Examining the effects of soil type and fungicide use on American elm seedling germination, survival, and development

Background/Question/Methods

     Forest community composition and dynamics can be linked with microbial-driven subsurface processes, including interactions with mycorrhizal and pathogenic organisms. Along with resource partitioning and nutrient cycling, these biotic interactions play a large role in determining the abundance and distribution of tree and plant species. However, the interactions between and relative importance of these controlling factors are not well understood. In this study, we focused on mycorrhizal soil type as a potential control over levels of pathogens that impact seedling germination, survival, and development. Our objectives were to examine the effect of different mycorrhizal soil types on seedlings. American elm seeds were planted on soils collected from forest plots dominated by arbuscular (AM) or ectomycorrhizal (ECM) trees. Various fungicides that target fungi or oomycetes were applied. We recorded germination, survival, and growth, which were analyzed by generalized linear modeling. AIC analysis was conducted to find the best-fitting model for each area of interest.

Results/Conclusions

     Neither soil type nor fungicide treatment had a significant impact on seed germination. However, seedlings grown on AM dominated soil had lower survival when treated with fungi-targeting fungicides compared to treatments that targeted oomycetes. Conversely, seedling survival on ECM dominated soil was not affected by fungicide treatment. Soil type had the greatest influence on the type of hyphae present. Seedlings grown on AM soil had slightly higher AM hyphal colonization than those grown on ECM soil, while seedlings grown on ECM soil had much higher non-AM hyphal colonization compared to those grown on AM soil. Aboveground biomass was unaffected by soil or treatment type, while below ground biomass was influenced by soil type. Seedlings grown on AM soil had larger root biomass compared to those grown on ECM soil. Examining the hyphal colonization and biomass data together showed a strong association between root growth, soil type, and hyphal colonization. From this, we conclude that roots grown in ECM soil were stunted due to the large presence of non-AM hyphae, which may be pathogenic or parasitic in nature. Alternatively, roots of the American elm seeds may have been stunted due to lack of AM fungi, the normal symbiont for this tree.