The International Space Station is being used to map the Earth's surface as part of the Global Ecosystem Dynamics Investigation (GEDI).
Tropical forests are not immune to the growing stress caused by climate change, according to a new study carried out by researchers at Harvard University. The study used laser satellites on the International Space Station to map the Earth's surface as part of the Global Ecosystem Dynamics Investigation (GEDI).
The results provide a deeper and broader insight into how global warming is affecting the canopy height and health of tropical forest trees in Asia, Africa and South America, using canopy size as a key indicator of forest health and carbon storage capacity.
The forest canopy, the “upper layer” of mature trees, is essential to the stability of the ecosystem: “The forest canopy is a very important indicator of forest health and ecosystem productivity,” explains the study's lead author, postdoctoral fellow Shaoqing Liu (Shaoqing Liu). “Taller canopies are usually associated with large carbon stocks and greater above-ground biomass. Tall canopies protect the microclimate.” Tall trees can often help to cool the air during periods of extreme heat.
Using GEDI's space-based Lidar (Light Detection and Ranging), the research team obtained global, highly detailed data on tropical forests, with a level of coverage and accuracy that far exceeds earlier studies, which were mostly carried out at a local or regional scale.
“Over the past ten years, NASA has used the International Space Station as a convenient platform for evaluating new types of remote sensing measurement from space,” says the study's principal author, Paul Moorcroft (Paul Moorcroft). “The Global Ecosystem Dynamics Investigation's waveform Lidar is an excellent example of this approach.”
GEDI (pronounced “jedi”) uses laser pulses to measure the vertical structure of forests. This includes canopy height and the density of leaves and branches throughout the forest stand.
“Our study demonstrates that climate, topography and soil properties account for almost three quarters of the variation in tropical forest canopy height,” said S. Liu. According to her, the study found that the most critical determinants of canopy height, at both local and regional scales, are elevation, dry season length and solar radiation. Not all tropical forests respond to climate change in the same way. The southern part of the Amazon, for example, is at pronounced risk.
S. Liu added that the tropical forests of the southern Amazon are vulnerable to climate change because the dry season is becoming ever longer: “The dry season is the decisive factor determining the height of tree canopies in the forest.”
As climate models predict that dry periods will become even longer in future, S. Liu warned that canopy height in the forests of the southern Amazon may decline considerably. Other regions show different trends. “In the central Amazon, where the climate is relatively wet, the first and most important factor is tree height,” the researcher explained. “The same applies to areas of Africa, where terrain (elevation) matters more than drought or heat.”
The findings have significant implications for forest conservation and climate strategy. According to the study's principal author, P. Moorcroft, understanding environmental influences on the height of tropical forests is important for assessing the carbon sequestration and conservation value of different areas of tropical forest.
“To understand how tropical forests will respond to climate change, it is also important to understand the environmental factors that influence variation in forest canopy height,” said P. Moorcroft.
Shaoqing Liu believes the study can help inform policy decisions on where nature conservation efforts should be concentrated: “When it comes to climate change policy, we see that tropical forests are not only biodiversity “hotspots”, they are also extremely important for carbon storage. Protecting them is essential to mitigating climate change.”
“We hope to help policymakers identify and prioritise the areas threatened by climate change.” S. Liu also plans to extend this research beyond undisturbed primary forests. She hopes the study will also take in other types of forest and wooded land, in order to better understand how the Earth's woody plants are adapting to, or struggling with, the pressures created by climate variability.
The full study is published in the journal Proceedings of the National Academy of Sciences.