research
overview
The McCarthy-Neumann Lab studies how plant–soil feedbacks shape forest regeneration, community composition, and long-term forest change. We combine greenhouse experiments, field studies, permanent forest plots, and spatial analyses to understand how trees modify their local soil environments and how those soil legacies influence future seedlings. Much of our current work focuses on oak–hickory forests of the Cumberland Plateau, where mature oak canopies can appear stable even as regeneration shifts toward more mesophytic, arbuscular-mycorrhizal species.
Current Research Program
Long-term forest change in upland oak–hickory forests
Long-term permanent plots on the Cumberland Plateau allow us to ask how mature oak–hickory forests have changed over nearly five decades. In the project “Tree Compositional Change in Upland Hardwood Forests Aligns with Mycorrhizal Type,” we examine changes in forest biomass, species composition, size-class structure, and functional traits, with particular attention to mycorrhizal type, fire tolerance, and shade tolerance.
This work shows that oak–hickory forests can continue accumulating biomass while the regeneration layer shifts toward different functional groups. By comparing canopy trees, small stems, and seedlings, we ask whether current forest structure is masking a future transition away from ectomycorrhizal, fire-adapted species such as oaks and hickories.
Long-term forest change in upland oak–hickory forests
Long-term permanent plots on the Cumberland Plateau allow us to ask how mature oak–hickory forests have changed over nearly five decades. In the project “Tree Compositional Change in Upland Hardwood Forests Aligns with Mycorrhizal Type,” we examine changes in forest biomass, species composition, size-class structure, and functional traits, with particular attention to mycorrhizal type, fire tolerance, and shade tolerance.
This work shows that oak–hickory forests can continue accumulating biomass while the regeneration layer shifts toward different functional groups. By comparing canopy trees, small stems, and seedlings, we ask whether current forest structure is masking a future transition away from ectomycorrhizal, fire-adapted species such as oaks and hickories.
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McCarthy-Neumann et al., 2026
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Neighborhood effects, density dependence, and oak recruitment
A second part of our current research asks how the identity and mycorrhizal type of nearby adult trees influence sapling recruitment. We are using spatial analyses of mapped forest plots to test whether saplings recruit near or away from conspecific adults and whether those patterns are shaped by shared mycorrhizal associations. These projects examine conspecific density dependence, con-mycorrhizal density dependence, and the possibility that adult trees create microbial or resource environments that favor some seedlings while limiting others.
We are also testing how abiotic context alters these relationships. Light, soil moisture, and soil resources may change whether neighborhood effects are negative, neutral, or positive. To connect spatial patterns with mechanism, we are developing a field transplant experiment focused on Quercus montana seedlings planted beneath canopy trees representing three mycorrhizal types: Quercus montana (EM associated), Acer rubrum (AM associated), and Oxydendrum arboreum (ErM associated). Seedlings are planted in control and fungicide-treated plots under relatively low- and high-light environments to test how canopy-tree mycorrhizal identity, pathogens, and light availability interact to affect oak seedling establishment and performance.
A second part of our current research asks how the identity and mycorrhizal type of nearby adult trees influence sapling recruitment. We are using spatial analyses of mapped forest plots to test whether saplings recruit near or away from conspecific adults and whether those patterns are shaped by shared mycorrhizal associations. These projects examine conspecific density dependence, con-mycorrhizal density dependence, and the possibility that adult trees create microbial or resource environments that favor some seedlings while limiting others.
We are also testing how abiotic context alters these relationships. Light, soil moisture, and soil resources may change whether neighborhood effects are negative, neutral, or positive. To connect spatial patterns with mechanism, we are developing a field transplant experiment focused on Quercus montana seedlings planted beneath canopy trees representing three mycorrhizal types: Quercus montana (EM associated), Acer rubrum (AM associated), and Oxydendrum arboreum (ErM associated). Seedlings are planted in control and fungicide-treated plots under relatively low- and high-light environments to test how canopy-tree mycorrhizal identity, pathogens, and light availability interact to affect oak seedling establishment and performance.
McCarthy-Neumann, unpublished data
Plant–soil feedback legacies shaping oak regeneration
Plant–soil feedbacks may persist after an adult tree dies or is removed, creating belowground “legacy effects” that continue to influence seedling establishment. This project asks how long soil legacies last after tree mortality or harvest and whether legacy duration differs between ectomycorrhizal oaks and arbuscular-mycorrhizal maples.
We utilize nearly 50-years of long-term forest data to investigate the effect of natural mortality and greenhouse experiment to test whether soils formerly associated with oaks continue to support oak seedling survival and growth after canopy removal, and whether maple-associated soils lose negative feedbacks after adult tree removal.
Plant–soil feedbacks may persist after an adult tree dies or is removed, creating belowground “legacy effects” that continue to influence seedling establishment. This project asks how long soil legacies last after tree mortality or harvest and whether legacy duration differs between ectomycorrhizal oaks and arbuscular-mycorrhizal maples.
We utilize nearly 50-years of long-term forest data to investigate the effect of natural mortality and greenhouse experiment to test whether soils formerly associated with oaks continue to support oak seedling survival and growth after canopy removal, and whether maple-associated soils lose negative feedbacks after adult tree removal.
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Fire, mycorrhizal associations, and plant–soil feedbacks
Prescribed fire is a major tool for restoring oak–hickory forests, but we know less about how fire alters the soil microbial legacies seedlings experience. This project asks whether prescribed fire changes the direction or strength of plant–soil feedbacks for tree species with different mycorrhizal associations.
We are focusing on Quercus alba and Acer rubrum to test how seedling survival and growth differ in conspecific versus heterospecific soils and whether these responses vary with time since prescribed fire. If fire temporarily reduces microbial biomass, pathogens, or mycorrhizal fungi, then both positive and negative feedbacks may become more neutral shortly after burning. This work will help determine whether fire changes belowground conditions in ways that favor or hinder oak regeneration.
Prescribed fire is a major tool for restoring oak–hickory forests, but we know less about how fire alters the soil microbial legacies seedlings experience. This project asks whether prescribed fire changes the direction or strength of plant–soil feedbacks for tree species with different mycorrhizal associations.
We are focusing on Quercus alba and Acer rubrum to test how seedling survival and growth differ in conspecific versus heterospecific soils and whether these responses vary with time since prescribed fire. If fire temporarily reduces microbial biomass, pathogens, or mycorrhizal fungi, then both positive and negative feedbacks may become more neutral shortly after burning. This work will help determine whether fire changes belowground conditions in ways that favor or hinder oak regeneration.
Student-led and Collaborative Projects
Plant–soil feedbacks in mountain laurel thickets
Student-led research in the lab examines how dense mountain laurel (Kalmia latifolia) thickets influence hardwood regeneration. This project combines spatial analyses of a permanent forest plot with greenhouse experiments testing whether Kalmia affects oak recruitment through plant–soil feedbacks, microbial effects, and interactions with light.
This work asks whether Kalmia thickets suppress oak regeneration directly through low-light conditions, indirectly through soil microbial legacies, or through both mechanisms. It also compares responses of Quercus alba with Oxydendrum arboreum to ask whether ericaceous-conditioned soils broadly alter regeneration patterns by mycorrhizal type.
Student-led research in the lab examines how dense mountain laurel (Kalmia latifolia) thickets influence hardwood regeneration. This project combines spatial analyses of a permanent forest plot with greenhouse experiments testing whether Kalmia affects oak recruitment through plant–soil feedbacks, microbial effects, and interactions with light.
This work asks whether Kalmia thickets suppress oak regeneration directly through low-light conditions, indirectly through soil microbial legacies, or through both mechanisms. It also compares responses of Quercus alba with Oxydendrum arboreum to ask whether ericaceous-conditioned soils broadly alter regeneration patterns by mycorrhizal type.
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Schutte, et al., in press
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Long-term effects of dogwood anthracnose in Appalachian forests
Collaborative work with researchers at The University of the South examines the long-term effects of dogwood anthracnose on flowering dogwood mortality and resilience in Appalachian forest communities. Flowering dogwood is an important subcanopy species that contributes to nutrient cycling and provides fruit resources for wildlife, but dogwood anthracnose caused severe declines across parts of the southern Appalachians.
This project revisits historical forest plots to ask whether dogwood has persisted, declined, or recovered across contrasting Appalachian forest habitats. By comparing older surveys with contemporary censuses, we are assessing how disease effects vary with forest environment and whether dogwood populations remain resilient in some portions of the landscape.
Collaborative work with researchers at The University of the South examines the long-term effects of dogwood anthracnose on flowering dogwood mortality and resilience in Appalachian forest communities. Flowering dogwood is an important subcanopy species that contributes to nutrient cycling and provides fruit resources for wildlife, but dogwood anthracnose caused severe declines across parts of the southern Appalachians.
This project revisits historical forest plots to ask whether dogwood has persisted, declined, or recovered across contrasting Appalachian forest habitats. By comparing older surveys with contemporary censuses, we are assessing how disease effects vary with forest environment and whether dogwood populations remain resilient in some portions of the landscape.
Broader Research Foundations
Previous and continuing work in the lab has examined plant–soil feedbacks as mechanisms maintaining tree species diversity, linking soil microbes with tree seedling shade tolerance, shaping the persistence of soil legacies after tree mortality or harvest, and influencing climate-driven range shifts and invasion dynamics. Across these projects, we use mechanistic greenhouse and field experiments to identify the processes underlying community-level patterns in forest ecosystems.
Previous and continuing work in the lab has examined plant–soil feedbacks as mechanisms maintaining tree species diversity, linking soil microbes with tree seedling shade tolerance, shaping the persistence of soil legacies after tree mortality or harvest, and influencing climate-driven range shifts and invasion dynamics. Across these projects, we use mechanistic greenhouse and field experiments to identify the processes underlying community-level patterns in forest ecosystems.