Old-growth forest 'greens up' during the dry season.© Getty

Some trees in the Amazon rainforest grow fastest not in the wet season, but in the dry, sunny part of the year, researchers have found. The discovery underlines the importance of preserving old-growth forest that is likely to be more resistant to drought.

The effect only occurs with pristine rainforest, rather than in re-planted areas. With logging continuing to deplete the forest, researchers note, the area could become too dry for even intact areas of forest to cope.

Researchers studied colour satellite images of the eastern and central Amazon collected over five years to survey the amount of vegetation produced in different seasons and in different areas.

They found that many areas of intact old-growth forest 'green up' during the dry season, which runs from July to November. Pasture lands, on the other hand, are parched and brown during the dry spells, and show fastest plant growth during wetter months.

A diet of sunshine

Most plant ecosystems, including crops, grow fastest when water is plentiful. But Huete and his colleagues think that the oldest trees in the Amazon have roots that extend deep enough to find water even in dry months, making sunlight the crucial factor for speedy growth.

The discovery highlights a unique characteristic of rainforests such as the Amazon, says Alfredo Huete of the University of Arizona in Tucson, who led the survey, published in Geophysical Research Letters¹.

It is unclear whether this effect extends to the entire rainforest or merely to those areas with the deepest soils, says rainforest expert Oliver Phillips of the University of Leeds, UK. More research will be needed to see whether this is a universal 'rainforest phenomenon' he says.

The same dry-season growth spurt does not seem to happen in areas where there are only younger trees. "Old-growth trees are doing something that secondary regenerating forest can't do," Phillips says.

Deeply rooted

Once the original trees are gone, the road to recovery would be an arduous one, Huete explains. "It would be a slow, delicate process as tree roots would not develop fast enough to carry the forest through the dry seasons," he says.

And the forest does face severe challenges from deforestation. Unless fresh action is taken, around 40% of the Brazilian Amazon will be lost by 2050, says Britaldo Silveira Soares-Filho, of the Federal University of Minas Gerais in Belo Horizonte, Brazil.

He and his team argue in Nature this week that public wildlife reserves, the mainstay of preservation in the forest today, will not be enough to prevent severe depletion of Brazil's rainforests². They say that private landowners will have to be persuaded to manage their farming and cattle ranching sustainably, by only cutting down as many trees as needed.

As trees are felled, the forest also becomes drier due to a decrease in rainfall. Around 70% of the Brazilian Amazon rainforest needs to be retained in order to maintain the region's current rainfall regime, the researchers say.

Leaf growth is an important process, not just for the rainforest itself, but for the planet. Growing trees suck carbon dioxide out of the atmosphere and convert it to the building-blocks of new plant material, sequestering the gas that would otherwise contribute to the greenhouse effect.

"Anything that disrupts the carbon cycle of such a huge reservoir is of concern," Huete says.

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• References

  1. Huete A. R., et al. Geophys Res. Lett., doi:10.1029/2005GL025583 (2006).
  2. Soares-Filho B. S., et al. Nature, 440 . 520 - 523 (2006). | Article |