( s study shows that the reaction to drou...)World's largest rainforest drying experiment completes first phase

s study shows that the reaction to drought is instead a decrease in the rate at which tree trunks grew in diameter. Many small trees, measuring 4 to 10 inches in diameter, simply stopped growing during the end of the dry season of 2000, following the first period of rainfall exclusion, as the trees of similar size continued to grow in the "control" forest plot. The trees slow down the amount of water that they lose from their leaves by closing their stomates. The trees adjust to the resulting reduction in photosynthesis by diverting less of the sugar and other carbohydrates that they make to wood production. This finding has important implications for climate change since the amount of carbon removed from the atmosphere during tropical droughts will decrease significantly. In addition, there are implications for sustainable forest management, because the time one must wait after harvesting timber from a forest before a second harvest will increase.

Third, the observed sensitivity of large canopy trees in the forest to drought is greater than expected. Once the moisture that is stored in deep soil is depleted, then the large trees that tower 130 to 150 feet above the ground, basking in full sunlight, begin to falter and die. The death of large trees ?trees that may take centuries to reach the top of the forest canopy and have trunks greater than 10 inches in diameter -- increased from about one percent per year before the rainfall exclusion began to nine percent in the fourth year of the experiment, when soil water was depleted. This sensitivity of large trees to drought means that a decline in rainfall will likely push this tall, green, lush rainforest towards a shorter, more stunted forest.

As the forest becomes shorter and its leaf canopy more open, compromising its remarkable resistance to fire, it is clear that drought in tandem with fire can swiftly push the tall, dense rainforests of the region towards savanna scrub. The amount of carbon that c
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Source:Woods Hole Research Center

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