The mycorrhizal associations of forest trees drive soil carbon and nitrogen cycling. Forests dominated by ectomycorrhizal trees tend to have higher soil C:N and a higher proportion of particulate organic matter than those dominated by arbuscular mycorrhizal trees. However, the mechanisms behind these patterns are unclear. Across our forest sites in NH, IL, WI and GA, we find the expected patterns in soil organic matter only where forests are dominated by trees in the Pinaceae and Fagaceae, and the ectomycorrhizal fungal community is dominated by taxa with certain functional traits (ie, exploration type, melanin concentration and the potential to produce peroxidases).  This work has been published in Ecology. We then conducted a theoretical modeling experiment, incorporating mycorrhizal processes into the Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment (CORPSE) model. We found that ECM fungi suppress decomposition in temperate deciduous and boreal forests with relatively recalcitrant litter inputs, and with ECM fungi that produce oxidases and necromass-degrading enzymes. This work is published in Soil Biology and Biochemistry.  

We are now testing two mechanisms -- litter decomposability and mycorrhizal fungal function -- in field and growth chamber experiments. 

Collaborators: Caitlin Hicks Pries, Rick Lankau and Ben Sulman.

Funding: DOE Terrestrial Ecosystem Science

Temperate forests have experienced repeated disturbances, including pest and pathogen introductions, timber harvesting and agriculture. What role has nitrogen fixation played in the recovery these forests over the last century? In southern Appalachian forests, fixation is largely governed by a single N-fixing tree species (black locust; Robinia pseudoacacia), which declines rapidly after early succession. We analyzed long-term vegetation data from the Coweeta Hydrologic Laboratory and found that historical land use and high levels of N fixation favor the growth of arbuscular mycorrhizal trees over ectomycorrhizal trees. This work is published in the Journal of Applied Ecology, featured in UGA Today and discussed in The Applied Ecologist.

As part of this study we developed a framework for scaling symbiotic N fixation from trees to the landscape over succession, published in Journal of Ecology. MS student Sarah Ottinger conducted a greenhouse study, and found that N availability and light regulate symbiotic N fixation by black locust seedlings. This work is published in Oecologia. MS student Jessie Motes examined how historical N fixation driven by land use disturbance affects contemporary patterns in N cycling, microbial gene and the dominance of mycorrhizal trees. 

Collaborators: Chelcy Miniat, Katherine Elliott, Jessie Motes and Sarah Ottinger. 

Funding: USDA FS SRS, NSF: CWT LTER

The exclusion of fire from fire-adapted systems is a relatively recent human disturbance. How does long-term fire exclusion affect the resiliency of forests to the reintroduction of fire? We are answering this question in southern Appalachian forests, where fire exclusion has led to the accumulation of soil organic horizons. If fine roots of trees are concentrated in organic horizons they are vulnerable to consumption by fire, which may lead to delayed mortality if trees lack sufficient root biomass to acquire resources for growth. 

As a part of this project, MS student Dana Carpenter found that forests dominated by ectomycorrhizal trees had deeper organic soil horizons, greater fine root consumption and a higher probability of delayed tree mortality following wildfire compared to stands dominated by arbuscular mycorrhizal trees. Her research was published in Ecosystems. 

PhD student Melanie Taylor is exploring how mycorrhizal patterns in leaf litter decomposition, soil chemistry and soil fauna communities are influenced by the return of fire.

Collaborators: Joe O'Brien, Kevin Hiers, Louise Loudermilk, Mac Callaham, Dana Carpenter and Melanie Taylor 

Funding: USDA FS SRS

Ecosystems are experiencing novel disturbance regimes as a result of introduced disease, increased fire frequency and a warming climate. How do novel disturbance regimes affect the resiliency of the ecosystem? In Big Sur forests an exotic pathogen, Phytophthora ramorum, causes the emerging infectious disease Sudden Oak Death (SOD), which has lead to widespread tree mortality. Coincident with the emergence of SOD and severe drought, fire frequency and severity have increased across the landscape. We are monitoring the vegetation, pathogens, and soil nutrients of the long-term field plots to understand if disease and fire are changing forest assemblages and biogeochemical cycles, and to predict how the joint disturbances of fire and disease and will re-emerge across the landscape. 

Collaborators: David Rizzo, Allison Simler-Williamson, Kerri Frangioso, Richard Cobb and Ross Meentemeyer

Funding: NSF: PCE & ES and UGA Faculty Research Grant