Terrestrial laser scanning to reconstruct branch architecture from harvested branches

Abstract

Quantifying whole branch architecture is critical to understanding tree function, for example, branch surface area controls woody gas exchange. Yet, due to measurement difficulty, branch architecture of small diameter branches (e.g. <10 cm (Formula presented.)) is modelled, subsampled or ignored. Methods that use terrestrial laser scanning (TLS) are now being widely applied to analyse tree and plot-level tree architecture; however, resolving small diameter branches in-situ remains a challenge. Currently, it is suggested that accurate reconstruction of small diameter branches can only be achieved by harvest and measurement in controlled conditions. Here we present a new TLS workflow for rapid and accurate reconstruction of complete branch architecture from harvested branches. The workflow sets out scan configuration, post-processing (including a novel reflectance filter) and fitting of quantitative structure models (QSM) to reconstruct topologically coherent branch models. This is demonstrated on 595 branches (scanned indoors to negate the impact of wind) and compared with 65 branches that were manually measured (i.e. with measuring tape and callipers). Comparison of a suite of morphological and topological traits reveals a good agreement between TLS-derived metrics and manual measurements where RMSE (%RMSE) for total branch length = 0.7 m (10%), volume = 0.09 L (43%), surface area = 0.04 m2 (26%) and N tips = 6.4 (35%). Scanning was faster and invariant to branch size compared with manual measurements which required significantly more personnel time. We recommend measuring a subsample of tip widths to constrain the QSM taper function as the TLS workflow tends to overestimate tip width. The workflow presented here allows for a rapid characterisation of branch architecture from harvested branches. Increasing the number of branches analysed (e.g. many branches from a single tree or branches from many species globally) could allow for a comprehensive analysis of the ‘missing link’ between the leaves and larger diameter branches.