downed trees in flooded igapó forest on the Rio Negro in Brazil
Octavio Campos Salles/Alamy Stock Photo

There’s a strange tree-killer on the loose in the Amazon: logjams

The Amazon rainforest is a hotbed of biodiversity and a major absorber of the world’s carbon dioxide. But large swaths of this jungle are dying every year, more than can be explained by the traditional culprits of deforestation, logging, and farming. Now, one researcher has used more than 30 years of satellite images to show that flooding caused by logjams—large pieces of timber partially or completely obstructing river channels—may be the missing factor. Naturally occurring logjams in the Bolivian Amazon kill off almost as much rainforest as human activity—just under 1000 hectares per year in one part of Bolivia—the scientist reports in a forthcoming issue of Earth Systems Dynamics. To find out how and where logjams did their dirty work, the researcher looked at images of rainforest in western Bolivia from 1984 to 2016, searching for areas that reflected colors of infrared light that typically signify bare soil and sand. He found that 22 rivers showed repetitive flooding caused by logjams. The sometimes-massive pileups reroute water flow, erode riverbanks, and transport massive amounts of sand, eventually killing nearby trees. Each flood can destroy thousands of trees, which die after the soil surrounding their roots is washed away. Logjams can also change the balance of plant species in the Amazon: After trees die, some of the flooded areas turn into savanna, and the uptick in sunlight hitting the ground can favor quick-growing trees like mahogany, at the expense of slower-growing species like Brazil nuts. The die-offs are also bad news for the rest of us—more savanna means fewer trees are absorbing carbon dioxide, one of the major greenhouse gases responsible for global warming.