Published online 20 January 2009 | Nature | doi:10.1038/news.2009.39

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Secondary forests are worth saving

Biodiversity there isn't as rich as untouched rainforest, but should still be conserved, some argue.

Tropical forest that has regrown after clear-cutting can become almost as biodiverse as untouched forest, according to new research. But some scientists question whether these so-called 'secondary' forests can truly forestall the extinction of species left homeless by deforestation.

By comparing preserved and regrown forest in Costa Rica, a team led by ecologist Robin Chazdon of the University of Connecticut in Storrs has found that 90% of tree species from the original landscape can also be found in secondary forest. The results, presented at a tropical biodiversity symposium last week in Washington DC, suggest these regrown areas may be worthy of conservation, even though they were once cleared.

A seedling in Costa Rica of the tree species Calophyllum brasilense, which regenerates substantially in secondary forests.Robin Chazdon

"You really need to be looking at this entire spectrum and trying to manage it all, not just focusing on the pristine biodiverse gems," says Thomas Lovejoy, biodiversity chair of the Heinz Center in Washington, DC, who was not associated with the study.

The symposium, hosted by the Smithsonian Institution, brought together biologists, policymakers, and environmental organization representatives to address controversy arising from a 2006 paper. In that, Joseph Wright of the Smithsonian Tropical Research Institute (STRI) in Balboa, Panama, and Helene Muller-Landau of the University of Minnesota in St Paul suggested that tropical extinctions may not be as dire as predicted. As farmers migrate to cities, the study argues, forests may have a chance to recover from agricultural clearing1.

Secondary cut-backs

One key question is whether these abandoned areas will be able to reassemble the same richness of species as the old primary forest. Chazdon and her colleagues assessed tree biodiversity changes in northeastern Costa Rica by surveying 18 hectares of preserved old-growth forest and 11 hectares of secondary forest, which ranged from 10 to 45 years old.

When they looked for trees with a diameter greater than 10 centimetres, they found only 59% of the old-growth tree species in the regrown areas. But when they extended their search to seedlings and saplings, the number rose to 90%. "We needed to start looking at the little guys," says Chazdon.

But the future is bleak for many secondary forests, says Carlos Peres, a conservation ecologist at the University of East Anglia, UK, who has worked with Chazdon. Much of the regrown area in the Amazon is cut back within five years, he says, and never reaches the 100 or 200 years necessary to become mature forest.

William Laurance, a conservation biologist at STRI, notes that the presence of seedlings doesn't necessarily translate into stable long-term populations. "I get concerned when people emphasize too much that the glass is half-full," he says. "It's definitely also half-empty."

Chazdon says that while the study presents a "promising picture for this region of Costa Rica", other parts of the world may respond differently. Costa Rica has fairly young, nutrient-rich soil, and much of the animal life is still intact. In Singapore, she says, trees are having trouble regenerating, possibly because large birds that act as seed dispersers have gone extinct in the area. And most of the regrown areas surveyed by her team are within 100 metres to two kilometres of primary forest, so species don't have to travel far to resettle.

Logging regrowth

Just how much secondary forest exists worldwide is a difficult question to answer. Scientists have used satellite data to estimate the amount of forest cover, but the technology is not yet advanced enough to distinguish between old and regrown trees.

Gregory Asner, an ecologist at the Carnegie Institution of Washington who is based in Stanford, California, and his colleagues attempted to come up with an estimate by searching the scientific literature for regional studies about secondary forest. About 1.7% of the world's tropical forest area has been reported as being in regrowth, they concluded.

The number is conservative because there are undoubtedly regrown areas that have not been reported in the literature, says Asner, who presented the results at the symposium. But, he says, it suggests that "regrowth is still in the geographic minority compared to deforestation and logging". Asner's team used satellite images, logging concession maps, and other data to calculate that 28% of tropical forests had been logged or were slated for logging. While other studies have tracked logging operations in Africa and the Amazon, this is the first global estimate of its kind.

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Wright, co-author of the controversial 2006 study, said at the symposium that the greatest threat to tropical species today is climate change. When he maps global climate data onto forest area data, Wright says, virtually no forest exists in areas with a mean annual temperature above 28 degrees Celsius. Tropical rainforests average 24 to 26 degrees and are expected to heat up by three degrees by the end of the century. "It's already the hottest place on the planet, and it's about to get hotter," he says.

Asner's and Wright's data will appear an upcoming special issue of Conservation Biology. 

  • References

    1. Wright, S. J. & Muller-Landau, H. C. Biotropica 38, 287-301 (2006). | Article |
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