Athens, Ga. – The longleaf pine forests that once dominated the Southeastern coastal plain are now considered one of the most endangered ecosystems in North America. Once covering 90 million acres from Virginia to Texas, longleaf now is found in only about 3 percent of its historic range. Many of those remaining acres are on U.S. military installations, where land managers are tasked with keeping the forests healthy while meeting their primary objective of supporting military readiness.
To help the U.S. Department of Defense accomplish these dual goals, University of Georgia ecosystem ecologist Nina Wurzburger is leading a study to explore how the soil-based process of nitrogen fixation facilitates the tree’s recovery from physical disturbances, including military training exercises and fire. The five-year project is supported by a $1.39 million grant from the Strategic Environmental Research and Development Program of the Department of Defense in partnership with the U.S. Department of Energy and the U.S. Environmental Protection Agency.
According to Wurzburger, understanding the natural processes of recovery in longleaf ecosystems is important for many reasons. One of those is conserving biodiversity: Longleaf forests harbor a number of endangered and rare species of animals and plants and store a significant amount of carbon.
“Longleaf pine forests serve as a strong carbon sink for the region,” said Wurzburger, an assistant professor in the Odum School of Ecology. “And nitrogen fixation provides a continual supply of new nitrogen to the forests, keeping them highly productive.”
Nitrogen fixation—the process by which soil microorganisms take nitrogen from the atmosphere and convert it into usable forms for plants—enhances pine growth, allowing them to keep more carbon dioxide out of the atmosphere.
The study also is important because longleaf forests need management. The longleaf ecosystem depends on periodic fire; without it, fire-intolerant species soon take over.
“These forests require a particular burning frequency,” Wurzburger said. “But there’s a delicate balance between burning enough to help longleaf grow and burning too much and depleting the soils of nitrogen.”
Fire, though necessary, removes nitrogen from soil and places it back in the atmosphere. Fortunately, longleaf forests contain plenty of nitrogen fixers that can replenish these losses. These include leguminous plants like indigo, partridge pea and prairie clover that associate with nitrogen-fixing bacteria in their roots; soil crusts—thin layers of lichen, algae, mosses and bacteria that cover the open ground; and certain free-living bacteria in the soil. Training conducted at military installations can impact these nitrogen fixers, however.
“There hasn’t been any systematic examination of nitrogen fixation, what controls it and how fire and other disturbances influence it,” Wurzburger said. “Longleaf pine forests are ideal systems for understanding these processes.”
Wurzburger and her colleagues will establish a series of 2.5-acre plots at Fort Benning in Georgia and Eglin Air Force Base in Florida, representing different combinations of fire frequencies and levels of military training impact. They then will measure the rates of nitrogen fixation, with particular attention to how the types of nitrogen fixers respond to the disturbance regimens, and determine whether nitrogen inputs are sufficient to replenish losses from disturbance.
They also will conduct experiments to determine how the combination of different soil nutrients affects nitrogen fixation.
“We know from previous work in the tropics that other nutrients—phosphorus and molybdenum—play a role in regulating nitrogen fixation,” Wurzburger said. Understanding how disturbance influences the interaction of all these nutrients will fill an important knowledge gap for ecologists and land managers alike.
“What we learn will help land managers understand soil ecology, particularly nitrogen fixation, as a natural mechanism for recovery in longleaf ecosystems,” she said. “It will help guide better management practices for conservation of these last remaining longleaf stands. So the benefit of this project is really threefold: to protect endangered ecosystems, help maintain the carbon balance in the region and allow these forests to play a role in military training.”
Wurzburger’s fellow investigators include Robert Mitchell, senior scientist at the Joseph W. Jones Ecological Research Center, and Lars Hedin, professor of terrestrial biogeochemistry at Princeton University.