Amazon forest shows unexpected resiliency during drought

Terra Daily: Drought-stricken regions of the Amazon forest grew particularly vigorously during the 2005 drought, according to new research. The counterintuitive finding contradicts a prominent global climate model that predicts the Amazon forest would begin to "brown down" after just a month of drought and eventually collapse as the drought progressed. "Instead of 'hunkering down' during a drought as you might expect, the forest responded positively to drought, at least in the short term," said study author Scott R. Saleska of The University of Arizona. "It's a very interesting and surprising response." UA co-author Kamel Didan added, "The forest showed signs of being more productive. That's the big news."

The 2005 drought reached its peak at the start of the Amazon's annual dry season, from July through September. Although the double whammy of the parched conditions might be expected to slow growth of the forest's leafy canopy, for many of the areas hit by drought, the canopy of the undisturbed forest became significantly greener -- indicating increased photosynthetic activity.

Saleska, a UA assistant professor of ecology and evolutionary biology, and his colleagues at the UA and at the University of Sao Paulo in Brazil used data from two NASA satellites to figure out that undisturbed Amazon forest flourished as rainfall levels plummeted…."A big chunk of the Amazon forest, the southwest region where the drought was severest, reacted positively," said Didan, a NASA-EOS MODIS associate science team member.

The study, "Amazon Forests Green-up during 2005 drought," is online in the current issue of Science Express, the early-online version of the journal Science. The paper will be published in the October 26, 2007, issue of Science.

…Global climate models predict the Amazon forest will cut back photosynthesis quickly when a drought starts. That slowdown in plant growth would create a positive feedback loop -- as the forest shuts down more and more, it removes less and less carbon dioxide from the atmosphere. The CO2 ordinarily sequestered by growing trees would remain in the atmosphere, increasing global warming and further accelerating the forest's decline and additional CO2-fueled warming.

By contrast, the UA-led team's findings suggest the opposite happens, at least in the short-term. The drought-induced flush of forest growth would dampen global warming, not accelerate it. During the 2005 drought, Amazon forest trees flourished in the sunnier-than-average weather, most likely by tapping water deep in the forest soil. To grow, trees must take up carbon dioxide, thus drawing down the levels of atmospheric CO2. That negative feedback loop would slow warming from greenhouse gases.

Evolutionarily, the forest's resilience in the face of a single drought year makes sense, Saleska said. During El Nino, which occurs about every four to eight years, the Amazon forest receives significantly less rain than average. The limit of the forest's resiliency is unknown, Saleska said, adding, "But if you take away enough water for long enough, the trees will die."