How do mycorrhizal fungi affect soil organic matter?
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 are now testing two mechanisms behind these patterns -- litter decomposability and mycorrhizal fungal function -- by combining field and growth chamber experiments with process-based model simulations of soil organic matter.
Collaborators: Caitlin Hicks Pries, Rick Lankau and Ben Sulman.
Funding: DOE Terrestrial Ecosystem Science
The legacy of nitrogen fixation decades after forest disturbance
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. The long-term effect of this transitory nitrogen fixation is unclear. If fixed N remains in the plant-soil system it may continue to facilitate forest biomass accretion long after N-fixers have declined. We are analyzing long-term vegetation data from the Coweeta Hydrologic Laboratory and quantifying nitrogen fixation across a forest chronosequence, to estimate historical N fixation and test for legacy effects in forest recovery.
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.
The return of fire after long-term exclusion
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.
Funding: USDA FS SRS
Are tree disease and fire changing forests of Big Sur California?
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.
Funding: NSF: PCE & ES and UGA Faculty Research Grant