Abstract
Forest plantations are globally important for the economy and are significant for carbon sequestration. Properly managing plantations requires accurate information about stand timber stocks. In this study, we used the area (ABA) and individual tree (ITD) based approaches for estimating stem volume in fast-growing Eucalyptus spp forest plantations. Herein, we propose a new method to improve individual tree detection (ITD) in dense canopy homogeneous forests and assess the effects of stand age, slope and scan angle on ITD accuracy. Field and Light Detection and Ranging (LiDAR) data were collected in Eucalyptus urophylla x Eucalyptus grandis even-aged forest stands located in the mountainous region of the Rio Doce Valley, southeastern Brazil. We tested five methods to estimate volume from LiDAR-derived metrics using ABA: Artificial Neural Network (ANN), Random Forest (RF), Support Vector Machine (SVM), and linear and Gompertz models. LiDAR-derived canopy metrics were selected using the Recursive Feature Elimination algorithm and Spearman’s correlation, for nonparametric and parametric methods, respectively. For the ITD, we tested three ITD methods: two local maxima filters and the watershed method. All methods were tested adding our proposed procedure of Tree Buffer Exclusion (TBE), resulting in 35 possibilities for treetop detection. Stem volume for this approach was estimated using the Schumacher and Hall model. Estimated volumes in both ABA and ITD approaches were compared to the field observed values using the F-test. Overall, the ABA with ANN was found to be better for stand volume estimation ( r y y ^ = 0.95 and RMSE = 14.4%). Although the ITD results showed similar precision ( r y y ^ = 0.94 and RMSE = 16.4%) to the ABA, the results underestimated stem volume in younger stands and in gently sloping terrain (<25%). Stem volume maps also differed between the approaches; ITD represented the stand variability better. In addition, we discuss the importance of LiDAR metrics as input variables for stem volume estimation methods and the possible issues related to the ABA and ITD performance.
Highlights
Forest plantations are globally important for the economy and for carbon sequestration
To improve the tree detection performance, we proposed a new algorithm we call Tree Buffer Exclusion (TBE), to be applied after the tree detection methods
Our results show that younger stands were more affected for tree height retrieval with Light Detection and Ranging (LiDAR)
Summary
Forest plantations are globally important for the economy and for carbon sequestration. Foresters often need to sample trees in the field to obtain reference mean values of tree height and volume, which they later extrapolate to the entire stand This approach, effective, is generally laborious, costly, and may incur errors due to not measuring the entire population [1,2,3]. LiDAR sensors work by tracking the emission and return of laser pulses (light emitted energy), determining the sensor-target distance based on the return time lag. This technique demonstrates the potential to improve estimates of forest parameters (e.g., diameter at 1.3 m above ground (dbh), volume, and biomass) since it extends spatial analysis beyond the x- and y-axes, generating three-dimensional information [6]. Aerial platforms are often used (named Airborne Laser Scanner—ALS) for the benefit of covering larger areas to provide spatialized estimates (or “wall-to-wall” estimates) [2]
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