Terrestrial laser scanners (TLSs) have successfully captured various properties of individual trees and have potential to further increase the quality and efficiency of forest surveys. However, TLSs are limited to line of sight observations, and forests are complex structural environments that can occlude TLS beams and thereby cause incomplete TLS samples. Boucher et al. evaluate the prevalence and sources of occlusion that limit line of sight to forest stems for TLS scans, assess the impacts of TLS sample incompleteness, and evaluate sampling strategies and data analysis techniques aimed at improving sample quality and representativeness.
The scientists used a large number of TLS scans (761), taken across a 255 650-m2 area of forest with detailed field survey data: the Harvard Forest Global Earth Observatory (ForestGEO) (MA, USA). They matched sets of TLS returns to stem positions in the field surveys to derive TLS-observed stem sets, which they then compared with two additional stem sets derived solely from the field survey data: a set of stems within a fixed range from the TLS and a set of stems based on 2-D modelling of line of sight. They compared stem counts and densities between the stem sets, and four alternative derivations of area to correct stem densities for the effects of occlusion are evaluated. Representation of diameter at breast height and species, drawn from the field survey data, were also compared between the stem sets.
The team concludes: “The availability of the detailed field survey data allowed us to attribute a portion of the observed influence of occlusion directly to stems, and to conclude that non-stem sources of occlusion (such as other vegetation, other objects, and topography) were dominant in this forest. Viewed from another perspective, the prevalence of non-stem occlusion means that even extremely detailed stem survey data would not have allowed us to prepare an effective set of TLS scanning positions a priori, no matter which overall sampling strategy we had employed. Therefore, we see it as a necessity to develop the technology and protocols to monitor the information collected during TLS surveys while they are in progress. Then, we can adapt TLS sampling strategies on the fly, to ensure the information requirements to retrieve the forest properties of interest are met.”
Boucher, P.B., Paynter, I., Orwig, D.A., Valencius, I., Schaaf, C., 2021. Sampling forests with terrestrial laser scanning. Annals of Botany. https://doi.org/10.1093/aob/mcab073