From branches to entire trees and forests: a new laser scanning-based model improves estimations

Forests are known as the “lungs of the Earth”, covering around 31% of the global land area. They provide unique habitats, people’s livelihoods and climate change mitigation possibilities. Forests are known to be carbon sinks as they “soak up” carbon dioxide and other greenhouse gases. But how does one quantify how much carbon is sequestered and stored in forests? Essentially, it comes down to tree size. 

PhD student Man Hu and colleagues from the University of Helsinki and Natural Resources Institute Finland combined terrestrial laser scanning (TLS) and Tree Structure Models to reliably estimate the branch biomass of Scots pines (Pinus sylvestris) in Finland. The researchers developed a new mathematical model that successfully extracts individual branch biomass from TLS imagery. 

Imagine how you would measure all the above-ground biomass of a pine tree by hand. Cutting each branch, measuring the length and weight would take a long time. While scanning techniques can save time and labour, they are computationally complicated and unreliable for more delicate features, such as individual branch measurements. The latest study overcomes many of these problems. 

Hu and colleagues used two datasets to measure the biomass of Scots pines in Finland. First, they scanned trees with LiDAR and felled them in the Lapinjärvi research forest. The scan produced a 3D point cloud of individual trees and used previous TLS-based models (e.g. TreeQSM) to calculate tree volume and structure. The TLS technique is expensive, and the data can be noisy in a forest as trees overlap. 

Next, the researchers destructively measured the felled trees. These measurements, along with some previous measurements, were used to create a model that improves the accuracy of the TLS-derived data. The authors used almost 14,000 branches from 122 Scots pine trees for the analysis.

The researchers found that their new model is more accurate at estimating branch biomass than four previous models because of its approach. For example, the widely used TreeQSM software tries to reconstruct branches as cylinders, but that procedure needs high-quality and numerous data points in the scanned image. The new method relies on the pipe model theory (PMT) that relies on the proportional changes in branch basal area and stem cross-sectional area. 

“In this study we present a new method that not only estimates branch biomass precisely, but also presents an opportunity to estimate individual branch attributes using TLS data,” Hu and colleagues wrote. 

The age range of the measured trees varied was 22-113, which could have influenced the model’s accuracy. Whilst the model is not perfect compared to the manual measurements, it is a significant improvement. 

“The good performance in Scots pine trees shows the great potential in extending the method to more species and larger areas.”

Whilst protecting and restoring forests are at the forefront of slowing down global climate change, quantifying the above-ground biomass of forest is a challenge. The age and composition of a forest will all impact its carbon sink size. The latest study shows how scientists are improving the latest technologies to provide reliable estimations from individual branches to entire trees that could be scaled up to whole forests.