FOREST DIRE
The concept is in textbooks and economic models: Central and South American woodlands are reservoirs that absorb and store carbon dioxide. Is it time to rewrite the scenario?
The sewing machine, it turns out, is one small piece of a major scientific undertaking to examine the relationship between tropical rain forests and global warming. McGlynn and several colleagues in the Ciclos Project in Costa Rica are trying to model how carbon and nutrients such as phosphorous flow through the ecosystem.
For years, Central and South American woodlands have been popularly regarded as carbon sponges that help combat greenhouse gas buildup by absorbing and storing atmospheric carbon. But now, Ciclos scientists and others are exploring more sinister ways in which the forests may be involved in climate change - by releasing more and more carbon as environmental conditions shift.
Their emerging conclusion is that relatively minor weather variations - particularly increases in temperature - could turn the forests into major emitters of carbon dioxide. The reason is that as the world continues to warm, as predicted by climate models, tropical rain forests are expected to shrivel, perhaps so much that they turn into grasslands with scattered trees.
As trees die, they release more carbon dioxide into the atmosphere, both through decomposition and fires that are more prevalent in warm and dry conditions. "With the climate changes that we expect ... we can't bank on the forest to suck up carbon," said McGlynn. "If anything, we can imagine that this forest ... is going to start shedding a bunch of carbon as the forest gets stressed."
Though scientists can't yet quantify many aspects of the carbon-climate relationship, they do know that tropical forests play a major role in regulating carbon worldwide. How the world's rain forests will behave when temperatures rise has enormous implications in global warming. "We count on (forest) uptake to balance part of our fossil fuel emissions," said David Schimel, a senior scientist with the National Center for Atmospheric Research in Boulder, Colo. "If it were to slow down, we would see even more rapid increases in atmospheric carbon dioxide concentrations."
TREE DEATH
Carbon is continuously being exchanged between land and atmosphere. In the ocean, it's stored as dissolved carbon dioxide; on land, plants take carbon dioxide from the air, turn it into new growth, then release it through respiration.
Humans are the source of some seven gigatons of carbon per year, mostly through the burning of fossil fuels, explained Jay Gulledge, senior research fellow at the Pew Center on Global Climate Change in Arlington, Va. The ocean, coupled with plants and the rest of the land-based ecosystem, soak up less than half that amount, and the remainder builds up in the atmosphere.
That's where the climate comes in. Rising levels of atmospheric carbon dioxide are effectively creating a blanket that traps the world's heat, increasing temperatures on Earth. The more carbon dioxide released into the atmosphere, the warmer the world is expected to become.
In the rain forests, in a worst-case scenario, rising temperatures could spark more tree deaths, and that would release more carbon dioxide. Fewer trees can absorb less carbon dioxide, so more of it would be trapped in the atmosphere. Earth's blanket would grow thicker, and temperatures would continue to escalate. In this way, a spiraling "positive feedback" would begin with more forest die-offs, more carbon releases and more global warming.
This kind of feedback is being closely monitored in other areas. A recent study in the journal Nature reported that the amount of the greenhouse gas methane emitted from thawing Siberian permafrost was much larger than previously thought, increasing concerns that even more methane will be released as temperatures rise.
At this point, there's still relatively little data about carbon loss in tropical forests, and the possibility of a downward carbon-climate spiral is based mostly on computer models, explained Steve Oberbauer, Ciclos Project coordinator at Florida International University in Miami.
However, there is compelling research from David and Deborah Clark, veterans at the La Selva Biological Station in Costa Rica where McGlynn and other Ciclos scientists focus their studies. The Clarks' work, including a few decades of tree measurements, have shown that trees grew less and died more during warmer years.
In 2003, the Clarks, with Stephen Piper and the late Charles Keeling of the Scripps Institution of Oceanography, reported on tree growth in La Selva from 1984 to 2000. During the 1990s, the "warmest decade globally in the instrumental record," the forest suffered a major depression in tree growth, the researchers wrote in the Proceedings of the National Academy of Sciences.
The findings, they said, indicated the forest was remarkably sensitive to increasing temperatures. Warmer weather, they concluded, had the potential to produce the positive feedback effect that would lead to more carbon dioxide accumulating in the atmosphere. "Such a feedback in future years would accelerate global warming."
How the La Selva findings will apply in places like the Amazon, larger and with more variable climates than La Selva, is uncertain.
"It's sort of like the blind man examining the elephant," said Gulledge. "You have your hands on one part of the system and don't really see the rest of the system."
MEASURING CARBON
Carbon flux towers, which measure carbon movement between land and air, are erected around the world to gather data in forests and other ecosystems. At La Selva, the tower has measured big differences from year to year in how the forest functioned. In 1998, the carbon was nearly in balance, meaning the forest acted neither as a major carbon sink nor source. The next year showed "moderate" ecosystem absorption of carbon, and more absorption in 2000, Oberbauer said.
However, Oberbauer is concerned that the towers don't capture much of the nighttime respiration and therefore understate the loss of carbon. His project is trying to measure the major components of carbon emissions and provide more detailed accounts of the carbon "budget."
Within a few years, Ciclos scientists hope to better quantify the amount of carbon the La Selva forest takes up or loses and link that to the climate over time. That's no easy task, since carbon is exchanged at several levels, including the soil, the litter, the roots and the canopy.
The weather will have a lot to say about the project's conclusions.
Oberbauer is hoping for an El Nino year - which results in warm and dry weather in Costa Rica - to measure, in real time, how hot temperatures affect carbon and other compounds in the forest.
For instance, an El Nino year followed by heavy rains likely would wash dead plant material out of the forest, reducing the forest's nutrients and its capacity to rebound. "We know that the trees aren't doing so well in an El Nino year," Oberbauer said. "We'd like to be able to put numbers on that."
ANSWERS IN LEAF LITTER
Back in San Diego, McGlynn is trying to do just that by focusing on the litter layer above the soil at La Selva. He and his students sewed about 800 mesh bags for his summer trip to Costa Rica.
The bags were filled with plant litter and left at research plots around the forest. McGlynn's research assistant is collecting them, weighing and charting decomposition at different spots.
In general, said McGlynn, it takes about six months for a leaf to break down in the dark, wet ecosystem. Some carbon is mineralized in the ground. Some is ingested by animals, including ants and mites. Some becomes gas during decomposition. "The main result that we have so far ... is that there are complex relationships, and the only way we are going to solve them is by looking at each component of the ecosystem and trying to understand how they fit together," he said. Other scientists are asking similar questions.
Alan Townsend and Cory Cleveland, at the University of Colorado, are looking at the effects of phosphorus and nitrogen added to plots in Costa Rico's Golfo Dulce Forest Reserve. The additions increased emissions of carbon dioxide into the atmosphere by nearly 20 percent annually, they reported in Proceedings of the National Academy of Sciences in June. The reason: The increased nutrients boosted activity among microorganisms that break down leaves and limbs that fall to the forest floor.
"This was not something we really expected to see, at least not across the board and in the size that we did," Townsend said.
Through increased use of synthetic fertilizers, humans are dramatically changing the amount of both nitrogen and phosphorus in the world, he said, just as they are carbon. "Soils may respond to those changes in a much more sensitive way than we might have guessed. That means that your potential for storing more carbon (in tropical forest soils) is not that good."
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The Ciclos Project
Purpose: To study links between carbon, water and nutrient cycles in old-growth rain forests.
Place: La Selva Biological Station, Costa Rica. It covers nearly 4,000 acres in an area that gets more than 13 feet of rain a year.
Funding: National Science Foundation grant of $1.6 million that runs from 2005 to 2009.
Started: 1996 as the Carbono Project to focus on the carbon cycle.
Principal Investigator: Steve Oberbauer, Florida International University.
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