Three-dimensional measurements of the central Brazilian Amazon rainforest have given NASA researchers a detailed window into the high number of branch falls and tree mortality that occur in response to drought conditions.
In a rainforest as vast as the Amazon, estimating the number of dying or damaged trees, where only branches may fall, is extremely difficult and has been a long-standing challenge.
Analyzing the three surveys, the team used the LiDAR data to detect new gaps in the canopy where a tree or branch had fallen in the months between observations.
During the non-El Niño period from 2013 to 2014, the branch and tree fall events altered 1.8 percent of the forest canopy in the study area, a small number on the surface but scaled up to the size of the entire Amazon, it’s the equivalent of losing canopy trees or branches over 38,000 square miles, or the area of Kentucky.
Tree and branch mortality was 65 percent higher during the El Niño drought period from 2014 to 2016, or 65,000 square miles, the size of Wisconsin.
"Because it’s a big forest, even a subtle shift in an El Niño year has a big impact on the total carbon budget of the forest," said Morton, referring to the balance between how much carbon dioxide trees remove from the atmosphere to build their trunk, branches, and leaves as they grow versus the amount that returns to the atmosphere when trees die and decompose.
Surprisingly, the scientists found that deaths for all tree sizes, as well as the number of smaller branch falls, increased at about the same rate.
If droughts were to preferentially kill large trees, it would boost the total amount of carbon that’s lost from drought as opposed to other disturbance types," he said.
To understand the relationship between the gaps seen by the airborne LiDAR system from above and the multiple layers of canopy and understory below, Morton’s colleague Veronika Leitold at Goddard and a team of collaborating scientists at the Brazilian Agricultural Research Corporation and the Federal University of Western Pará conducted field measurements underneath observed gaps in the canopy to measure the woody material that had fallen to the ground.
If the number of trees present declines on a large scale, that adds up to a lot of carbon dioxide left in the atmosphere to contribute to greenhouse warming, which can feed the cycle of the Amazon seeing more droughts in the future.