Rainforest "greens" in drought

Satellite data reveal surprising resilience of the Amazon forests

Differences in growth (above) and precipitation (below) in 2005 compared to the average. Red marks each a reduction from the average, yellow the normal value and green areas with an increase of growth or precipitation. © University of Arizona
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The rainforests of the Amazon seem to be much more resilient to dryness than previously thought. A recent study published in "Science" shows that forests grew even faster during the 2005 drought. In doing so, she refutes a climate model that predicts a "yellowing" of the rainforest after only a month of drought and a collapse in the event of a prolonged drought.

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The drought in 2005 reached its peak at the beginning of the annual dry season from July to September. Although this double blow of the drought was expected to actually slow the growth of the canopy, in many affected areas the opposite was the case: the treetops were much greener and photosynthetic activity increased. That revealed satellite data that scientists from the University of Arizona and the University of Sao Paulo evaluated.

Satellite data reveal "greening" of the rainforest

They used data from two satellites, the NASA satellite Terra, which measured the greenness of the vegetation, and a second, the Tropical Rainfall Measuring probe, which monitored rainfall in the tropics. Using the monthly maps of these satellites, the researchers determined the changes in Amazonian vegetation over the course of nine years. When they evaluated the 2005 maps and compared them to the data for the same months in the previous maps, they were in for a big surprise.

"Rather than 'shut down' during a drought, as you would expect, the rainforest responded positively to the drought, at least in the short term, " explains Scott R. Saleska, of the University of Arizona. "It's a very interesting and surprising reaction." Co-author Kamel Didan adds: "The forest showed clear signs of higher productivity. That's the surprising news. "Ad

Contradiction to climate models

This result clearly contradicts the previous climate models. These predict that the Amazon will reduce its photosynthetic activity very quickly when a drought begins. This decline in productivity in turn triggers a positive feedback, as less carbon dioxide from the atmosphere is bound up in the plants. And the more greenhouse gas stays in the atmosphere, the more it promotes warming and thus dryness.

But at least in the short term, the rainforest does not seem to follow these forecasts, as the new data show. Instead, the D rre-induced growth spurt could even increase the buffering effect of the forest and bind more CO2 than normal. Obviously, despite the drought, the trees were able to tap water reservoirs deep underground and thus grow in spite of the drought.

Limit of resistance still unknown

From an evolutionary point of view, this behavior seems to make sense, according to the researchers. Periodic dryness is not uncommon in the Amazon: every four to eight years, during an El Nino, it rains less than normal in the Amazon Basin, only to return to moist normality thereafter, However, the resilience of the rainforest has its limits, as the scientists state. "If enough water is left behind for a long time, the trees will die." However, it is not yet known how long drought will last.

(University of Arizona, 24.09.2007 - NPO)