Rainforest crowns are lighter than expected
Tropical forests have an unexpectedly weak layered structure
Layer change: The vegetation of most tropical forests is layered differently in many places than previously assumed, as researchers have now discovered using space laser data. Accordingly, the density of most rainforests does not increase towards the canopy, but reaches a single density peak at around 15 meters. This also means that the forests and treetops are more exposed to sunlight than previously thought and therefore possibly even more vulnerable to rising temperatures.
Most of the world’s species live in the canopy of the rainforest , yet we know less about this habitat and the structure of tropical forests than we do about the surface of Mars or the bottom of the ocean. Until now, it has been assumed that the lush flora of the rainforest can be divided into specific vertical layers. Accordingly, the vegetation is particularly dense both halfway up and in the treetops. Between these two peaks and towards the ground there should again be thinner overgrown layers.
A space laser maps the tropics
Researchers led by Christopher Doughty from Northern Arizona University are now questioning this assumption, however, because using state-of-the-art methods they have succeeded for the first time in analyzing the structure of all of the world ‘s major rainforests . The data for this comes from a NASA space laser attached to the International Space Station (ISS). As part of the GEDI mission (Global Ecosystem Dynamics Investigation), it has been firing an invisible laser beam at the world’s forests thousands of times a day since the end of 2018.
The strength and shape of the reflection of this laser radiation provides information about the density of the vegetation at the different heights of the forest. In this way, a 3D map of all tropical forests can be created, which even includes individual leaves and branches and thus provides information about the vertical vegetation structure within the forests. With the help of the GEDI data, Doughty and his colleagues were able to check for the first time whether the previously assumed vegetation structure actually corresponds to reality.
Reality doesn’t follow the textbook
The result: “The most common forest structure shows a minimum vegetation density at around 40 meters, followed by an increase to 15 meters, followed by a decrease to the soil layer,” the researchers report. In other words: Contrary to previous assumptions, most tropical forests have a density peak at a height of 15 meters, but no second one in the canopy. According to the analysis by Doughty and his team, this applies to 80 percent of the Amazon rainforest and 70 percent of the forests in Southeast Asia and the Congo Basin.
“It was really surprising to see the dominance of this type of structure because it’s different from what we’ve read in the classic textbooks on the subject,” Doughty says. The researchers also found textbook structure types with two density peaks, in which the canopy forms the densest layer of vegetation, but they were rare. Doughty and his colleagues were only able to identify this growth type in around three to six percent of the tropical forest area.
Consequences of sparse canopies unknown
Overall, however, the study results also mean that the tropical canopies are much lighter than expected in many places and are therefore also more susceptible to rising temperatures. How the animals living in these lofty heights deal with the possible heat stress is still unclear. But Doughty reckons the GEDI data will help learn more about their lifestyles and adaptations to climate change.
The laser scans could also be useful in determining more precisely the carbon storage capacity of forests and thus their contribution to the fight against climate change. (Environmental Research Ecology, 2023; doi:10.1088/2752-664X/ace723 )
