Missing the forest for the trees

Covering 2.7 million square miles, the Amazon Basin is home to an estimated 16,000 different tree species and 390 billion individual trees. Yet one tree was interesting enough to lure a group of researchers on a six-day trek through the rainforest just so they could survey it with their own eyes.

The tree first appeared as a lidar signature in data that was gathered during an airborne survey of a small section of the forest. Initially, the signature confused Eric Bastos Gorgens, a forest engineer at the Federal University of the Valleys of Jequitinhonha and Mucuri in Diamantina, Brazil. Most of the lidar survey showed a trackless forest canopy that reached between 130 and 165 ft above the ground. But there were occasional anomalies that reached substantially higher, and one that towered 290 ft above the forest floor.

Gorgens and a team of researchers hypothesized that the largest outlier signal in their current survey must be a giant tree. But neither the vertical nor horizontal resolution of the airborne lidar system was high enough to distinguish individual specimens. The survey data, which covered only 0.01% of the Amazon region, largely underscored just how incredibly ignorant we still remain about vast portions of the planet we live on.

A GeoSlam portable lidar image showing the trunk of a giant tree deep in the Amazonian rainforest. Courtesy of E.B. Gorgens and J. Rosette, 2023.

Therefore, the only way to confirm that a giant tree was causing the errant lidar signature was to travel 160 miles up the Jari River and hack through 12 more miles of hot, dense, viper-ridden rainforest to take a look around.

One expedition, launched in 2019, came within 3 miles of the giant tree’s location before the terrain and diminishing food supplies forced the team to turn back. Last September, a second expedition, led again by Gorgens, returned better equipped and with better knowledge of the surrounding terrain. Braving fevers, river rapids, and pit vipers during eight days, they finally arrived at the base of the giant tree — or at least they thought it was their tree. The dense forest canopy made it difficult to confirm.

Photonics, in the form of a conventional drone-mounted digital camera, elevated their perspective above the canopy where they confirmed they had found their forest giant, which belonged to the species Dinizia excelsa.

Because much of the Amazon Basin remains unsurveyed, even larger giant trees might yet be discovered. The Global Ecosystem Dynamics Investigation mission, launched in 2018, captured full-waveform lidar data for the entire region from the lofty perspective of the International Space Station. This data suggests that there may be trees growing over 300 ft high deeper in the rainforest. But without the resolution of airborne lidar — or the resolution of even more intrepid and better equipped expeditions — these trees remain legends.

The research was published in Nature (www.doi.org/10.1038/d41586-023-01828-x).