"Our work is the first to demonstrate that the loss of tropical tree biomass continues to accelerate if the forest is droughted over decadal time-scales," Lucy Rowland of the University of Edinburgh, UK, told environmentalresearchweb. "Understanding tropical forests’ responses to longer term droughts is essential if we are to improve our understanding of how tropical forests will respond to changes in the climate."

The amount of biomass loss from tree death increased substantially after ten years of reduced soil moisture, Rowland and colleagues found. After seven years with the test site receiving half the natural rainfall, the cumulative biomass loss through mortality was 17%. After 13 years of the treatment, the biomass loss was 41%.

"The largest trees are most vulnerable to mortality during drought," said Rowland. "The larger trees are more vulnerable because their xylem tissue is much more prone to be blocked by air bubbles than for smaller trees. This finding is an important clue to the mechanism through which the trees die, something that was previously unknown for these tropical forests, and which stands to significantly advance how well we can predict the future of tropical forests."

Trees continued to store sugars and grow before they died, the team found, indicating that a shortage of carbohydrates wasn’t what killed them.

Rowland and colleagues studied trees in the long-term drought project in Brazil’s Caxiuanã National Forest Reserve in Pará State. Set up in 2002, the experiment is the world’s longest running tropical forest drought study. An array of transparent plastic panels creates a structure like a greenhouse roof that prevents half of the natural rainfall reaching the ground.

"The rationale for droughting a 1 hectare (100 m × 100 m) section of rainforest was to try to understand the biological and ecological changes the forest would undergo following a 50% reduction in rainfall, which some areas of rainforest may experience under certain climate change scenarios," said Rowland." This project … is the only experiment that can allow scientists to study how tropical forest trees respond to drought over decadal timescales."

Earlier studies had found that after several years of drought, the mortality of smaller trees dropped off and growth rates recovered.

"Our finding demonstrates that if future climate change droughts tropical forests then the health of these will continue to decline, losing carbon over decadal timescales rather than reaching a new stable state," said Rowland. "Large amounts of carbon normally stored in tropical trees will continue to be released as carbon dioxide into the atmosphere over decades as trees die and rot. These inputs of carbon dioxide into the atmosphere would feedback to accelerate the greenhouse effect."

Now Rowland plans to continue study at the Caxiuana tropical forest drought experiment. She will also improve how we represent tropical forest responses to drought in climate change models in a project based at the University of Exeter, UK.

The team reported the results in Nature.

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