Laser technology measures forest canopy height from space

There’s good reason why tropical forests are often called the lungs of the planet. They produce oxygen through photosynthesis and absorb massive amounts of carbon dioxide. In this way, they help regulate Earth’s climate.

But these vital ecosystems are not immune to the growing stress of a changing climate.

A new study, conducted by researchers from Harvard University, used a special laser detection system mounted aboard the International Space Station. This remote sensing technology is able to measure the 3D structure of objects on Earth’s surface. It is used as part of the Global Ecosystem Dynamics Investigation (GEDI).

The new findings offer a deeper, more complete look at how global warming is impacting the height – and health – of tropical forest canopies across Asia, Africa, and South America, using canopy height as a key indicator of forest health and carbon storage capacity.

Why forest height matters

The forest canopy – the uppermost layer formed by mature trees – is essential for ecosystem stability.

“The canopy is a very critical indicator of forest health and ecosystem productivity,” explained Shaoqing Liu, a postdoctoral fellow and lead author of the study.

“In general, taller canopies are associated with high carbon storage and greater above-ground biomass. Tall canopies can buffer the microclimate.”

In fact, tall trees can even help cool the air during extreme heat events.

By using GEDI’s space-based LiDAR (Light Detection and Ranging), the research team was able to gather global data on tropical forests in great detail with a level of coverage and accuracy that far surpasses earlier studies, which were mostly conducted on local or regional scales.

A unique eye from space

“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, senior author of the study.

“The Global Ecosystem Dynamics Investigation waveform LiDAR is a prime example of this approach.”

GEDI – pronounced like “Jedi” – uses laser pulses to measure the vertical structure of forests. This includes not just canopy height, but also the density of leaves and branches throughout the forest column.

“Our study demonstrates that climate, topography, and soil properties account for almost three-quarters of the variation in tropical forest canopy height,” said Liu.

In addition, the study revealed that elevation, dry season length, and solar radiation are the most important drivers of canopy height both locally and regionally.

Forests face big climate challenges

Not all tropical forests are reacting to climate change in the same way. The southern Amazon, for example, faces a distinct risk.

Liu noted 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.”

With climate models predicting even longer dry seasons in the future, Liu warned that we may see significant reductions in canopy height in the southern Amazon. Other regions show different trends.

“In the central Amazon, because it is relatively moist, the first important driver is actually elevation,” Liu explained. The same holds true in parts of Africa, where elevation appears to play a stronger role than drought or heat.

Policy implications and future goals

The findings have significant implications for forest conservation and climate strategies. According to Moorcroft, understanding the environmental controls of tropical forest height is important for assessing the carbon sequestration and conservation value of different tropical forest areas.

“Understanding the environmental drivers of forest canopy height variation is also crucial for understanding how tropical forests will respond to climate change,” said Moorcroft.

Liu believes that the research can help inform policy decisions on where conservation efforts should be concentrated.

“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,” said Liu.

“We hope to help policymakers identify areas that are vulnerable to climate change and prioritize them.”

Liu also plans to expand this research beyond untouched primary forests. He hopes to include other types of woodlands and forested areas to build a fuller picture of how Earth’s greenery is adapting – or struggling – to cope with climate pressure.

The full study was published in the journal Proceedings of the National Academy of Sciences.

Image Credit: NASA

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