A single dry season in the Amazon rainforest can reduce the forest's carbon dioxide uptake for years after the rains return, according to a new study published in the journal Nature. This is the first study to quantify the long-term legacy of a drought in the Amazon.
A research team at NASA's Jet Propulsion Laboratory in Pasadena, California, and other institutions used satellite lidar data to map the damage and mortality caused by a severe drought in 2005. In years of normal weather, the forest undisturbed may be a "natural carbon sink", "absorbing more carbon dioxide from the atmosphere than it returns to it." But starting in the drought year of 2005 and running through 2008, the last year for available lidar data, the Amazon basin lost an average of 0.27 petagrams of carbon (270 million metric tons) per year, with no signs of recovery from its function as a carbon sink.
At approximately 2.3 million square miles (600 million hectares), the Amazon is the largest tropical forest on Earth. Scientists estimate that it absorbs up to one-tenth of human fossil fuel emissions during photosynthesis. "The old paradigm was that whatever carbon dioxide we put into [man-made] emissions, the Amazon could help absorb a lot of it," said NASA's JPL's Sassan Saatchi, who led the study.
But severe drought episodes in 2005, 2010 and 2015 are causing researchers to reconsider that idea. "The ecosystem has become so vulnerable to these hot and episodic drought events that it can change from sink to source depending on severity and extent," Saatchi said. "This is our new paradigm."
Drought from the ground
For scientists on the ground in the Amazon, "the first thing we see during a drought is that trees can lose their leaves," Saatchi said. “These are rain forests, the trees almost always have leaves. Therefore, the loss of leaves is a strong indication that the forest is stressed ”. Even if the trees eventually survive defoliation, this damages their ability to absorb carbon while under stress.
Observers on the ground also note that droughts tend to disproportionately kill tall trees first. Without adequate rainfall, these giants cannot pump water over 100 feet high from their roots to their leaves. They die from dehydration and eventually fall to the ground, leaving gaps in the forest canopy far above.
But any observer on the ground can monitor only a small part of the forest. There are only about a hundred plots used for research and some towers for long-term monitoring of the Amazon forests. "Detailed measurements at these sites are extremely important to understanding the function of the forest, but we can never use them to tell what this giant ecosystem is doing in a timely manner," Saatchi said. To do that, he and his colleagues turned to satellite data.
Drought from space
The research team used high-resolution lidar maps derived from the Geoscience laser altimeter system aboard Ice, Cloud, and the Earth Elevation Satellite (ICESat). These data reveal changes in the canopy structure, including leaf damage and gaps. The researchers developed a new analysis method to convert these structural changes into changes in aboveground biomass and carbon. Pixels showing burned or deforested areas were removed to calculate carbon impact of drought only in intact forests.
They found that after the drought, downed trees, defoliation, and canopy damage produced a significant loss in canopy height, with the most severely affected region declining an average of about 35 inches (0.88 meters) in the year. post-drought. The less severely affected regions of the forest declined the least, but all continued to decline steadily throughout the remaining years of the data record.
Saatchi noted that half of the forest's rain is produced by the forest itself: water that transpires and evaporates from vegetation and soil, rises into the atmosphere and condenses and rains during the dry season and the transition to the wet season. A drought that kills forest trees not only increases carbon emissions, reduces rainfall and extends the length of the dry season. Those changes increase the likelihood of a future drought.
If droughts continue to occur with the frequency and severity of the last three events in 2005, 2010 and 2015, Saatchi said, the Amazon could eventually transform from a rain forest to a dry tropical forest. That would reduce the carbon absorption capacity of the forest and its biological diversity.
The Nature article is titled "Post-Drought Decline of Amazon's Carbon Sink." Co-authors are affiliated with UCLA, Boston University, Oregon State University in Corvallis, and the United States Forest Service International Institute of Tropical Forestry in Río Piedras, Puerto Rico.
Jet Propulsion Laboratory, Pasadena, California
Written by Carol Rasmussen
NASA Earth Science News Team
Original article (in English)