American Scientist

JOBS NOT TREES yells a bumper sticker in the timber country of the western United States, crisply stating one side of a long-running dispute between loggers and "tree-hugging" environmentalists. There is, however, a middle ground in this debate: the system of forestry called selective logging. Only trees of desired species are removed from the forest, leaving other trees intact and ensuring the continued health of the ecosystem.

But recent reports on selective logging's effects on a forest's carbon-storing ability may erode the middle ground. In the journal Science (November 11, 2005),  a research group led by modeler Daniel Bunker at Columbia University recently reported that carbon storage in a selectively logged forest could be reduced by up to 70 percent if certain species are permanently removed.

Bunker's analysis is not the only evidence that selective logging may be more damaging than realized. The Carnegie Institution's Gregory Asner, working with colleagues in Puerto Rico and Brazil, measured forest degradation in the Brazilian Amazon caused by selective logging. By tweaking remote sensing methods, they reported in an earlier issue of Science (October 21) that selective logging is degrading the Amazon rain forest at twice the rate previously estimated.

In a finding consistent with Bunker's, Asner's team calculated that selective logging adds 25 percent more carbon to the atmosphere than accounted for by deforestation alone, contributing to the "greenhouse effect" thought to drive climate change. "Logging is widespread and cause[s] an important gross loss of carbon from the Brazilian Amazon each year," says Asner, an ecologist.

Several scientists urge caution in interpreting these results. Wulf Killmann, director of the Forest Products and Economics Division at the Food and Agriculture Organization (FAO), says of Bunker's results that "It would be erroneous to assume that modeling studies based on [data collected at the Smithsonian Tropical Research Institute's Barro Colorado Island site in Panama] would apply to tropical forests worldwide." Michael Keller, a biogeochemist with the International Institute of Tropical Forestry, agrees. "[Bunker's results are] a limiting case only," he says. And Asner's publication prompted a swift defense from Killmann. "Selective logging is not in principle that destructive," he asserted in a press release.

Killmann is not alone in this view. Richard Rice and fellow conservation biologists at Conservation International reported in a 1997 Scientific American article that the effects of selective logging on tropical forest in Bolivia were "relatively mild" and that associated activities "disrupt less than 5% of the land."

Evidently, more work is needed to better estimate selective logging's impact in the tropics. Given the notoriously complex balance sheet for the global carbon budget, scientists will have a tough job figuring out an appropriate forest management strategy to optimize carbon storage. "You have to consider time periods, carbon storage in wood products, and future silvicultural treatments, methodologies for keeping track of carbon storage, carbon stock change, et cetera," notes Gary Bull, a forestry expert at the University of British Columbia.

Another complication is the growth rate of tropical trees and hence the time taken for the forest to regenerate after logging. Greg Asner contends that there are "misconceptions about this issue." Regeneration takes a lot longer than some people think, he says. "In just a few years, the foliage from secondary plant regrowth will obscure satellite sensors, making the forest look like it has grown back." Field studies show that this growth accumulates little biomass. Bunker notes, "As the forest recovers, it would likely be dominated by fast-growing yet low-density pioneers and lianas, neither of which will store much carbon." Thus, decades pass before the forest returns to its former carbon-absorbing ability. "This is no surprise, considering recent studies showing that trees in mature forests of the Amazon can be 300 to 800 years old," Asner says.

One such study was published in December  in the Proceedings of the National Academy of Sciences. The study's lead U.S. scientist, ecologist Susan Trumbore of the University of California, Irvine, says modelers need to rethink the Amazon forest's role in determining global carbon dioxide levels. Trumbore says that trees in the Amazon are older and grow more slowly than scientists thought, so previous studies have overestimated the Amazon rain forest's capacity to absorb carbon.

To reduce logging's impact on tropical rain forests' carbon-absorbing capacity, managers must adhere to recommended methods, according to Greg Asner. "Under best practices of surgical 'reduced-impact logging,'" he says, "the carbon losses can be kept low."

But in reality, says Bunker, "There is often little incentive for [logging crews in the field] to follow the rules closely … [so it] may be difficult to reliably enforce codes in practice."

In fact, Richard Rice says that  "unregulated logging is pretty much the norm in most tropical developing countries." Killmann's FAO group admits the point. "There is clearly a shortcoming in the implementation of better logging techniques … in the 'real world.'"

The barriers to improving forestry practices in the tropics are daunting. "Tropical forests are a major resource for many countries [that] want to benefit from them economically," Bunker says. "All too often decisions are made by bureaucrats and multinational corporations with very short-term interests … multinational logging companies bring their own crews and equipment—the local economy gains nothing."

Asner alludes to the same problem. "The scientific and forestry knowledge regarding how to do low-impact logging in the field now exists. The barriers now are thus political and economic." And Rice suggests that such impediments will continue to hamper efforts to implement best practices. "Approaches to 'careful' or 'sustainable' logging have had next to no discernible impact despite years of effort and hundreds of millions of dollars of public and private investment."

Whatever practices are used, tropical rain forest destruction remains a major cause of carbon dioxide emissions, significantly reducing the planet's capacity to absorb atmospheric carbon produced by human activities. Rather than advocate leave-alone policies, a more realistic role of scientists and others concerned with the future of tropical forests is, according to Killmann's FAO group, to promote use that is "wiser and more sustainable."