Capturing the effects of El Niño in three dimensions
Last updated: 23 Aug, 2018
Ask a scientist how they study the effects of drought on the Amazon rainforest, and 'lasers' might not be the first thing you expect them to say.
Droughts kill trees in rainforests. In a rainforest as vast as the Amazon, estimating the number of dying or damaged trees is extremely difficult. Traditionally, researchers hike in and can only survey a limited area.
NASA researchers have been using lasers – or, more specifically, light detection and ranging (LiDAR) technology mounted on an aeroplane – to create a three-dimensional model of the rainforest canopy. By measuring 300,000 bounces of laser light per second, the researchers were able to create a detailed model of the forest over a much greater area than they could cover on foot.
By analysing data taken from three surveys in 2013, 2014 and 2016, 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, they detected fallen trees or branches that had altered 1.8 percent of the forest canopy.
This might seem like a small area but, scaled up to the size of the entire Amazon, it's the equivalent of losing canopy trees or branches over nearly 10 million hectares. This area increased by 65 percent between 2014 and 2016.
"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 Earth system scientist Doug Morton at NASA’s Goddard Space Flight Center, and a co-author on the paper.
"Climate projections for the Amazon basin suggest warmer and drier conditions in coming decades," said Morton. "Drought events give us a preview of how tropical forests may react to a warmer world.”
The research team lays out a transect tape along which they measured the amount and location of woody debris on the forest floor, Tapajós National Forest, Brazil.
Credit: NASA/Veronika Leitold
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. This painstaking effort to measure downed branches and trees was essential to estimate the total amount of carbon lost when trees or branches fall in a tall, multi-layered Amazon rainforest.
"[This] is one of the first studies to use repeated LiDAR during the drought and have people go into the field and conduct all the measurements," said ecologist Paulo Brando at the Woods Hole Research Center in Falmouth, Massachusetts, and the Amazon Environmental Research Institute in Brazil, who was not involved in the study. "This combination is extremely powerful to understand not only what is happening, but why it is happening, and why it’s changing during the drought."
Read the paper: Leitold, V., Morton, D. C., Longo, M., dos‐Santos, M. N., Keller, M. and Scaranello, M. (2018) El Niño drought increased canopy turnover in Amazon forests. New Phytologist 219: 959-971. doi: 10.1111/nph.15110
This piece contains material taken from an original press release.