How big is that tree? Estimations are tricky but Terrestrial Laser Scanning helps!

Have you ever admired the trees along city roads or standing tall in parks? Besides looking pretty, urban trees provide a range of important ecosystem services, such as carbon sequestration. Carbon sequestration by a tree can be calculated from its aboveground biomass and can inform city development planning. Measuring the aboveground biomass of trees is labour intensive and compared to forest trees, urban trees grow differently (e.g., faster, develop larger crowns), so allometric equations might not be entirely accurate to make any predictions. 

Dr Daniel Kükenbrink and colleagues from five Swiss institutes measured felled trees from eight cities by traditional methods and high resolution Terrestrial Laser Scanning (TLS). The scientists reconstructed the 3D structure of urban trees and developed accurate species-specific Quantitative Structure Models (QSMs) to estimate aboveground biomass of 25 tree species. Dr Daniel Kükenbrink has previously reconstructed small-scale and large-scale forests in 3D based on laser scanning.

The researchers scanned and destructively measured 55 felled trees from eight Swiss cities that belonged to 25 species. The trees were scanned by a Terrestrial Laser Scanner (see video) that produced a point cloud consisting of 45 million points. The tree height, trunk diameter, crown width, total weight of the tree, basic density of coarse and fine wood and biomass were determined destructively. Kükenbrink and colleagues compared the tree metrics derived from the point cloud data and estimated with QSMs, previous allometric equations and the direct measurements.

The aboveground biomass (e.g., tree height, crown metrics) was accurately estimated from the TLS readings but non-circular trunks, trunk growth with multiple stems, ivy growth and smaller branches posed certain problems. For example, the ivy led to a 56% overestimation of the trunk diameter. The estimations from the TLS were more accurate than the predictions from allometric equations when compared to the field measurements. The total coarse wood volume, crown volume and trunk diameter measurements at lower heights had the highest correlation with aboveground biomass.

This study showed that combining terrestrial laser scanning (TLS) with quantitative structure modelling (QSM) can accurately provide urban tree metrics and aboveground biomass (AGB) estimates. Whilst the researchers acknowledge that using TLS and processing the data is still a complicated process, equipment and software developments will help streamline the process in the future. As the TLS cannot penetrate the foliage, the estimations are better when leaves have already fallen off the trees. For now, it provides a great opportunity to develop reliable allometric equations for urban trees. 

“Monitoring urban trees is becoming increasingly important especially with the increase in urban areas in mind,” Kükenbrink and colleagues wrote.

“To this reason, TLS has been added to the 2019 refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories as a promising new technology for the inclusion in national greenhouse gas inventories.”

In Switzerland, approximately 9.4 million tonnes of carbon are estimated to be stored in trees within settlements but species specific modelling could provide more accurate estimates and help city developers utilise urban trees for their climate change mitigation functions.