From January 2000 to July 2004, rainfall was excluded from a one-hectare (2.2 acre) plot in the middle of the Tapajs National Forest, in Brazil. A total of 6 feet of rainfall was diverted with six thousand 2' by 6' clear plastic panels suspended 3 to 12 feet above the soil. The panels were removed during the five-month dry season each year. To sort out the forest responses to the "umbrellas" from the normal variation in tree growth, tree death, leaf production, and other aspects of forest behavior, researchers compared this dry plot of forest with a similar plot, from which rainfall was not excluded. These two forest plots were compared for a year prior to installation of the plastic panels to register any differences in behavior that already existed when we began the experiment.
According to Daniel C. Nepstad, a senior scientist with The Woods Hole Research Center, "This experiment provides researchers with a peek into the future of this majestic forest, a future that will most likely be drier because of global warming, El Nio episodes, and even the drying effects of rainforest clearing and burning itself."
First, the biggest surprise noted thus far has been the great tolerance that this forest presented in the face of the severe drought that was created. As the moisture stored in the soil that sustained the forest during prolonged dry seasons was depleted in the dry plot, the trees simply absorbed water from deeper in the soil with their extensive root systems, avoiding most of the visible symptoms of drought stress. By the end of the five-year period of exclusion, many trees in the forest were drawing in water from more than 40 feet deep in the soil.
Second, it was anticipated that when the trees ran out of water in the soil, they would shed their leaves. In fact,
Contact: Elizabeth Bran
Woods Hole Research Center