By Alexandria Herr
In the tropical heat of Danum valley, Malaysia, in early August of 2018, Alexander Shenkin and Chris Chandler stood at the base of a mammoth trunk. A quick estimate with a laser hypsometer confirmed what Shenkin and Chandler already suspected to be true: they were standing next to the world’s tallest tropical tree. At 100.8 meters tall, the tree reaches a bit higher than the Big Ben. Even a tree like this can be easy to miss from the dense understory of the forest floor, but new technologies are allowing scientists to identify these giants with greater ease; Chandler initially spotted the tree using Airborne LiDAR, a laser sensor flown over the canopy of the forest. Chandler, a PhD researcher at University of Nottingham, and Shenkin, a postdoctoral researcher at Oxford, are a part of a movement of scientists bringing technology, including lasers and drones, into some of the most challenging ecosystems on earth.
LiDAR, short for Light Detection and Ranging, functions not too dissimilarly from bat echolocation. The sensor sends out laser pulses at an incredible speed, 500,000 per second, and measures the time that it takes for the light to bounce back. This information is then transformed to a ‘point-cloud’, a map of millions of pinpricks in space that together represent a 3-D view of a tropical forest. Tropical ecology hasn’t always been this high-tech. LiDAR technology originated in the early 1960s, and was first used by scientists at the National Center for Atmospheric Research to measure clouds. The dawn of the 1970s brought LiDAR into the public eye when the Apollo 15 mission used laser altimetry in 1971 to map the surface of the moon. Even after its application on the moon, LiDAR didn’t make it to tropical forests until 1978, when scientists from the Canadian Forest Services tested LiDAR technology as a means of measuring forest canopies in Costa Rica. Since the 70s, LiDAR has exploded within ecology, and has been used to study everything from wildfires to bird habitats. Using this technology, scientists have been able to gain insight into the dense canopies of tropical forests unlike ever before.
Before LiDAR, understanding exactly how much carbon was stored in a tropical forest was no easy task. The best way of measuring the biomass of a tree is to physically cut it down and take a tape measure to it, branch by branch. This process is arduous, requires huge man power, and above all, is extremely risky. Tree felling in the tropics is “one of the most dangerous jobs in the world,” according to Shenkin, as the tree could land on the crew or take nearby trees down with it. Measuring a single tree this way can take upwards of two days. Only after compiling measurements of thousands of trees from different studies can scientists finally build equations called allometries, which describe the different relationships between a tree’s diameter, height, and mass. LiDAR isn’t necessarily more accurate; even now, the most accurate way to confirm tree height is still the old fashioned way (in January 2019, Unding Jami, a Southeast Asia Rainforest Research Partnership climber and researcher, scaled the tree in Malaysia with a measuring tape to confirm the world record). But the advantage of LiDAR technology is speed; the process of building allometries with LiDAR is much faster than hand measurement. More data to build these equations will ultimately give scientists the tools to produce better measures of how much carbon the tropics actually contains.
In ecology, tropical forests are another kind of final frontier. They are the most diverse but least understood terrestrial ecosystem on earth. Despite covering only 7% of the Earth’s surface, they contain more than half of the world’s species, many of which are still undiscovered. Beyond their unique biology, tropical forests have also become vitally important in the context of climate change. When a ton of CO2 is pumped into the atmosphere, only half of it actually remains there. The other half is absorbed by the biosphere: 30% by the land and 20% by the ocean. In fact, trees are the most inexpensive and efficient Carbon Capture and Storage technology on the market; through the process of photosynthesis, they suck CO2 out of the atmosphere and store it in plant tissue. Tropical forests, which have more photosynthetic productivity per unit area than any other ecosystem on the planet, are major engine of this carbon sink. Like giant lungs belted around the earth’s tropics, they account for almost half of all CO2 absorbed by the land.
This carbon sink may be in danger of disappearing. Until now, the amount of CO2 absorbed by tropical forests has been increasing with rising carbon emissions, a phenomenon dubbed ‘carbon fertilization’. Scientists warn, however, that the increasing dry season and temperature in the tropics could lead to tree die-off, turning the carbon capture function of the forests into a source of carbon emissions. This potential reversal of carbon flow in tropical forests is one of the tipping points that some scientists are concerned could lead to exponential climate change. This danger is part of the reason it is vital that scientists keep a pulse on how tropical forests are responding to rising temperature and CO2 levels, and also why remote sensing technologies like LiDAR have proven to be so useful in the context of ecology.
LiDAR has also provided new insight into basic nature of tropical tree structure. The evolution of tropical tree biology is deeply influenced by one basic constraint: the need for light to fuel photosynthesis. Light and water are essential to the survival of plants, but while water is usually abundant in tropical rainforests, competition for light is intense on the forest floor. As soon a mature tree dies, leaving behind a patch of light, there is a sudden race for seedlings to fill the gap. Life is creative, so different species have developed a variety of strategies to survive the brutal fight for light on the forest floor. Some have developed a long-lived strategy and are able to tolerate low-light conditions as they slowly emerge into the canopy. Others have a live-fast die young lifestyle, shooting up as fast as possible in the race for the sunlight, but unable to sustain their quickly-built tissues for nearly as long as the denser, slow-growing trees. For Shenkin, using LIDAR to build detailed representations of tropical forests is a tool that can give ecologists more insight into tree architecture and ecology. An engineer by training, Shenkin wants to understand how tropical trees are built on a very fundamental level, and what limits their height in the canopy. “It’s a big mystery how all these different species are put together,” said Shenkin. Understanding how trees are constructed might give insight, on a micro-level, into how tropical forests could respond to changing climatic conditions, including drought or increasing temperature. “Tree architecture could say a lot about how trees may or may not be resilient to future changes in the environment,” said Shenkin, “we don’t even know all the import of it because we don’t understand it in the first place. It’s so unexplored.”
Alexandria Herr is an MSc student in Environmental Change and Management at the Environmental Change Institute, Oxford. In the fall she will be beginning a PhD in Environment and Sustainability studies at the Institute on Environment and Sustainability at UCLA. Her research interests lie at the intersection of climate, justice, and environmental narrative. She is currently a head editor of Anthroposphere.
Editor's Note (7/5/19): An earlier version of this article incorrectly said that Alexander Shenkin was a postdoctoral student and Chris Chandler a PhD student. This has now been rectified.
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