NASA's space laser offers solutions to a puzzle surrounding the rainforest canopy.

More than the surface of Mars or the bottom of the ocean, we know less about the rainforest canopy where the majority of the world's species reside. GEDI, a NASA satellite laser that has delivered the first-ever detailed structure of the world's rainforests, is set to change that, though.

The majority of species on earth live in tropical forests and depend heavily on the canopy, yet we still know so little about them, according to Doughty. These results will help us comprehend how vulnerable tropical forest animals are to climate change. "Rainforest structure matters because it controls how animals access resources and evade predators."

The study of forest canopies has advanced significantly. Tropical woodlands were called horror vacui (nature abhors a vacuum) by early Western explorers because the vegetation was "anxious to fill every available space with stems and leaves."

Later, as researchers dug further into tropical woods, they divided the luxuriant vegetation into layers that included a thick upper crown, a thick middle layer, and a thin layer in between. But this was only seen in a few cases.

Most tropical forest structures are yet unknown.

The Global Ecosystem Dynamics Investigation (GEDI) then appeared. "A key difference between GEDI and many other satellites is its measurement of three-dimensional canopy structure," said Hao Tang, a professor in the Department of Geography at the National University of Singapore (NUS) and a co-author of the study.

According to Tang, who is also a principal investigator at the NUS Centre for Nature-based Climate Solutions, "Conventional satellites frequently lack the precise vertical information provided by GEDI, although offering vital data on land cover and canopy greenness. Understanding ecosystem dynamics, carbon storage, and biodiversity—things that are difficult to discern from standard satellite images—requires vertical information.

NASA's GEDI, which was launched in late 2018, fires an indistinct laser into the forests of Earth hundreds of times per day from the International Space Station. A detailed 3D map showing where the leaves and branches are in a forest and how they change over time can be produced depending on the quantity of energy returned to the satellite.

Researchers will be able to better grasp how much carbon and biomass trees store and lose when disturbed—important knowledge for comprehending the Earth's carbon cycle and how it is changing.

In their analysis of GEDI data from all tropical forests, Doughty, Tang, and the other authors of the paper discovered that the structure was more open to sunlight and simpler than previously believed.

Data also disproved the fullest-at-the-top hypothesis of early researchers by showing that the majority of tropical forests (80% of the Amazon and 70% of Southeast Asia and the Congo Basin) have a peak in the number of leaves at 15 meters rather than at the canopy top.

Although there are differences between forests, a fundamental result that seems to apply in every case was that forests become shorter, less stratified, and have lower biomass when less optimal conditions (such as greater temperatures or poorer fertility) are present.

Given how this structure type contrasts with what we learned in the traditional texts on the subject, Doughty said it was "really surprising" to observe its predominance.

These discoveries "will not only help us understand how the millions of species that live in a rainforest canopy might acclimatize to changing temperatures but also how much carbon these forests hold and how effective they are at battling climate change.

Early explorers have recognized the stratified nature of tropical forest structure, but it had not been able to quantify stratification across all tropical forests until recently thanks to satellite-based LiDAR (GEDI, or Global Ecosystems Dynamics Investigation LiDAR).

According to some estimates, tropical forest canopies are home to the majority of the world's species, hence understanding stratification is crucial.

Stratification has the potential to alter a species' sensitivity to anthropogenic climate change by altering the vertical microenvironment. Here, we discover that the majority of footprints (60–90%) do not include many layers of vegetation based on analysis of each GEDI 25m diameter footprint in tropical forests (after screening for human effect).

The most typical forest form, referred to as a "one peak footprint," has a minimum plant area index (PAI) at about 40 meters, a rise in PAI until about 15 meters, and a drop in PAI to the ground layer. Large geographic areas

Patterns in forest structure can be found both within the Amazon basin (60–90% at one peak) and between the Amazon (79–9% standard deviation) and SE Asia or Africa (72–14% vs. 73–11%). Tree height (r2=0.12) and forest biomass (r2=0.14) are highly associated with the number of canopy layers.

Given the complex and heterogeneous local structural to regional climatic interactions, environmental variables such as maximum temperature (Tmax) (r2=0.05), vapor pressure deficit (VPD) (r2=0.03), and soil fertility proxies (e.g. total cation exchange capacity -r2=0.01) were also statistically significant but less strongly correlated.

The Pebas Formation and Ecoregions are two borders that clearly define structural changes on a continental scale. In general, less-than-ideal conditions (such as greater temperatures or poorer fertility) result in shorter, less-stratified forests with lower biomass.