Non-native forest insects and pathogens (henceforth “pests”) cause significant damage to forests across the US. It is hypothesized that native tree richness is related to pest invasion, but the effect of mycorrhizal associations with tree species on pest invasion has not been investigated. Two main types of mycorrhizal fungi—arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi—vary in their rates of nutrient cycling. AM trees tend to have higher nutrient availability in both surrounding plant soils and plant tissues than EM trees, which could mediate pest invasions. We combined mycorrhizal association data from over 130,000 Forest Inventory and Analysis (FIA) plots with county-level pest richness data from across the USA to test the hypothesis that higher nutrient availability in AM associated hosts leads to increased pest richness in forests dominated by AM trees. We conducted this study in the context of several known drivers of invasion (e.g. propagule pressure, habitat composition) using spatially auto-regressed models and Chi-squared contingency table tests.
Results/Conclusions
We did not detect a significant relationship between mycorrhizal associations and pest richness in our analysis. Drivers of pest richness varied by guild and region. Across all guild-specific analyses, AM basal area did not influence pest richness. When analyzing richness of all pests, however, we found significant, positive relationships between pest richness and EM basal area in the eastern US and between pathogen richness and EM basal area in the western US. The results of the Chi-squared tests indicated that a significantly higher proportion of pests across all guilds except sap suckers feed on EM associated hosts. Our results indicate that factors other than mycorrhizal associations drive pest richness, but that abundance of EM trees might have context-specific influences. Inclusion of additional mycorrhizal variables (e.g., soil/nutrient availability, richness) in future analyses may elucidate these complex relationships and allow a better understanding of how nutrient dynamics related to mycorrhizal associations play a role in forest pest invasions.