Soil microorganisms can alter the growth, reproduction, and survival of trees, yet we have a limited understanding of how soil microorganisms affect the nature and functioning of entire forests. All tree species host fungi that grow inside root tissue in a symbiosis called “mycorrhiza”. Mycorrhizal fungi help trees acquire soil nutrients and resist infection by pathogens, while trees provide the mycorrhizal fungi with a carbon and energy source. Most trees can be categorized into one of two major types of mycorrhizal symbiosis, known as “arbuscular mycorrhizal” or “ectomycorrhizal” symbioses. Although the structures corresponding with these categories are belowground and microscopic, these different types of tree species tend to differ in ways that could have much broader consequences. These differences including nutrient demands, leaf and root chemistry, decomposability, carbon allocation and dependence on the mycorrhizal fungi, and protection from root pathogens.

The overarching goal of this project was to determine how the mycorrhizal type of dominant trees can have a “mycorrhizal spillover” effect on soil biogeochemistry and the soil microbiome, resulting in different stresses experienced by all members of the forest community. Given the differences between arbuscular mycorrhizal and ectomycorrhizal tree species, our study sought to explore how the mycorrhizal type of dominant tree species influences nutrient availability, pathogens, and the broader plant and soil microbiome, thereby affecting the growth and survival of other plants in the area.

We tested our ideas by sampling trees and soils in forests in New York, Ohio, and Indiana. In each area, we compared forest plots dominated by trees with arbuscular mycorrhizas to forests dominated by trees with ectomycorrhizas. We found that soils in ectomycorrhizal-dominated forests have decreased soil mineral nutrient availability, lower abundance and diversity of plant pathogens and arbuscular mycorrhizal fungi, and greater abundance and diversity of ectomycorrhizal fungi. These patterns were stronger than effects of nutrient fertilization, soil depth, or topographic position, but may depend in part on local climate, as differences between the mycorrhizal types were greater in warm and dry sites. We also conducted an experiment whereby tree roots were removed from small areas, and then allowed to grow back into soil under differing experimental conditions. Initial results indicate that arbuscular and ectomycorrhizal tree species forage in soil via different mechanisms. Ectomycorrhizal species had slower regrowth of root abundance, but traits indicating greater dependence on mycorrhizal fungal symbionts.

These differences in the soil microbiome, biogeochemistry, and root foraging help explain spatial patterns of tree diversity observed in our study and other locations. Arbuscular mycorrhizal-dominated forests tend to exhibit increased diversity and greater distances among adults of the same species, consistent with greater abundance of pathogens leading to rapid buildup of diseases that affect offspring near adult trees of the same species. Ectomycorrhizal-dominated forests tend to exhibit the opposite patterns, with lower diversity and smaller distances among adults of the same species, consistent with greater reliance on mycorrhizal fungi in soils with lower levels of available nutrients. Collectively, our findings help us understand how shifts in tree species composition can alter the soil environment in ways that can feedback to affect the functioning of whole forests, which are particularly important in the face of shifting species ranges and new combinations of species due to climate change, land-use change, and species invasions. This guides development of new conceptual models about how forests will respond to environmental change over ecological and evolutionary timescales.

Broader impacts of the project included the training of dozens of undergraduate students, graduate students, and lab personnel. In addition, we established and continued partnerships with Cuyahoga Valley National Park (Ohio) staff. The latter project was catalyzed by the establishment of long-term ecosystem restoration experiment (16 tree species planted at former mine sites to assess the effects of tree-mycorrhizal type on ecosystem recovery), and public engagement. The restoration project itself also led to education of multiple graduate students and new insights into restoration of damaged soil and vegetation. Other broader impacts included high school science teacher training (bringing climate change resources and curricular material into the classroom) via the Summer Science Institute, located at Indiana University.

Last Modified: 02/04/2024
Modified by: Chris B Blackwood