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How does a modern scientist measure the exact size of a tree?

a tree from two angles in different colours with colour scale beside
A typical 3D model generated by a LiDAR scanner. The colours in this picture scale the height of the tree, which is one of Linda’s savanna trees. (Image: Linda Luck)

Have you ever stood at the base of a tree and marveled at how big it is? What if you were tasked with establishing just that; calculating exactly how big a tree is from base to tree-top, branch to branch, each and every leaf. Sounds hard right? Now imagine you have to work out the size of every tree in an entire forest… Impossible? And why would anyone ever embark on such an exercise?

Well, one PhD student at Charles Darwin University (CDU) is aiming to do just that - and has a cutting-edge method for doing so. Linda Luck from the Research Institute for the Environment and Livelihoods (RIEL) wants to know how big a lot of trees are, because tree size and development has important ramifications in the fight against climate change.

Trees are principally made up of carbon and are therefore a great natural source of carbon storage, dragging carbon dioxide (a greenhouse gas) out of the air and into the wood. Knowing the size and speed of growth (or decline) of trees is therefore useful in calculating the carbon storage potential of various natural environments.

Linda’s research uses LiDAR, a relatively new technology with massive potential. LiDAR stands for Light Detection and Ranging and is similar in many ways to radar technology. LiDAR measures the distance from a LiDAR scanner (which comes in many shapes and sizes) to the surfaces of the environment all around the scanner, often in every direction (within a limited range). Once you start your LiDAR scanning, it will shoot out millions of laser pulses, each of which generate a point in 3D-space that can be interpreted by a computer into a model of the area scanned. Unfortunately, LiDAR can’t see through solid objects, but if you set your LiDAR scanner up in multiple places around a stand of trees, you can get a readout of every side of those trees. The computer can then render a (often rainbow-coloured) 3D image of the trees you have scanned, such as the one seen above.

So, LiDAR can generate some rainbow-colored images of 3D trees. Why does that matter? Well, when you have a 3D model of a tree, you can use a computer to determine its size. The data LiDAR generates is incredibly detailed and a computer can use it to determine the shape (and therefore volume) of all the tree’s features. Now you can much more accurately estimate how much carbon the tree is storing and monitor how that tree is growing through time. In Linda’s own words, “LiDAR makes previously unobtainable tree measurements available to ecologists and land managers for the purpose of monitoring tree development and carbon stocks.”

Linda’s research is not the first to use LiDAR to measure tree biomass, but she hopes to streamline the process, particularly when it comes to the computing. “I am finding ways to make data processing and analysis more streamlined and less expensive, so I can apply these methods to answer ecological questions such as: How does tree biomass and therefore carbon content vary in space and time?” Traditionally, estimates of tree biomass rely on far cruder measurements such as ‘diameter at breast height’ (DBH), a measure of the circumference of a tree trunk taken at 130 cm above ground level. This method relies on a formula to estimate the average biomass for a tree of a given DBH. This works well in pine forests - where each individual tree is nice, straight, and uniform, but it is less accurate in savanna trees where damage from fires, storms and termites, make for some gnarly branches and twisted trunks. LiDAR has the potential to greatly improve tree biomass estimations in savannas, and therefore our understanding of the carbon cycle.

Linda’s research is focusing on the savanna trees of Litchfield National Park in the Northern Territory under the supervision of Lindsay Hutley (CDU), Shaun Levick (CSIRO) and Kim Calders (Ghent University).

By Robin Leppitt and Linda Luck

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