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Researchers turn to drones for that big-picture view of the forest canopy

  • Scientists need to collect data fast to understand how forests are changing due to climate change and deforestation.
  • In a recent study, scientists flew drones over the forest canopy to learn more about tree mortality. The drones revealed new patterns because of the large areas they can cover. According to one researcher, a single drone can cover an area in a few days that would take a team a year on foot.
  • Drones are also helping local and Indigenous communities monitor forest fires and deforestation as well as harvest resources more sustainably.
  • Yet experts say that the useful tool should complement, and not replace, fieldwork done on the ground.

Tropical forests across the world are hurting. Climate change is intensifying drought and wildfires worldwide, and deforestation continues to challenge conservation efforts to save intact forests. This means scientists need to collect data fast and efficiently.

But this often isn’t feasible in remote areas where forests cover large swaths of land. Moreover, remote sensing using satellites can’t always penetrate the cloud-covered canopies of the tropics. In a recent study, scientists piloted drones to learn more about tree mortality across one of the most well-studied research stations in the world: Barro Colorado Island in the Panama Canal, home to the Smithsonian Tropical Research Institute (STRI). K.C. Cushman, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory, who authored the study, says that patterns emerged only because they were able to collect a lot of data across an area much larger than the plots typically studied on the ground.

“It speaks to how important it is to look at these processes over really large areas using tools like drones,” she says.

The kind of soil, age of the forest and steepness of terrain explained most of the differences in the natural disturbances that the scientists saw. Disturbance is more common in older forests, where the canopy trees vary in height and can be exposed to more wind. On steep terrain, trees are more exposed to weather. But trees can form deeper roots in certain soils, helping them stand strong during storms.

And while the proof-of-concept study shows the usefulness of drones, experts say they shouldn’t replace fieldwork on the ground.

A drone image of the dense tropical forests on Barro Colorado Island in the Panama Canal. Image courtesy of the Smithsonian Tropical Research Institute.

Top of the canopy

In the tropics, only about 2% of large trees die each year in intact forests. That’s why scientists need to cover a large area to accurately understand how forests are changing, says Daniel Zuleta, a postdoctoral researcher with the Forest Global Earth Observatory (ForestGEO), who was not involved in the study.

It would take more than a year for a research team to cover what a drone can do in a few days, he says.

Drones can capture a lot of high-resolution images fast, making it plausible to track trends and changes over time. Milton García, a co-author on the recent study and research manager at the STRI, flew the drone over 1,500 hectares (3,700 acres) and took images over the course of five years for the research. After analyzing the images, the team came up with a model of canopy height and tracked how it changed over time.

“That’s really opened up a lot of possibilities for the kinds of questions that we’re able to address … and the kind of scales we can work in to address these challenges,” Cushman says.

Helping forests and people

There are many other ways that researchers and conservationists use drones to study forests. For example, researchers can use drones to study leaf phenology — the timing of leaves emerging and dropping — as well as to map individual trees.

In the Amazon, drones are an important tool for conservationists, “keeping the forest there, keeping the forest healthy,” says Felipe Spina, a conservation analyst with WWF-Brazil.

Milton García pilots a drone that allows the team of scientists to study tree mortality from the air. Image courtesy of K.C. Cushman.

WWF-Brazil is currently training Indigenous communities to map forest resources, like the Brazil nut, so they can make sure they are harvesting sustainably, Spina says. They are also training communities to zone for fire risk and monitor forest fires and deforestation from a safe distance.

They can send in a drone and take pictures of the deforested area and even cars from a safe place where they can’t be seen, Spina says.

“We can use the technology to both gather the data, but also to help and protect the people,” he says.

Not without limitations

Although drones are a great tool to get data fast, cover a lot of ground and repeat data collections over time, they are not without limitations.

Standard drones that capture red, green and blue light, or RGB, can’t capture images from underneath the canopy, Zuleta says. This means the drone can’t detect smaller trees growing underneath the canopy that tend to die off more frequently than larger trees, he adds.

Zuleta also says it’s hard to distinguish between dead and living stems on a tree, all of which are a big part of the carbon balance in a forest.

However, scientists can get measurements from the middle of the canopy using drones equipped with lidar, a laser-based technology that works on the same principles as radar, but this is very expensive, Spina says.

It’s also hard to operate a drone in dense tropical forests with frequent rain, wind and clouds. Battery life is also an issue when trying to cover large swaths of forest such as in the Amazon.

K.C. Cushman and Helene Muller-Landau check out a treefall on the ground on Barro Colorado Island in the Panama Canal. Although drone images are a great way to get data fast, experts agree that drones should not replace on-the-ground observations. Image courtesy of the Smithsonian Tropical Research Institute.

‘There’s nothing better than being there on the ground’

With these limitations in mind, Cushman, Zuleta and Spina all agree that drones should complement work done on the ground, not replace it.

“There’s nothing better than being there on the ground close to what you’re studying,” Spina says. “However, when you’re working in a forest environment, it’s very complex and sometimes even impossible to reach all the areas that are very remote.”

That’s why drones are a great tool to assess threats like wildfires and deforestation before a team is sent in, he adds.

Ground-based collections will always be valuable to “notice aspects of natural history in the field that you might miss,” Cushman says.

Regulations also remain a big hurdle when operating a drone, Spina says. Even if researchers do have a drone that can fly for three hours, legally they may not be able to do it, he says, adding that both the technology and the legislation need to evolve.

Banner image: Conservationists are also training Indigenous communities to map forest resources as well as monitor deforestation and forest fires. Sending in a drone allows the communities to assess risk from a safe distance. Here, Awapy Uru-eu-wau-wau prepares a drone for takeoff from a boat in the Candeias River in the Brazilian Amazon. Image courtesy of WWF-UK.

Citations:

Cushman, K. C., Detto, M., García, M., & Muller-Landau, H. C. (2022). Soils and topography control natural disturbance rates and thereby forest structure in a lowland tropical landscape. Ecology Letters. doi:10.1111/ele.13978

Zuleta, D., Arellano, G., Muller-Landau, H. C., McMahon, S. M., Aguilar, S., Bunyavejchewin, S., … Davies, S. J. (2021). Individual tree damage dominates mortality risk factors across six tropical forests. New Phytologist, 233(2), 705-721. doi:10.1111/nph.17832

Park, J. Y., Muller-Landau, H. C., Lichstein, J. W., Rifai, S. W., Dandois, J. P., & Bohlman, S. A. (2019). Quantifying leaf phenology of individual trees and species in a tropical forest using unmanned aerial vehicle (UAV) images. Remote Sensing11(13), 1534. doi:10.3390/rs11131534

Krůček, M., Král, K., Cushman, K. C., Missarov, A., & Kellner, J. R. (2020). Supervised segmentation of ultra-high-density drone lidar for large-area mapping of individual trees. Remote Sensing12(19), 3260. doi:10.3390/rs12193260

Editor’s note: This story was supported by XPRIZE Rainforest as part of their five-year competition to enhance understanding of the rainforest ecosystem. In respect to Mongabay’s policy on editorial independence, XPRIZE Rainforest does not have any right to assign, review, or edit any content published with their support.