- Using airborne light detection and ranging technology, more commonly known as “lidar,” a team of researchers remotely studied tree death and canopy gaps across the Brazilian Amazon.
- Gaps in the forest, the researchers found, were mostly driven by water stress, soil fertility, floodplains and forest degradation. The data also pointed to strong correlations between the patterns of tree gaps and water deficit, a lack of water that can slow down photosynthesis.
- In the southeastern and western Amazon, a pattern of 20 to 35% higher gap dynamics emerged, meaning trees are dying and creating gaps more frequently there than in other regions.
- Aircraft lidar is helpful for studying remote areas of the Amazon and could be used effectively to monitor for illegal logging and deforestation, as well as for calibrating satellite technology.
In the skies above far-flung corners of the Brazilian Amazon, a small plane aims laser beams down at the treetops to create a real-time topography down to the ground. Its goal is simple: find gaps in the forest.
“It may sound like a Star Wars movie, but this is just another application of lasers in our day-to-day lives,” Ricardo Dalagnol, a scientist at the Brazilian National Institute for Space Research (INPE), told Mongabay in an email. “In practice, millions of laser beams are shot from an airplane over the forest, some beams hit the trees and some hit the ground. With this information, we can map trees and gaps.”
Using this technique of airborne light detection and ranging technology, more commonly known as “lidar,” a team of researchers from INPE and the universities of Leeds and Birmingham in the UK. remotely studied tree death and canopy gaps — holes in the green cover that extend from the tree tops down to the understory or ground. Their findings have been published in the journal Scientific Reports.
Dalagnol, the study’s lead author, said reports of increased tree mortality in the Amazon motivated the research. The rate of tree death has been increasing in the Amazon over the past few decades, and this has consequences for its people and ecosystems, as well as for the global climate.
“This is worrying given potential effects of climate change and human pressure over the undisturbed forests,” Dalagnol said. “Using this technology, we wanted to map exactly where canopy gaps and tree mortality were happening more frequently and to reach areas not previously studied before.”
A small plane equipped with lidar made 610 flight lines across the Amazon region. Each flight line measured an area of forest 300 meters wide by 12.5 kilometers long (984 feet wide by 7 miles long) down below. The locations of these flight lines were chosen to cover a broad range of the forest as well as to sample along environmental gradients such as differences in water, soil fertility and forest degradation. The team also used 181 field plots to “ground truth” the lidar measurements and found the lidar method to be highly accurate.
Gaps in the forest, the researchers found, were mostly driven by water stress, soil fertility, floodplains and forest degradation. In areas with more fertile soil, trees grow, and therefore die, a lot faster than in areas with nutrient-poor soil. These more frequent tree deaths mean a faster turnover of what are known as dynamic gaps, or new gaps in the forest. The data also pointed to strong correlations between the patterns of tree gaps and water deficit, a lack of water that can slow down photosynthesis.
“Gaps are an indicator of [tree] mortality, carbon, and state transition,” Eben Broadbent, an assistant professor at the University of Florida and co-director of the GatorEye Unpiloted Flying Laboratory, who was not involved in the study, told Mongabay. “This might involve a tropical forest changing into more of a savannah…and this is a concern, and actually a very real possibility, especially when you start considering feedback from and fire and climate change.”
More gaps per area of forest were found closer to human disturbance, such as in the infamous “arc of deforestation.” But overall, the effects of human disturbance did not show up as strongly in the study as the researchers expected. The effect of humans was likely underestimated in the analysis, the authors said, because there still isn’t a very accurate way to measure human-caused forest disturbance apart from measuring the distance to non-forested areas.
The authors found a correlation between long-term gaps and dynamic (newer) gaps. Areas with lots of long-term gaps in the forest also had more new gaps opening up. The southeastern and western Amazon are drier, with less standing water. In these areas, a pattern of 20 to 35% higher gap dynamics emerged, meaning trees are dying and creating gaps more frequently here than in other regions.
“I think the driving message is that there’s a lot of degradation and disturbance mortality happening in these forests,” Broadbent said. “The thing about all these factors that they correlate with increased [long-term] gaps … is that these are factors that are going to change the climate change. And so, it’s really a big deal to highlight this and to start to quantify them.”
Aircraft lidar is helpful for studying remote areas of the Amazon. For instance, lidar data have been collected and used by the Brazilian Forest Service (BFS) to monitor forests under selective logging management since at least 2015, Dalagnol said.
“Besides the new insights on forest dynamics and tree mortality for the Amazonian region, this study also highlights the scarcity of data that we currently have, and how important airborne lidar data can be to complement existing field efforts and [to] reach places we have never stepped foot [in] before,” Dalagnol said.
Lidar could also be used to monitor for illegal logging and deforestation, but is not yet practical because of its high cost. High-resolution, high-frequency satellite imagery, now free and accessible through platforms such as Global Forest Watch, is still a much more cost-effective way to monitor for deforestation. But the level of detail lidar can provide, such as individual tree height and potentially tree species, isn’t available with satellite technology.
“Everything is slowly moving towards satellites…We will have high-res lidar data from satellites within the next ten years,” Broadbent said, “but large lidar data sets like this are important for calibrating those satellites.”
“Lidar is an amazing technology,” Dalagnol said, “and there is still so much we can discover from that.”
Dalagnol, R., Wagner, F. H., Galvão, L. S., Streher, A. S., Phillips, O. L., Gloor, E., … Aragão, L. E. (2021). Large-scale variations in the dynamics of Amazon forest canopy gaps from airborne lidar data and opportunities for tree mortality estimates. Scientific Reports, 11(1). doi:10.1038/s41598-020-80809-w
Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough