Abstract
The use of terrestrial laser scanning (TLS) to provide accurate estimates of 3D forest canopy structure and above-ground biomass (AGB) has developed rapidly. Here, we provide an overview of the state of the art in using TLS for estimating forest structure for AGB. We provide a general overview of TLS methods and then outline the advantages and limitations of TLS for estimating AGB. We discuss the specific type of measurements that TLS can provide, tools and methods that have been developed for turning TLS point clouds into quantifiable metrics of tree size and volume, as well as some of the challenges to improving these measurements. We discuss the role of TLS for enabling accurate calibration and validation (cal/val) of Earth observation (EO)-derived estimates of AGB from spaceborne lidar and RADAR missions. We give examples of the types of TLS equipment that are in use and how these might develop in future, and we show examples of where TLS has already been applied to measuring AGB in the tropics in particular. Comparing TLS with harvested AGB shows r2 > 0.95 for all studies thus far, with absolute agreement to within 10% at the individual tree level for all trees and to within 2% in the majority of cases. Current limitations to the uptake of TLS include the capital cost of some TLS equipment, processing complexity and the relatively small coverage that is possible. We argue that combining TLS measurements with the existing ground-based survey approaches will allow improved allometric models and better cal/val, resulting in improved regional and global estimates of AGB from space, with better-characterised, lower uncertainties. The development of new, improved equipment and methods will accelerate this process and make TLS more accessible.
Highlights
Terrestrial laser scanning (TLS) is a relatively new technology, developed since the early 2000s for surveying applications, i.e. for measuring very precise distances and angles
The utility of TLS for measuring tree properties was explored in the early 1990s by the forestry community, for providing new ways to measure properties already measured for forest surveying
While TLS has proved useful for these applications (Hopkinson et al 2004; Calders et al 2015; Newnham et al 2015), it is the potential to provide cm and even mm-accurate 3D structural detail, for estimating 3D full-tree volume, and from this above-ground biomass (AGB), that has seen a rapid increase in interest in TLS for measurement of AGB
Summary
Terrestrial laser scanning (TLS) is a relatively new technology, developed since the early 2000s for surveying applications, i.e. for measuring very precise distances and angles. The utility of TLS for measuring tree properties was explored in the early 1990s by the forestry community, for providing new ways to measure properties already measured for forest surveying. These include tree height, diameter-at-breast-height (DBH), crown size and basal area. Where D and H are DBH and height, respectively, is the wood density (g/cm3), and a, b are empirically derived constants (Feldpausch et al 2010; Chave et al 2014) This ASE approach broadly represents a tree as a tapered cylindrical volume. Other ASEs contain far fewer samples, typically a few dozen in the case of most locally calibrated models (Stovall et al 2018)
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