by On a hillside overlooking cabbage fields outside the northern Thai city of Chiang Mai, a drone’s rotors begin to whirr, lifting it over a patch of forest.
It moves back and forth on top of the rich canopy, transmitting photos to be woven into a 3D model that shows the health of the woodland and helps estimate how much carbon it can absorb.
Drones are part of an increasingly sophisticated arsenal used by scientists to understand forests and their role in the battle against climate change.
The basic principle is simple: woodland absorbs and stores carbon dioxide, the greenhouse gas that contributes the most to climate change.
But how much they absorb is a complicated question.
Forest size is a key part of the answer — and deforestation has led to a 12 percent drop in tree cover worldwide since 2000, according to Global Forest Watch.
But composition is also important: different species sequester carbon differently, and the age and size of trees also play a role.
Knowing how much carbon is stored by forests is crucial to understanding how quickly the world needs to reduce emissions, and most current estimates mix high-level imagery from satellites with small, labor-intensive ground surveys .
“Usually, we’d go into this forest, we’d put in the pole, we’d have our piece of wire, five meters long. We’d walk around in a circle, we’d measure all the trees in a circle,” explained Stephen. Elliott, director of research at Chiang Mai University’s Forest Restoration Research Unit (FORRU).
But “if you have 20 students stopping with tapes and poles … you’re going to get trashed below,” he said, referring to the layer of vegetation between the forest floor and the canopy.
That’s where the drone comes in, he said, taking in the model of the Phantom hovering above.
“With this, you don’t protect in the forest.”
– ‘All trees’ –
Three measurements are needed to assess the absorptive capacity of a tree: height, girth and wood density, which vary by species.
With an assistant looking through binoculars for birds that might collide with the drone, the machine flies a path plotted into a computer program.
“We collect data or capture (images) every three seconds,” explained Worayut Takaew, FORRU field research officer and drone operator.
“The overlapping images are then rendered into a 3D model that can be viewed from different angles.”
The patch of woodland being surveyed is part of a decades-long project led by Elliott and his team that reforested around 100 hectares by planting a number of key species.
Their goal was not large-scale reforestation, but the development of good practices: planting native species, encouraging the return of animals that bring seeds from other species and working with local communities.
The drone’s 3D model is a strong visual representation of its success, especially compared to straggly non-contact control plots nearby.
But it is also being developed as a way to avoid labour-intensive land surveys.
“Once you’ve got the model, you can measure the height of every tree in the model. Not samples, every tree,” Elliott said.
A forest’s carbon potential goes beyond its trees, however, with leaf litter and soil also acting as stores.
So these are also collected for analysis, which Elliott says shows their reforested plots store carbon at levels close to nearby undamaged woodland.
– ‘Bigger and more precise’ –
But for all its bird’s-eye view, the drone has one major limitation: it can’t see under the canopy.
For that, researchers need technology like LiDAR — high-resolution remote sensing equipment that efficiently scans the entire forest.
“You can go into the forest … and really recreate the shape and size of each tree,” explained Emmanuel Paradis, a researcher at France’s National Research Institute for Sustainable Development.
He is leading a multi-year project to build the most accurate analysis yet of how much carbon Thailand’s forests can store.
It will survey five different types of forest, including some FORRU plots, using drone-mounted LiDAR and advanced analysis of the microbes and fungi in the soil that feed trees.
“The aim is to estimate at the country level … how much carbon can be stored in one hectare anywhere in Thailand,” he said.
The stakes are high at a time when debate rages over whether current estimates of the world’s forest carbon capacity are correct.
“I think many people, and I am a little of this opinion, that these estimates are not accurate enough,” said Paradis.
“Overly optimistic estimates can lead to over-optimism and over-optimism about the potential of forests to store carbon,” he said.
The urgency of the issue is driving rapid developments, including next year’s launch of the European Space Agency’s Biomass satellite, designed to monitor forest carbon stocks.
“The technology is changing, the satellites are more and more accurate … and the statistical technologies are more and more accurate,” said Paradis.
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