Abstract
Unmanned aerial vehicle (UAV) laser scanning, as an emerging form of near-ground light detection and ranging (LiDAR) remote sensing technology, is widely used for crown structure extraction due to its flexibility, convenience, and high point density. Herein, we evaluated the feasibility of using a low-cost UAV-LiDAR system to extract the fine-scale crown profile of Larix olgensis. Specifically, individual trees were isolated from LiDAR point clouds and then stratified from the point clouds of segmented individual tree crowns at 0.5 m intervals to obtain the width percentiles of each layer as profile points. Four equations (the parabola, Mitscherlich, power, and modified beta equations) were then applied to model the profiles of the entire and upper crown. The results showed that a region-based hierarchical cross-section analysis algorithm can successfully delineate 77.4% of the field-measured trees in high-density (>2400 trees/ha) forest stands. The crown profile generated with the 95th width percentile was adequate when compared with the predicted value of the existing field-based crown profile model (the Pearson correlation coefficient (ρ) was 0.864, root mean square error (RMSE) = 0.3354 m). The modified beta equation yielded slightly better results than the other equations for crown profile fitting and explained 85.9% of the variability in the crown radius for the entire crown and 87.8% of this variability for the upper crown. Compared with the cone and 3D convex hull volumes, the crown volumes predicted by our profile models had significantly smaller errors. The results revealed that the crown profile can be well described by using UAV-LiDAR, providing a novel way to obtain crown profile information without destructive sampling and showing the potential of the use of UAV-LiDAR in future forestry investigations and monitoring.
Highlights
The crown profile of a tree is the maximum outer edge of the crown branches and the minimum boundary that encapsulates the whole crown and can characterize both the shape and size of the crown [1]
By comparing the predicted value with the reference value (Figure 6), we found that the main source of the difference is the lower part of the crown, which directly reflects the lack of a description of the lower part of the crown by the Unmanned aerial vehicle (UAV)-light detection and ranging (LiDAR) data
We explored the possibility of modelling the crown profile of Larix olgensis using UAV-based high-density LiDAR data, which are able to quickly characterize the crown extent in three dimensions without destructive sampling
Summary
The crown profile of a tree is the maximum outer edge of the crown branches and the minimum boundary that encapsulates the whole crown and can characterize both the shape and size of the crown [1]. The shape and size of the crown affect tree physiological processes, such as photosynthesis, respiration and transpiration, due to the utilization of light and precipitation [2,3]. The crown profile reflects a tree’s characteristics and growth (such as its species, age, and size) [1,4,5,6]. A variety of models have been used to describe crown shape and size and predict crown width at any location within the crown. Such models were initially based on simple geometric shapes.
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