Abstract
Terrestrial laser scanning (TLS) has been adopted as a feasible technique to digitize trees and forest stands, providing accurate information on tree and forest structural attributes. However, there is limited understanding on how a variety of forest structural changes can be quantified using TLS in boreal forest conditions. In this study, we assessed the accuracy and feasibility of TLS in quantifying changes in the structure of boreal forests. We collected TLS data and field reference from 37 sample plots in 2014 (T1) and 2019 (T2). Tree stems typically have planar, vertical, and cylindrical characteristics in a point cloud, and thus we applied surface normal filtering, point cloud clustering, and RANSAC-cylinder filtering to identify these geometries and to characterize trees and forest stands at both time points. The results strengthened the existing knowledge that TLS has the capacity to characterize trees and forest stands in space and showed that TLS could characterize structural changes in time in boreal forest conditions. Root-mean-square-errors (RMSEs) in the estimates for changes in the tree attributes were 0.99–1.22 cm for diameter at breast height (Δdbh), 44.14–55.49 cm2 for basal area (Δg), and 1.91–4.85 m for tree height (Δh). In general, tree attributes were estimated more accurately for Scots pine trees, followed by Norway spruce and broadleaved trees. At the forest stand level, an RMSE of 0.60–1.13 cm was recorded for changes in basal area-weighted mean diameter (ΔDg), 0.81–2.26 m for changes in basal area-weighted mean height (ΔHg), 1.40–2.34 m2/ha for changes in mean basal area (ΔG), and 74–193 n/ha for changes in the number of trees per hectare (ΔTPH). The plot-level accuracy was higher in Scots pine-dominated sample plots than in Norway spruce-dominated and mixed-species sample plots. TLS-derived tree and forest structural attributes at time points T1 and T2 differed significantly from each other (p < 0.05). If there was an increase or decrease in dbh, g, h, height of the crown base, crown ratio, Dg, Hg, or G recorded in the field, a similar outcome was achieved by using TLS. Our results provided new information on the feasibility of TLS for the purposes of forest ecosystem growth monitoring.
Highlights
Forests change over time and across space due to natural phenomena and anthropogenic processes
If an increase or decrease in tree and forest attributes was recorded in the field with conventional mensuration tools, a similar outcome was achieved by using bi-temporal Terrestrial laser scanning (TLS) data
It is known that TLS is capable of characterizing forest structure in detail in space
Summary
Forests change over time and across space due to natural phenomena and anthropogenic processes. Biotic changes, such as forest growth and damage, affect forest structure and tree stems and branches grow annually in width and height. That is often combined with the spread of pathogenic fungi, causes defoliation and changes in the bark whereas winds fall trees or cut tree parts [2] and fire burns low vegetation, needle mass, and possibly reshapes the structure of trees [3,4]. There are abiotic changes that affect the forest structure These include land-use changes as well as silvicultural and harvesting operations. Spatiotemporal information is needed to improve the understanding of, or quantify, the consequences of these natural phenomena, processes, and human activities with varying temporal and spatial patterns and dimensions
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