Lasers used to measure the mass of trees in forests

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New laser scanning technology is being used by UCL scientists to provide insights into the structure and mass of trees, a development that will help plot how much carbon they absorb and how they might respond to climate change.

The new approach, dubbed Terrestrial Laser Scanning (TLS), was pioneered by UCL’s Dr Mat Disney and colleagues and has enabled trees to be “weighed” very accurately by estimating their volume from the precise 3D data.

The system uses lasers to collect hundreds of thousands of points of data a second from the canopy which are processed to build a three-dimensional picture of the tree revealing its structure and its volume, which allows estimates of mass.

It revealed that a seemingly ordinary Sycamore tree in Wytham Woods near Oxford, for example, had nearly 11km of branches, double that of the much larger tropical trees measured by the team.

It is hoped the information will give a more accurate picture of the amount of carbon absorbed by forests, as part of efforts to reduce greenhouse gases, as well as help predict how trees might respond to climate change.

Dr Disney, who pioneered the technology, said that previously tree mass could only be measured by chopping it down and weighing it, which kills the tree, or estimating it manually from the ground, a difficult endeavour.

“Getting the height of the tree and the volume of the crown are really hard to do from the ground, particularly big tropical trees when you can’t see the top of them,” he said

The new approach, which the scientists have used to analyse trees in places ranging from Gabon and Borneo to the UK and northern California, accurately captures the fine-scale 3D structure of the tree.

The new laser technology, which cost between £75,000 and £150,000, can pinpoint branches to an accuracy within millimetres from a range of nearly 1km. It fires hundreds of thousands of laser pulses per second, and energy scattered back to the sensor enables 3D images to be constructed.

This can tell researchers about the tree’s “life history”, including its survival strategy, how it grows and competes with other trees, and, because the tree does not have to be cut down, its development can be monitored over time.

Accurate assessments of so-called ‘Above Ground Biomass’ (AGB) will underpin international efforts to mitigate the impacts of climate change and could help assess more accurately how much carbon is being stored in forests, and consequently the value that should be paid to countries not to cut them down.

Dr Disney said: “If the accuracy of TLS-derived estimates of AGB is demonstrated across a wide range of tree species and forest types, they are likely to become invaluable for monitoring of carbon stocks and fluxes. This is particularly important for international forest monitoring and protection agreements.”

Forests are earmarked to provide around a quarter of greenhouse gas emissions reductions under international efforts to cut climate change, but estimates of how much carbon is stored in tropical forests could be as much as 45 billion tonnes out, the researchers said.

Last year, a study concluded that trees in metropolitan areas have been growing faster than trees in rural areas worldwide due to the ‘heat island effect’. 

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