Forests around the world are supported and structured by the mycorrhizal fungal communities that expand trees' access to soil water and nutrients. As forest communities respond to increased disturbance and changing climatic conditions, these fungal symbionts will play a critical role in shaping which seedlings thrive, and where. In this project, we investigate how soil history and neighboring plant species interact to influence the community composition and function of a seedling’s ectomycorrhizal portfolio. Seedlings from two widespread and ecologically important genera, Pseudotsuga macrocarpa and Quercus chrysolepis, were grown in each of three soil conditions: collected from under mature P. macrocarpa, collected from under mature Q. chrysolepis, or sterile greenhouse soil. At the end of a year of growth, seedlings were uprooted and washed, measured for size, and had root tips harvested for ectomycorrhizal community characterization. Seedlings were then repotted into larger pots containing sterile soil, but with a neighboring seedling sharing the pot. These mixed seedlings were allowed to grow for another year, in which time ectomycorrhizal fungi could grow between seedlings. Finally, half of these seedling pairs were exposed to a simulated drought, over which time we will continue collecting growth and health data prior to a final harvesting.
Results/Conclusions
Over the course of the first year, soil treatments had significant effects on the growth and fungal colonization of seedlings, with field-inoculated soils leading to larger and healthier seedlings particularly in the Pseudotsuga seedlings. Ectomycorrhizal community composition varied significantly in Bray-Curtis diversity between field-inoculated soils and between species.
Seedlings planted into sterile soil had extremely low rates of fungal contamination when assessed under a microscope and with root tip sequencing. Prior to the onset of spring in the year following co-planting, the vast majority of differences in growth between seedling cohorts could be explained best by the initial soil conditions of the seedling irrespective of the neighboring seedling or that seedling’s soil history. The simulated drought has led to consistently significant differences in levels of soil moisture in the pots, which should provide an environmental disturbance sufficient to induce differences in seedling and fungal growth over the course of the experiment.
