“A Balancing Act: Fire and Biological Nitrogen Fixation in the Longleaf Pine Ecosystem"  by Dr. Nina Wurzburger

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. Biological nitrogen fixation (BNF) can replenish N losses, but whether and how BNF balances fire-induced N losses in frequently-burned longleaf pine has remained unknown. Across 54 1-hectare plots of longleaf pine at Fort Benning and Eglin Air Force Base representing a 227-year gradient of stand recovery, we quantified N losses from fire, patterns of N demand and availability, and quantified BNF by legumes, soil crusts, and asymbiotic bacteria. We found surprisingly low rates of BNF. Although individual fire events temporarily stimulated BNF by legumes, BNF was insufficient to balance N losses from fire and soil N stocks declined over stand age. Progressive N loss from the ecosystem may signal a decline in resiliency and present a long-term concern for land managers. 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.

“Exploiting Theory to Guide Practice: Using Mechanistic Models to Streamline Monitoring and Inform Longleaf Pine Management on Department of Defense Properties”  by Dr. Joseph O'Brien

Longleaf pine (Pinus palustris) ecosystems are remarkably rich in plant species and represent the dominant upland forest type in several southeastern military installations. Management of these forests on installations is critical both to fulfill the military mission and to conserve this unique natural resource. The project involved field experiments and data mining exercises designed to help managers meet their objectives for monitoring the impact of various activities on the understory plant community. Project results will also aid the development of modeling tools to help evaluate different management scenarios based on the intimate link between overstory structure, fire, and understory plant diversity. The project goals were as follows: (1) understand how the accuracy and effectiveness of sampling and monitoring programs are affected by the scale and timing of measurements, (2) increase plant sampling efficiency and efficacy by identifying and developing statistical approaches for dealing with complex spatial patterns of species distributions, and (3) use state-of-the-art technology to understand mechanisms driving patterns of plant and insect diversity and link this information to management relevant scales. The overarching goal was to develop an understanding of how these important forests function in order to predict the outcomes of various fire- and stand-management practices relative to understory diversity. In this way the project will help maximize the effectiveness of fire, timber, and biological diversity management on military bases.

Dr. Nina Wurzburger is an Associate Professor in the Odum School of Ecology at the University of Georgia. She is an ecosystem ecologist who studies how nutrients such as nitrogen, phosphorus, and molybdenum affect the cycle of carbon. Nina has a keen interest in plant-soil relationships – specifically, how microbial symbioses regulate nutrient access to plants and the ecosystem. Her research spans the globe from tropical forests to arctic tundra. Nina served as a Principal Investigator for a SERDP project that studied the nitrogen cycle of longleaf pine savannas to determine if and how biological nitrogen fixation replenishes the nitrogen lost from fire and land-use disturbances. This SERDP project is the first to quantify the nitrogen balance of recovering longleaf pine stands over a gradient of age (227 years) and fire frequency. Prior to her appointment at the University of Georgia, Nina was a postdoctoral associate at Princeton University and the Smithsonian Tropical Research Institute. She earned a bachelor’s degree in environmental science and a master’s degree in soil science from the University of California at Davis. She earned a doctorate in forest resources from the University of Georgia.

Dr. Joseph O'Brien is a Research Ecologist and the Fire Team Leader with the Center for Forest Disturbance Science of the United States Department of Agriculture (USDA) Forest Service Southern Research Station in Athens, Georgia. Since joining the Forest Service in 2002, his research has centered on fire science, specifically the spatial interactions among wildland fuels, fire behavior, and fire effects. He also is focused on the impacts of fire exclusion on ecosystem function and how to best restore fire into long-unburned fire dependent ecosystems. Joe’s graduate training and prior work experience have given him expertise in plant ecophysiology, plant ecology and forest management. He has global experience in fire ecology with active research in the Caribbean, Central America and Africa. He has worked as an ecologist in the southeastern United States for more than 30 years. He received a bachelor’s degree in biology from State University of New York (SUNY) Geneseo in 1986, and his master’s and doctoral degree in biological sciences from Florida International University in 1997 and 2001, respectively.