Amazon rainforest inhabitant, one very fine brown-hooded parrot looking handsome

Lidar used to make 3D model of the Amazon to understand drought impact

Image credit: Dreamstime

Nasa has used lidar technology to scan the Amazon rainforest in order to better understand the impact of the El Niño-driven drought in 2015-2016.

The three-dimensional measurements have given Nasa an in-depth insight into the number of branch falls and tree mortality that occur in response to drought conditions. They found that 65 percent more trees and large branches died due to the El Niño event compared to an average year.

Understanding the effects of prolonged drought gives scientists a better sense of what may happen to carbon stored in tropical forests if these events become more common in the future.

“Climate projections for the Amazon basin suggest warmer and drier conditions in coming decades,” said Nasa’s Doug Morton. “Drought events give us a preview of how tropical forests may react to a warmer world.”

Without regular rainfalls, trees are at risk as they cannot transport enough water from the soil to their canopies, which can reach 15 to 20 stories high. 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. Traditionally, researchers hike in and survey a few acres of trees to measure living trees and dead debris on the ground.

Morton and his colleagues took the bird’s eye perspective using lidar technology mounted onto an airplane to create a 3D reconstruction of the same forest canopy over three separate flights in 2013, 2014 and 2016.

With 300,000 laser pulses a second, the data provides an incredibly detailed depiction of the forest over a much greater area than they could cover on foot. Analysing 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 period from 2013 to 2014, the branch and tree fall events altered 1.8 per cent of the forest canopy in the study area. This appears to be a small number, but, scaled up to the size of the entire Amazon, is equivalent to losing canopy trees or branches over 38,000 square miles, or the area of Kentucky. Tree and branch mortality was 65 per cent higher during the El Niño drought period from 2014 to 2016, or 65,000 square miles.

“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. This means that the drought didn’t selectively kill a greater proportion of tall trees than smaller trees, as was previously thought from experiments that simulated drought conditions in small plots.

This is good news for the carbon budget, Morton said: “Large trees hold most of the carbon in any forest. 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."

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