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International audience; Terrestrial laser scanners provide accurate and detailed point clouds of forest plots, which can be used as an alternative to destructive measurements during forest inventories. Various specialized algorithms have been developed to provide automatic and objective estimates of forest attributes from point clouds. The STEP (Snakes for Tuboid Extraction from Point cloud) algorithm was developed to estimate both stem diameter at breast height and stem diameters along the bole length. Here, we evaluate the accuracy of this algorithm and compare its performance with two other state-of-the-art algorithms that were designed for the same purpose (i.e., the CompuTree and SimpleTree algorithms). We tested each algorithm against point clouds that incorporated various degrees of noise and occlusion. We applied these algorithms to three contrasting test sites: (1) simulated scenes of coniferous stands in Newfoundland (Canada), (2) test sites of deciduous stands in Phalsbourg (France), and (3) coniferous plantations in Quebec, Canada. In most cases, the STEP algorithm predicted diameter at breast height with higher R2 and lower RMSE than the other two algorithms. The STEP algorithm also achieved greater accuracy when estimating stem diameter in occluded and noisy point clouds, with mean errors in the range of 1.1 cm to 2.28 cm. The CompuTree and SimpleTree algorithms respectively produced errors in the range of 2.62 cm to 6.1 cm and 1.03 cm to 3.34 cm, respectively. Unlike CompuTree or SimpleTree, the STEP algorithm was not able to estimate trunk diameter in the uppermost portions of the trees. Our results show that the STEP algorithm is more adapted to extract DBH and stem diameter automatically from occluded and noisy point clouds. Our study also highlights that SimpleTree and CompuTree require data filtering and results corrections. Conversely, none of these procedures were applied for the implementation of the STEP algorithm.

Aim of study : To assess terrestrial laser scanning (TLS) accuracy in estimating biometrical forest parameters at plot-based level in order to replace manual survey for forest inventory purposes. Area of study : Monte Morello, Tuscany region, Italy Material and methods : In 14 plots (10 m radius) in dense Mediterranean mixed conifer forests, diameter at breast height (DBH) and height were measured in Summer 2016. Tree volume was computed using the second Italian National Forest Inventory (INFC II) equations. TLS data were acquired in the same plots and quantitative structure models (QSMs) were applied to TLS data to compute dendrometric parameters. Tree parameters measured in field survey, i.e . DBH, height, and computed volume, were compared to those resulting from TLS data processing. The effect of distance from the plot boundary in the accuracy of DBH, height and volume estimation from TLS data was tested. Main results : TLS-derived DBH showed a good correlation with the traditional forest inventory data ( R 2 =0.98, RRMSE=7.81%), while tree height was less correlated with the traditional forest inventory data ( R 2 =0.60, RRMSE=16.99%). Poor agreement was observed when comparing the volume from TLS data with volume estimated from the INFC II prediction equations. Research highlights : The study demonstrated that the application of QSM to plot-based terrestrial laser data generates errors in plots with high density of coniferous trees. A buffer zone of 5 m would help reduce the error of 35% and 42% respectively in height estimation for all trees and in volume estimation for broadleaved trees.

Terrestrial laser scanning (TLS) technology is a powerful tool for assessing tree growth based on time series analysis, as it allows a level of scrutiny not achievable using established destructive techniques.[br/] We applied TLS technology to 21 wild cherry trees grown in a research plot near Breisach (southern Germany) in order to build quantitative structure models (QSMs) for each tree. Scans were carried out over three subsequent years (2012-2014), so that three QSMs per each tree were constructed.[br/] Using the above approach, we were able to assess the annual growth of the individual wild cherry trees in terms of diameter and height, stem and branch volume, and the merchantable timber fraction. In addition, the growth of single branches of sample trees was detected and quantified. The availability of QSMs based on TLS-derived data allowed the accurate determination of crown length and width, as well as the volume reduction as the result of the tree pruning applied after the first scan (2012). The aboveground biomass (AGB) was assessed for each tree based on the QSM-derived volume and published wood density values for wild cherry, and then compared with AGB values estimated with standard allometric methods, obtaining a very high correlation (r(adj)(2) = 0.941).[br/] We concluded that the proposed approach is an effective non-destructive technique to accurately assess the increase of tree biomass, and discuss its future application in the forestry sector.