Lower canopies show struggle for tropical forests — Harvard Gazette

With their ability to store carbon, forests are often considered the lungs of the Earth, but they are vulnerable to the world’s ills, too. A new study, using NASA laser technology from the International Space Station, reveals the impact of climate change on global tropical forests with greater depth and breadth than ever before.

That study, “Environmental drivers of spatial variation in tropical forest canopy height: Insights from NASA’s GEDI spaceborne LiDAR,” published in the Proceedings of the National Academy of Sciences, used measurements from the Global Ecosystem Dynamics Investigation (GEDI), a LiDAR laser instrument on the International Space Station, to look at changes in tropical forest canopy height and study how this crucial measure has been affected by heat, drought, and other aspects of climate change.

The forest canopy, the upper layer of mature trees, “is a very critical indicator of forest health and ecosystem productivity,” explained Shaoqing Liu, a postdoctoral fellow in Organismic and Evolutionary Biology (OEB) and the first author on the paper.

“In general, taller canopies are associated with high carbon storage and greater above-ground biomass. Tall canopies can buffer the microclimate,” Liu said, even helping reduce the temperature during heat waves. The study looked at tropical forests in Asia, Africa, and South America — lands with minimal disturbances or human activities such as logging.

To measure changes in such forests, his group used laser measurements from GEDI, which allowed the group to study a wide swath of forests globally, whereas earlier studies had been limited to small areas.

“Over the past decade, NASA has been using the International Space Station as a convenient platform for evaluating new forms of space-based remote sensing measurements,” said Paul Moorcroft, professor of OEB and senior author of the study. “The Global Ecosystem Dynamics Investigation waveform LiDAR is a prime example of this approach.”

GEDI — pronounced “Jedi” — “can tell us the vertical structure of the forest canopy” such as leaf density, said Liu. “Our study demonstrates that climate, topography, and soil properties account for almost three-quarters of the variation in tropical forest canopy height. We also found the elevation, dry season, and solar radiation are the most important variants to determine the canopy height.”

The researchers discovered that “tropical forests in the southern Amazon area are vulnerable to climate change” because of increasingly prolonged dry seasons. “The dry season is the dominant driver determining forest canopy height in this area,” said Liu.

Because global climate model projections show that this area will have longer dry seasons, “We may see significant reductions in canopy height,” he added.

“Understanding the environmental controls of tropical forest height is important for assessing the carbon sequestration and conservation value of different tropical forest areas,” said Moorcroft. “Understanding the environmental drivers of forest canopy height variation is also crucial for understanding how tropical forests will respond to climate change.”

The impact of climate change is not uniform, however. Thanks to GEDI, the researchers were able to view differences in its manifestation and effect on the canopy. “In the central Amazon, because it is relatively moist, the first important driver is actually elevation,” said Liu. This was also true in Africa, the researchers found.

Looking ahead, Liu would like to move beyond studying the primary forest to examine more of the globe’s forest and woodland areas. He said he hopes these studies will influence policy.

“In terms of climate-change policies, we see the tropical forests are not only biodiversity hotspots, they are critical for carbon storage. Protecting them is essential for mitigating climate change,” he said. “We hope to help policymakers help identify areas that are vulnerable to climate change and prioritize them.”

Funding for this study was provided, in part, by a NASA grant.

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