PS 13-172 - Can the importance of nutrient limitation and pathogen damage on root growth and traits be predicted by the mycorrhizal dominance of forest plots?

Abstract: In temperate deciduous forests, trees generally develop either arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) associations with fungi, thereby creating a patchwork of AM and ECM stands. This project explores whether resource limitation or pathogen damage assumes greater importance in determining plant community composition and if this can be predicted by mycorrhizal dominance. Because AM soils are usually nutrient-rich and may be plagued by pathogens, we predict that root growth in AM stands is limited by plant pathogen activity, which will be alleviated by fungicide. Conversely, ECM soils tend to be nutrient poor but harbor fewer pathogens, so we predict that root growth in ECM stands is limited by nutrient availability, which should be alleviated with fertilizer. To test this hypothesis, we installed root in-growth cores in the ground and filled them with soil, with one third receiving a fertilizer treatment, one third receiving a fungicide treatment, and one third receiving water as a control. Treatments were applied monthly during the growing season in 48 forest plots dominated by either AM or ECM trees across three states (OH, IN, NY). After 1-2 growing seasons, the cores were harvested, and root biomass measured as an indicator of root proliferation, as well as a suite of root traits.

Overall, fine root responses to the different treatments were site-specific. At the Ohio site, fertilizer increased root growth in ECM plots (relative to controls) whereas fungicide elicited a stronger response from roots in AM plots – consistent with our hypotheses. At the other sites, fertilizer and fungicide had limited effects. Overall, alleviation of these two common limitations – nutrient impoverishment and pathogen pressure - failed to consistently stimulate root growth. On the other hand, mycorrhizal dominance of a forest plot appears to be consistently correlated with a suite of root traits. Root diameter was significantly greater in AM plots, while specific root surface area, root tissue density, specific root tip abundance and branching interval were significantly greater in ECM plots. None of the root traits showed consistent responses to the fertilizer and fungicide treatments, suggesting that interactions of mycorrhizal dominance and mechanisms of limitation are more complex than anticipated. An important next step is to identify the root species identities and separate community root trait responses from species-level responses to treatments. Continued progress will broaden our understanding of forest responses to environmental change, including species’ range shifts, nutrient enrichment and cycling, and terrestrial C modeling.