Saving the Forest for the Trees

While scientists debate the future of tropical forests in a warmer world, there's no question that protecting these biologically rich habitats is key to combating climate change

  • Laura Tangley
  • Dec 01, 2008

AT FIRST, it looks much like any other rain forest in the Tropics: a luxuriant jumble of greenery punctuated by the occasional giant tree, its trunk several feet across and crown a hundred feet high, pushing through the canopy to reach sunlight above. Here and there, the monotony of muted tones is broken by splashes of brilliant color--a scarlet bromeliad growing at the base of a palm or a rainbow-hued toucan ferrying fruit from one tree to another. On the forest floor, flecks of green seem to move on their own, fragments of leaves held aloft in the mandibles of leafcutter ants marching in long, orderly lines back to their nest.

A closer look at one huge tree, however--a magnificent, 160-foot-tall Dipteryx--hints that there's something different about this Panamanian forest. Tacked to the tree's trunk, just above the huge buttress roots that support it, is an aluminum tag with a four-digit number. The number corresponds to the tree's identity and precise location within an experimental plot at the center of Barro Colorado Island, a research site operated by the Smithsonian Tropical Research Institute (STRI). It turns out every single tree within this 50-hectare plot, from forest titans like Dipteryx to broomstick-thin saplings, is similarly identified, mapped and marked. More remarkable, every five years an army of scientists descends upon the plot, checking each of its roughly 240,000 trees to see if the plant has survived and, if so, how much it has grown--a process that can take a full year to complete.

Established in 1980, STRI's Barro Colorado plot was the first in what has grown into a global network of 25 permanent forest plots in 15 tropical countries. Within them, more than 100 researchers now monitor the survival and growth of more than 3 million trees of some 8,200 species. "It is a very intensive way to study rain forests," explains STRI biologist Stuart Davies, director of the Center for Tropical Forest Science (CTFS), which coordinates the network. "Our hope is that by rigorously sampling biodiversity in tropical forests worldwide, we will be able to understand the major processes affecting the forests."

Initially conceived to answer theoretical questions--such as why tropical forests have so many tree species--the network today is helping scientists determine on-the-ground effects of global warming, findings that have implications for policy makers trying to tackle the problem. Among the most important of these questions is whether tropical forests in a warmer world will be able to store more carbon dioxide, the most important greenhouse gas that is causing climate change.

For many years, conventional wisdom has held that the large amounts of CO2 humans are pumping into the atmosphere will act as a fertilizer on plants--particularly tropical trees--spurring their growth and allowing them to soak up much of the excess carbon that is dangerously altering climate. "It's been the one bright spot among the gloomy global warming predictions," says Davies. Yet recent findings from Barro Colorado, and from the second-oldest CTFS plot in Pasoh, Malaysia, suggest that these forests have not been growing faster, but slower, as the Earth has heated up. "The results suggest that tropical forests may not be acting as a carbon sink after all," says Davies.

Like all green plants, tropical forest trees powered by sunlight take in carbon dioxide and release oxygen during the process of photosynthesis. When they respire, they exhale CO2 and breathe in oxygen. Tree wood growth (along with leaf, root, flower, fruit and seed production) is a reflection of the difference between the rates of photosynthesis and respiration, and it provides one indication of how much carbon is being stored.

But does increasing CO2 enhance photosynthesis and make trees grow faster? Decades ago, after scientists first warned that atmospheric CO levels were rising, laboratory experiments began to demonstrate that plants grown in chambers with extra amounts of the greenhouse gas did indeed grow more rapidly than normal. Outside the lab, researchers theorized that the fertilizing effect of CO2 would have a particularly strong impact on trees in the Tropics. Though tropical forests cover only about a twelfth of the Earth's land surface, they are spectacularly productive ecosystems, accounting for about a third of all primary productivity on the planet. 

Over the following decades, several long-term studies of forest plots in the Amazon showed that trees in that region were growing faster, as predicted. Such results fueled the hope that faster-growing tropical forests--especially in vast regions such as the Amazon, Congo Basin and Southeast Asia--would act like sponges to soak up some excess CO2 emissions--and buy decision makers a little extra time to solve the problem.

But the recent findings from Panama and Malaysia may be sinking such hopes. "I was surprised by the magnitude of the results and how widespread they were," says Wake Forest University biologist Kenneth Feeley, lead author of the study, published last fall in Ecology Letters. At Barro Colorado, tree growth--measured by changes in trunk diameter--has slowed by 25 percent over the past 20 years, and it has decreased by approximately the same amount in Malaysia over the past decade. "We discovered declining growth in all size classes of trees, from huge emergents to understory saplings," says STRI biologist and study coauthor Joseph Wright. "It is not the result you'd get if CO2 was fertilizing the forests."

The findings remain controversial. Oliver Phillips, an ecologist at the University of Leeds in the United Kingdom, has led an analysis of tree growth in more than 100 permanent plots in the Amazon known as the RAINFOR Network. "That Amazon forests have on average gained biomass over many years is without doubt, and in many sites tree growth rates have increased," says Phillips. He cautions against "overinterpreting results from individual sites, which can be affected by factors such as fragmentation or local droughts."

Reinforcing the findings from Panama and Malaysia, a detailed study of a Costa Rican rain forest has yielded similar results. At the Organization for Tropical Studies' La Selva Biological Station, husband-and-wife ecologist team David and Deborah Clark have monitored the annual growth of 164 trees since 1984. The researchers discovered that during particularly hot years, such as those that occur during El Niño events, the trees exhibit "remarkably depressed growth," says Deborah Clark. "We found no relationship between tree growth and annual atmospheric CO2," she adds. 

The Clarks did note a strong relationship between tree growth and minimum nighttime temperatures: The hotter the nights, the slower the trees grew. According to the scientists, the finding suggests that higher temperatures may be speeding up the trees' respiration rates at night, increasing the loss of CO2 from the forest.

Wright finds the Clarks' hypothesis a "plausible explanation" for slower tree growth in the CTFS plots. Another possibility is that the rate of photosynthesis may have decreased if trees were stressed by too much heat. Whether, and when, a forest was disturbed by humans also changes its response to higher CO2. "It's clear that forests in different parts of the world are behaving differently," says Wright.

Meanwhile, he and other scientists are finding additional signs that tropical forests are changing in ways that may decrease their ability to store carbon. In the Amazon, STRI biologist William Laurance, for example, has discovered that the mix of trees in 18 forest plots he's been monitoring has shifted over the past 20 years, with fast-growing species gaining an edge over slow-growing trees whose denser wood locks up more carbon.

In some of his Amazon plots, Phillips has found a doubling in the biomass of large lianas, parasitic woody vines that twist up the sides of trees to reach the sunlit canopy, shading out and sometimes killing their hosts in the process. On Barro Colorado, Wright has documented a 50 percent increase in liana biomass. "If you're getting more lianas, you'll definitely see a reduction in tree growth," he says.

While researchers continue these studies, they say uncertainties over tropical forests' role as a carbon sink should not obscure a more important point: Whether or not the forests will buy us time to solve the climate crisis, their destruction remains one of the most important sources of the pollutants that cause global warming. When rain forests are razed, they release into the atmosphere vast amounts of the carbon formerly stored in their trees and soil. Scientists say that between 20 and 25 percent of all CO2 emissions come directly from tropical deforestation. "That's more than the entire global transportation sector--all the cars, ships, planes and trains on Earth," says Laurance.

But he and other researchers also see mounting concern about climate change as an opportunity to protect tropical forests. Despite decades of concern about the loss of these lush woodlands--the most biologically diverse ecosystems on the planet--some 13 million hectares (32 million acres) of tropical forest are still destroyed every year, the equivalent of about 50 football fields a minute. Providing developing countries that house the forests financial incentives to conserve them could turn that around. According to a recent analysis published in the Proceedings of the National Academy of Sciences, paying developing nations about $1 billion annually would stop about a tenth of all tropical deforestation--and prevent the release of roughly half a billion metric tons of CO2 over the next 25 years.

When the Kyoto Protocol was negotiated a decade ago, participants rejected a proposal to allow industrialized countries to offset carbon emissions by paying developing nations to protect forests. According to Eric Palola, senior director of NWF's Forests for Wildlife program, one fear was that rich countries, especially the United States, would use the option as a way to avoid making cuts in emissions from power plants, the transportation sector and other big polluters back home.

But Palola says attitudes have changed. "Now decision makers realize we cannot afford to be choosy about global warming solutions," he says. Indeed, at a 2007 meeting to renegotiate the Kyoto treaty (which expires in 2012), representatives of more than 100 countries agreed to set up a system to compensate developing nations for "avoided deforestation"--with details to be worked out in time for the next climate convention in Copenhagen in 2009. Meanwhile, the World Bank has provided $300 million for demonstration projects in the Tropics.

The success of the new approach will be critical to solving what many scientists say are the two most important environmental problems of our time. "Collectively, our efforts to conserve tropical forests over the past 35 years have failed," says Columbia University biologist Don Melnick, an advisor to a coalition of rain forest nations that championed the recent agreement in Bali. "With attention to global warming, we may be looking at one of the last great opportunities."

Senior Editor Laura Tangley visited STRI in Panama while researching this story. To learn more about the institute's work, see

Can Tropical Species Take the Heat?

With temperatures rising fast at the planet's poles, Arctic species like the polar bear have become poster children for global warming. But biologists who work in the Tropics warn that animals and plants at low latitudes may be affected even more. Though tropical temperatures since the 1970s have increased by just .6 to .8 degree C, compared to 2 degrees C at the poles, "tropical organisms have evolved to be finely tuned to very specific and limited temperature ranges," said STRI biologist Joseph Wright at a recent meeting, "Climate Change and Biodiversity in the Americas," held in Panama. For tropical organisms, temperatures experienced throughout seasons and geographic ranges vary far less than they do for temperate plants and animals. "Tropical species may in fact be more vulnerable to global warming than any others on Earth," said Wright.

NWF Priority: Protecting Tropical Forests

NWF is working at several levels to stem the loss of tropical forests and slow the emission of greenhouse gases caused by deforestation. "The best way to do this is to ensure forest owners benefit more from keeping forests alive than cutting them down," says Barbara Bramble, NWF's senior advisor for international affairs. In 2008, for example, NWF announced its participation in The Climate Tree, a new partnership with the Tropical Forest Trust that is working in tropical countries to promote sustainable forest management. NWF also continues to be a leader in the international Forest Stewardship Council, which provides independent certification of environmentally friendly wood products in more than 80 countries, and in promoting responsible wood and paper sourcing through education and consumer action, including NWF's Garden Furniture Scorecard.

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Amazon Deforestation Spikes

There's more bad news from the largest tropical rain forest on Earth: According to Brazil's National Institute for Space Research (INPE), which monitors forest loss in the vast Amazon Basin, some 3,088 square miles of the region's forests were destroyed between August 2007 and August 2008--a 69 percent jump over the amount of forest razed during the previous 12-month period. INPE officials blame the increase on rising global demand both for soy and cattle.

To learn more about the forces behind Amazon forest loss--including recent changes brought about by global warming--read the National Wildlife article "Amazonia Drying."--Laura Tangley

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Invasive Species: Another Threat to Tropical Forests

As if the world's tropical forests did not already have enough problems, scientists have added a new one: invasive species, which pose a tremendous threat to entire forest ecosystems on tropical islands.

Conducting their research on the island of Hawaii--where roughly half of today's species are non-native--researchers from the Carnegie Institution and other organizations used remote-sensing technology carried on aircraft to gauge the impact of invasive plants on more than 220,000 square miles of forest. (Previous studies on the effects of invasive plants on forests were done in much smaller areas.) They found significant changes in the structural, biochemical and physiological properties of invaded forests--denser canopies or decreased soil fertility, for instance--that prove harmful to native plants.

"These new airborne technologies, which are sensitive enough to discern saplings and young trees, may make this problem more tractable," says Flint Hughes of the U.S. Forest Service, a coauthor of the study, which was published last spring in the Proceedings of the National Academy of Sciences. "They allow scientists to probe the makeup of forests over large areas and detect invasions at earlier stages."--Laura Tangley

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New Opportunities to Fight Forest Loss

Despite decades of concern about loss of the world's tropical forests, some 13 million hectares (33 million acres) of these biologically rich habitats are still destroyed every year, according to the United Nations. Yet behind these overall figures lies a potentially promising shift--away from deforestation driven primarily by poor, subsistence farmers toward forest loss caused by industries such as mining, timber and large-scale agricultural enterprises, reports a recent study. "Although this trend is pretty scary, it's also much easier to target a handful of global corporations than many millions of poor farmers," says William Laurance, a biologist at the Smithsonian Tropical Research Institute and coauthor of the study, "New Strategies for Conserving Tropical Forests," published last fall in the journal Trends in Ecology & Evolution.

Conservationists already are taking aim at corporate villains. "Green groups are learning to use public boycotts and embarrassment to target the corporate bad guys," says study coauthor Rhett Butler of, a leading tropical forest website. "And it works: We're seeing the global soy, palm oil and timber industries beginning to change their approach."

"Environmental groups are using carrots as well as sticks," adds Laurance. "Many multinational corporations are developing greener products because they're more profitable. The market for eco-friendly timber products is expected to be worth tens of billions of dollars in the U.S. by 2010, for example."

To find about NWF's programs to green the timber industry, check out our Forests page.--Laura Tangley

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