by Steven N. Koppes
photographs by James Strawser
Something like the seven biblical plagues has been visited upon the Coweeta Hydrologic Laboratory study area in western North Carolina.
In recent decades drought, insect outbreaks, plant disease and even a hurricane have afflicted the Coweeta watershed just three miles north of the Georgia border. But UGA and other researchers have discovered that these relatively rare, seemingly damaging events actually replenish the ecosystem with important nutrients.
Take, for example, the fall cankerworm outbreak of the early 1970s.
"It's called the fall cankerworm because the adults fly in the fall, but the larvae attack in early spring," said D.A. Crossley, a UGA professor emeritus of ecology. Crossley has conducted research at Coweeta since the late 1960s. He saw the fall cankerworm defoliate up to half the trees on a high-elevation Coweeta watershed.
The insects did kill some trees. Most trees, however, managed to sprout a healthy canopy of new leaves, thereby mobilizing nutrients stored in their roots, stems and twigs.
We increased nutrient cycling on the watershed as a result of that defoliation," Crossley said.
"The increased amount of nutrients that are released are mobilized again and spark that growth. This puts insects in a whole different light. Do you really want to control insects if they are actually going to produce increased growth in the forest?" Crossley asked.
Such questions about the response of forest and stream to natural and human-induced ecological disturbances can only be answered following years, even decades, of research. And that is the purpose of Coweeta's Long-Term Ecological Research (LTER) Program, a cooperative effort between UGA and the USDA Forest Service.
The Forest Service, which manages Coweeta, began conducting a steady stream of research there in 1934.
Early on, the Forest Service documented the harmful effects of land-use practices such as mountain farming, grazing and unrestricted logging on water flow in the forest. When UGA joined the effort in 1968 with support from the National Science Foundation, a new tributary of ecosystem data joined the mainstream of research at Coweeta. NSF's support has continued ever since, contributing to Coweeta's record as the longest continuously monitored landscape in North America.
Nature's perfect laboratory
Research at Coweeta today encompasses collaborations among 55 graduate and undergraduate students and 29 senior researchers from UGA, the Forest Service, Virginia Polytechnic Institute and State University, Duke University, University of Minnesota, Mars Hill College, University of Wisconsin at Madison and Portland State University. Much of the work focuses on the Coweeta basin itself, but the regional study area includes parts of western North Carolina, northern Georgia, eastern Tennessee and southern Virginia.
The bowl-shaped Coweeta basin covers 8.4 square miles of the Nantahala National Forest. The basin ranges in elevation from 2,250 feet at the Forest Service headquarters to the 5,250-foot peak of Albert Mountain a few miles distant. Enough water flows through the basin to supply a city of 30,000 people. Coweeta's waters flow steady and clear but will continue to do so, decades of research indicate, only if the basin's ecology remains in chemical balance.
Coweeta's geology and its rainfall, which varies from 71 to 94 inches annually across its elevations, make it a perfect natural laboratory for studying how the forest uses water. Impervious bedrock underlies the basin, so researchers could install weirs - water flow measuring devices - to precisely record forest water use. Would a pine forest use more water than a hardwood forest under similar circumstances? An ongoing study provides conclusive evidence that the answer is yes.
But the area's extraordinary biodiversity makes it an equally ideal setting for undertaking broader ecological research projects.
"The southern Appalachians are what we call a hotspot for biodiversity," Coleman said. The Coweeta basin especially abounds in soil mites, which feed on fungus, decaying leaf litter and nematodes. UGA graduate student Randi Hansen has found more than 160 soil mite species on a research plot measuring approximately 20 by 40 yards.
"That's a higher number than has been recorded from most any tropical site," Coleman said. "You always hear of the high species richness in the tropics. It's gospel. But it's not that simple."
Coweeta's soil mite diversity astounds even Crossley, who began studying mites five decades ago. "As ecologists, we know that many species shouldn't be able to co-exist on the same resource," he said.
Before joining the UGA faculty in 1967, Crossley conducted ecological research in a radioactive drainage system at Oak Ridge National Laboratory in Tennessee. Radioactive materials had been released into the environment at Oak Ridge. "We had wondered, how do you tell when an ecosystem is sick?" he said. "If it's running a fever, where do you stick the thermometer?"
Then came a landmark paper published in the journal Science by Yale University's Herb Bormann and Gene Likens. Bormann, formerly of Duke University, had visited Coweeta many times. "He told me once that Coweeta was the inspiration for his work on the watersheds up at Hubbard Brook in New Hampshire, where he and Likens did their outstanding work," Crossley said.
Bormann and Likens showed how elements such as calcium cycled within the watershed. Trees take up calcium and other elements from the forest floor. The calcium returns to the forest floor after the leaves drop and decompose.
"About 95 percent of the calcium involved in that cycle continued to cycle," Crossley said. "About 5 percent was lost each year. This was made up by erosion from bedrock and by atmospheric deposition. When the forest is disturbed - by cutting, by acid precipitation, something like that - these cycles are altered."
How ecosystems work
Initially, project scientists set out to measure ecological productivity - the amount of energy available through photosynthesis - and its controlling factors in a variety of environments worldwide. But as the program developed, scientists began to address a broader question: How do ecosystems work?
"The project was important for us because we learned a lot about how to do scientific research in that magnitude," Crossley said. "Most of us were familiar with one- or two-year studies of perhaps two people. These were teams of ecologists, which was not an entirely new idea, but at this level it was certainly new."
Research teams were necessary because of the wide range of expertise needed to fully examine the workings of an ecosystem. The specialties of the scientists who have overseen Coweeta's research over the years hint at the project's interdisciplinary scope.
First, during the International Biological Program years, came botanist Carl Monk, now retired. When the IBP gave way to the Long-Term Ecological Research Program in 1980, Crossley, a soil mite specialist, headed the UGA contingent. Crossley handed the reins to stream ecologist Judy Meyer in 1990, and she to David Coleman, a microbial and soil ecologist, in 1996.
Wayne Swank, a hydrologist and ecologist, served as the Forest Service research leader from 1966 to 1996. James Vose, who specializes in forest ecology, succeeded Swank.
Together they and their colleagues monitor research sites throughout the Coweeta watershed. Some sites are devoted to streamside habitats, which ecologists refer to as riparian zones. Other sites focus on terrestrial processes that occur at low, intermediate and high elevations, respectively. Some sites are left undisturbed, while others are experimentally manipulated in some way.
Natural and man-made disturbances
Experimental treatments have ranged from clearcuts and prescribed burning to excluding leaf litter or woody debris from stream beds with netting to record their effect on nutrient flow.
Researchers initiated a commercial clearcut experiment in a mixed hardwood section of Coweeta forest in 1976. Most trees on the watershed, including oaks, hickories, maples, black locusts and pines, were cut to see how their removal would affect forest floor nutrient dynamics.
The idea was to remove the trees without disturbing the forest floor. The Forest Service made this possible by developing a technique to raise and remove the logs by cable. "Normally, they'd be skidded across the forest floor," Crossley said. "It was our thesis that leaving the forest floor intact would reduce the loss of nutrient capital from clearcutting."
Mother Nature complicated the experiment by throwing in a couple of unexpected experimental parameters. First came unusually heavy rains, followed by an unusually severe drought. But the researchers' thesis proved to be largely correct.
Today, the clearcut watershed sports a lush stand of trees 30 to 40 feet tall and a diverse collection of plants on the forest floor. In fact, the young forest harbors more abundant species than do undisturbed areas. A book-length manuscript, now in preparation, will describe these results.
Unfortunately, Coleman said, the clearcut experiment could not be repeated today. The Forest Service has banned clearcutting. Crossley disagrees with the policy.
"Small clearcuts, not massive ones, are probably the best way to harvest forests in the southern Appalachians because there is such rapid regrowth from stump sprouts and root sprouts," he said. "The public does not want it, however, so we don't do it."
A bigger ecological threat to Coweeta blows in from metropolitan Atlanta. The Coweeta watershed sits within the atmospheric umbrella of Atlanta, which produces a huge volume of nitrogen oxides, sulfur oxides and other air pollutants. "We're like a big sponge sitting here absorbing these oxides," Coleman said.
Coweeta's older forests appear to have entered the initial stages of nitrogen saturation due to forest maturation and increasing input of atmospheric nitrogen. Coweeta researchers wondered what effect that would have in future decades on the vital microbial processes taking place on the forest floor.
"One of our LTER collaborators in southern Minnesota has found some net losses in biodiversity when you heavily fertilize some grassland plots," Coleman said. "We have to understand natural purification processes and what would happen if we get our system saturated."
Nitrogen comprises four-fifths of Earth's atmosphere. Like oxygen, which makes up most of the other 20 percent of the atmosphere, nitrogen is essential to life on Earth. Nitrogen serves both as food and fertilizer, but too much of the element, especially in the form of nitrates, can slow plant growth and make drinking water unsafe for humans.
"Stream nitrate concentration is a very sensitive indicator of forest ecosystem disturbance. It's the bottom line," Swank said.
And Coweeta's pure waters make nitrate leakage easier to detect there than at other locations, Crossley said.
"In the Northeast today, with as much nitrogen deposition as they have had, they couldn't see it. It would not be above the background level," he said.
Both natural and unnatural disturbances can result in stream water export of nitrates. An outbreak of locust stem-borers killed a stand of Coweeta's locust trees in 1979. The dead trees released a store of nitrogen from their roots, leaves and stems, increasing stream nitrate concentration by 300 parts per billion. "Other vegetation took up much of the nitrogen," Crossley said. "At the same time, much of it reached the ground and we detected it in the stream water."
Researchers also measured nitrate export increases in an area damaged by Hurricane Opal, which blew through North Carolina, including Coweeta, in October 1995. The hurricane hit exposed sites at high elevations, where the trees were poorly anchored in relatively thin soils.
Opal even damaged one of the Coweeta study sites, throwing a stand of oak and poplar trees downhill. The downed trees opened a research opportunity along with a large hole for rainwater in the forest canopy. The disturbance produced a nitrate leakage of 50 to 100 parts per billion - about the same concentration that Swank has measured following a forest harvest. Only 3 parts per billion were recorded before the hurricane.
Coweeta researchers are still recording a considerable concentration of nitrates draining into the groundwater through this gap. "This is a fascinating thing for people who like to think about infrequent events," Coleman said. "The last recorded hurricane damage in that part of western Macon County in North Carolina was in the 1830s."
Humans complicate the future
"I'm well-aware, now, of the importance of gap formation in the forest," Crossley said. Trees are struck by lightning, killed by insects or knocked down by hurricanes, opening gaps in the forest. The gaps allow new successions of plants to emerge. The relative abundance of species changes continually over the years for all kinds of reasons.
Half a century ago, the American chestnut trees dominated the region's forests. Then came the chestnut blight, a pathogenic fungus imported from Japan, which all but wiped them out. But the near extinction of the American chestnut has allowed many other species to flourish instead.
"We lost the chestnuts to disease," Crossley said. "We call these natural forests, but the major tree species is gone. What's undisturbed about the undisturbed part of our controls? I think it's safe to say there aren't any natural areas left at Coweeta," he said. "There are only some less-disturbed areas."
Increasingly, humans cause these disturbances, Coleman said.
"With all these [people] coming into the area, and that includes me coming into this area 14 years ago, we're going to have to try to somehow lessen our impact," he said. The alternative is to continue participating in a huge, uncontrolled ecological experiment. What would happen if, say, the human species burned enough fossil fuels to dramatically increase the carbon dioxide composition of the atmosphere?
Earth is a life-support system, not a marketplace. Organisms don't get to choose what gases to breathe if the supply of their favorite ones comes up short.
"Folks, we're talking oxygen and carbon dioxide. We've got some basic physiological processes going on here," Coleman said. "We're not going to substitute for them."
Steven N. Koppes, an award-winning writer, is the former UGA assistant director of research communications and associate editor of Research Reporter. He has a bachelor's degree in anthropology and a master's in journalism and is a science writer at the University of Chicago.