Feedback effects between plants and soil are known to influence plant community composition and ecosystem function, but predicting the outcome of these feedback effects presents a challenge for plant ecologists. Frameworks of plant-soil feedback using mycorrhizal associations and plant phylogeny to predict these dynamics have recently emerged, but their efficacy requires testing in established plant communities. In this study, a combination of linear mixed-effect models and point pattern analyses of stem-mapped forest plots from an experimental forest in Northeast Ohio were used to look for evidence supporting mycorrhizal and phylogenetic frameworks of plant-soil feedback. Individual trees in 30-m radius plots were identified and categorized based on size (adult or juvenile). Point pattern analyses were used to test for spatial over-dispersion (negative feedback) or clustering (positive feedback) between adult and juvenile trees. The phylogenetic framework was tested using the phylogenetic mark correlation function. The mycorrhizal framework was tested by classifying tree species as arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) and aggregating point pattern analysis results for each category. Additionally, plots (communities) were assigned to a mycorrhizal category based on whichever mycorrhizal type accounted for >50% of the plot basal area, and point pattern analyses were also aggregated by community mycorrhizal type.
Results/Conclusions
Our forest plots ranged in their mycorrhizal dominance from 92% AM-dominant communities to 92% ECM-dominant communities. Our point pattern analyses found strong evidence of mycorrhizally-structured plant-soil feedback between juvenile and adult trees in these communities, but no evidence of phylogenetically-structured plant-soil feedback. AM-dominant communities reflected spatial patterns consistent with negative feedback processes influencing the distribution of juvenile trees around adult trees, while ECM-dominant communities demonstrated spatial patterns consistent with positive feedback processes occurring between juvenile and adult trees. Interestingly, these results were observed at the community level and not the individual species level, suggesting that plant-soil feedback effects can spill over from dominant community members and affect the structure of entire forested communities. Our results support the use of mycorrhizal-based frameworks for studying the mechanisms driving plant-soil feedback in temperate hardwood forests and how these relationships influence community dynamics in a changing world.