soil microbial communities beneath tropical trees in a single population impact conspecific seedlings differently depending on the seedling’s relationship to the tree and provide an advantage to non-offspring seedlings. The relationship between the seedling and the tree that had cultured the soil microbial community determined both seedling growth and colonization by mutualistic fungi. Disadvantages in offspring seedling growth conferred by soil microbes were strong enough to generate a negative plant-soil feedback within the tropical tree population, theoretically promoting local genetic diversity in this tropical tree population. Additional studies are necessary to confirm genotype-specificity among tropical soil microbes and their impacts on seedlings in field conditions, but our results suggest that genotype-specific soil microbes exist in tropical forests and promote seedling diversity in tree populations.