The legacy of nitrogen fixation decades after forest disturbance
Temperate forests have experienced repeated disturbances, including pest and pathogen introductions, timber harvesting and land-use change. 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 (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, but few datasets offer a way to test this idea. 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.
The return of fire after a century of exclusion
The exclusion of fire from fire-adapted systems is a relatively recent anthropogenic perturbation. 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, and recent wildfires have consumed this organic matter. 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 is examining whether the mycorrhizal identity of trees and their effects on soil carbon and nitrogen cycling helps predict organic matter depth and the predisposition for fine root consumption caused by wildfire. Her research was published in Ecosystems. Continuing with this research, PhD student Melanie Taylor is examining how tree mycorrhizal associations and fire affect leaf-litter decay, soil chemistry and soil fauna communities.
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
Does nitrogen fixation facilitate biomass gains in tropical forests?
Wet tropical forests are linchpins in the global carbon cycle, removing more atmospheric CO2 annually than any other terrestrial biome. The extent to which soil nitrogen acts to constrain this carbon sink is unclear. We are analyzing long-term data and conducting research in Trinidadian forests where Pentaclethra macroloba, a leguminous N-fixing tree, accounts for ~30% of the stems in the forest. We are quantifying N fixation and analyzing stem growth across permanent forest plots to understand how N fixation is regulated at the tree level and whether fixed N at the plot level is facilitating a long-term pattern in biomass gain among non-fixing trees. Our work is published in Scientific Reports.
A balancing act : fire and nitrogen fixation in the longleaf pine ecosystem
Fire is a critical force in maintaining the structure and diversity of longleaf pine ecosystems, but it also removes a substantial amount of nitrogen (N), which can limit plant growth. N fixation can replenish N losses, but whether and how fixation balances fire-induced N losses in frequently-burned longleaf pine has remained unknown. We quantified N fixation and estimated N loss from fire across plots of longleaf pine at Fort Benning and Eglin Air Force Base. We found that fixation was insufficient to balance N losses from fire. Progressive N loss from the ecosystem may signal a decline in resiliency. An alternative possibility is that longleaf pine ecosystems have accumulated excess N as a result of land-use change and N deposition. In this case, fire may be a relief mechanism for excess N, critical for returning the ecosystem to its N-poor state. This work is published in Ecology.
As a part of this project, MS student Mike Ament examined how N fixation by the diverse community of herbaceous legumes associated with longleaf pine is regulated by soil nutrients. He found that phosphorus limits growth and fixation rates of all legumes, but there are substantial differences in fixation rates across species. His work was published in Oecologia.
Mycorrhizal fungi as drivers and modulators of ecosystem processes
The distribution of mycorrhizal associations across biomes parallels a distinct gradient of soil carbon and nitrogen stocks, which raises the question of whether mycorrhizal traits confer ecosystem properties. We examined how arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree seedlings and their fungi differ in strategies for nitrogen acquisition using isotopically-labeled organic matter in lab mesocosms. We found that AM and ECM strategies for N can manifest in differences in soil carbon pools and soil C:N. Our work is published in Ecology.
To test this idea across temperate forests, we analyzed FIA data to determine if forest mycorrhizal identity explained patterns in soil carbon and nitrogen, after accounting for potential effects of climate, phylogeny and leaf traits. We found that increasing ECM dominance was associated with higher soil C:N, which was explained by differences in soil N rather than soil C. This work was published in Journal of Ecology, along with other studies on mycorrhizal fungi and ecosystems in a Special Feature. This work also motivated an oral session for the ESA 2016 meeting, and a meeting report for New Phytologist.
In a related study, MS student Melanie Taylor sought to understand if mycorrhizal fungi have indirect effects on organic matter decomposition. She crossed soils derived from four AM and four ECM tree species with leaf and root litter originating from these species and monitored soil CO2 efflux in a laboratory microcosm experiment. Matching the mycorrhizal identity of litter and soil resulted in a difference in total respiration, such that combinations of AM litters with AM soils lost more C than did combinations of ECM litters with ECM soils. Her work was published in Journal of Ecology.
Funding: UGA Faculty Research Grant, NSF: CWT LTER