Tree-D modelling: Scanning the savanna with 3D imaging

How do you measure the quantity of carbon in strangely shaped trees? With a little bit of Tree-D modelling!

For National Science Week, we are celebrating a pioneering researcher who is employing a clever remote-sensing technology to help us learn more about tree carbon capture. Linda Luck, a PhD candidate at Charles Darwin University, is using 3D modelling to estimate the amount of carbon in trees across the Top End savanna, to get a better picture about how our woodlands store carbon, and how this can be lost following fires and storms. Luck is working at TERN’s tropical savanna ‘SuperSite’ at Litchfield National Park in collaboration with the CSIRO Land and Water group in Darwin.  

Trees are fantastic storers of carbon and are necessary for mitigating climate change. They act like a sieve, absorbing carbon and leaving the air cleaner. Knowing the quantity of carbon they can store gives a good indication of what the climate might look like in the future.

Estimating carbon is quite difficult in Australian trees, however, because they have unique shapes – also called tree architecture.

“Unlike many other vegetation types, individual trees have really quite different architecture from each other, even within the same species,” says Luck.

“So is the amount of carbon individual trees hold, because there’s just so many fires and storms and termites ravaging the trees.

“What we usually do to try and estimate the amount of carbon a tree holds, without chopping it down, is we measure the diameter (of the trunk) at 130 centimetres above the ground. From that, it takes an equation to estimate approximately how much carbon is in that tree.”

This method works well for trees of a uniform shape, such as pine trees, but not so well for the gnarly, twisted trees that dot the savanna.

Enter the 3D LiDAR (Light Detection and Ranging) scanner, a device that uses the reflection of lasers to estimate the location of matter. Millions of laser beams are emitted from the scanner, and they bounce back if they hit matter. The scanner can measure distance by calculating the time the laser takes to return, because the speed of the laser is always the same.

“Right now, I’m using the LiDAR technology to figure out a more streamlined way of processing (tree) data,” explains Luck.

“The data is amazing. You can get images of individual trees.”

The 3D imaging gives a full picture of the outside shape of individual trees and a clearer picture of volume, so the biomass can be calculated with much greater accuracy.

Read more: Using forests to fight global warming

This type of imaging, with its incredible detail, could also be useful in other areas of science.

“As far as I know, we’re already using it in agriculture in terms of measuring how fast the trees grow on an orchard, for instance,” says Luck.

“Or if you want to do carbon abatement schemes, it helps if you know exactly how your trees are evolving and changing over time.

“I knew nothing about this technology when I started. I just saw pictures and thought ‘Oh my god, I can see the potential here. I know nothing, teach me’.”

The process is still being perfected, and Luck is looking at ways to streamline it.

“Processing of the data is really still complex and really quite computationally expensive,” she says. “Just as a baseline for my current research of use, for about 100 trees, it took me maybe a week of processing.

“And that’s not using an average laptop; that’s using a mainframe computer in Canberra. That’s a massive amount of computing. It’s a huge computer!”

Beyond this, the savanna trees can also experience internal damage, which the scanners may not pick up from external scans.

“This technology can only see the trees on the outside, you can’t see inside the tree,” says Luck.

“And there’s termites that cause a fair bit of hollowing, and we really haven’t quite measured exactly how much hollowing there is.

“We’ve done some extensive surveys, but I think more research needs to be done in there. I know that there is some research in the works to be done in the future.”