Effects of Plot Size on Airborne LiDAR-Derived Metrics and Predicted Model Performances of Subtropical Planted Forest Attributes
Abstract
:1. Introduction
2. Materials and Methods
2.1. Study Area
2.2. Field Plot Data
2.3. Lidar Data
2.4. Comparative Analysis of Plot Size Effects
3. Results
3.1. Plot Size Effects on LiDAR-Derived Metrics
3.1.1. Height Metrics
3.1.2. Density Metrics
3.1.3. Vertical Structure Metrics
3.2. Plot Size Effects on Measured Forest Attributes
3.3. Plot Size Effects on the Performances of the Predictive Models of Forest Attributes
4. Discussion
5. Conclusions
- (1)
- The means of the 25th, 50th, and 75th height percentiles of laser point clouds, Hmean, Hcv, CC, 25th and 50th density percentiles, and LADcv of plots of different sizes for all four forest types showed irregular differences or no statistically significant difference from that of the 900 m2 plots. However, their standard deviations decreased as the plot size increased. In general, statistically significant differences in the means of Hmax, LADmean, and 75th density percentile were found between plots of various sizes and 900 m2 plots.
- (2)
- Except for the mean Hm, the measured forest attributes of plots of different sizes for all four forest types exhibited irregular variations and no statistically significant difference from those of the 900 m2 plots. However, their standard deviations decreased with the increasing plot size.
- (3)
- As the plot size increased from 100 m2 to 900 m2, the predictive errors (MPE and rRMSE) decreased at approximately the same rate for all forest types, and the model accuracies gradually improved at a similar rate for all forest types. These results were most likely due to the fact that the standard deviations of the LiDAR-derived metrics and measured forest attributes decreased as the plot size increased; that is, the variation in the independent and dependent variables of the model decreased with the increasing plot size, which improved the robustness of the model.
- (4)
- According to this paper, we preliminarily recommend that for a large-scale subtropical planted forest inventory, the plot sizes should be at least 600 m2 for all forest types.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Stratum | Sample Size | Stand Age (yr) | DBH | Height | Max. Height (m) | BA | Tree Density (Stem ha−1) | VOL | ||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Mean (cm) | CV (%) | Mean (m) | CV (%) | Mean (m2 ha−1) | CV (%) | Mean (m3 ha−1) | CV (%) | |||||
Chinese Fir | 22 | 19–28 | 15.04 | 14.78 | 13.37 | 13.41 | 16.45 | 24.78 | 19.75 | 1536 | 179.87 | 24.39 |
Pine | 29 | 7–24 | 17.83 | 21.57 | 13.14 | 26.94 | 14.91 | 26.51 | 28.69 | 1166 | 175.86 | 42.34 |
Eucalyptus | 25 | 2–9 | 11.11 | 15.26 | 16.02 | 20.99 | 18.67 | 17.6 | 35.42 | 1826 | 146.25 | 49.3 |
Broad-leaved | 28 | 7–56 | 14.35 | 27.36 | 11.37 | 31.84 | 13.7 | 20.44 | 39.28 | 1343 | 128.74 | 59.24 |
Protocol | 100 m2 | 200 m2 | 300 m2 | 400 m2 | 600 m2 | 900 m2 |
---|---|---|---|---|---|---|
1 | P1 | P1, P2 | P1, P2, P3 | P1, P2, P5, P6 | P1–P6 | P1–P9 |
2 | P2 | P2, P5 | P2, P5, P8 | P2-P5 | P2-P5, P8, P9 | P1–P9 |
3 | P6 | P6, P7 | P1, P6, P7 | P5-P8 | P1, P2, P5–P8 | P1–P9 |
4 | P5 | P4, P5 | P4-P6 | P4, P5, P8, P9 | P4–P9 | P1–P9 |
Forest Type | Plot Size (m2) | hp50 | Hmean | Hcv | CC | dp50 | LADcv | H | VOL | BA |
---|---|---|---|---|---|---|---|---|---|---|
Fir | 100 | 2.50 | 1.93 | 0.16 | 0.15 | 0.17 | 0.37 | 2.62 | 68.49 | 7.35 |
200 | 1.92 | 1.62 | 0.15 | 0.13 | 0.17 | 0.29 | 2.18 | 54.82 | 5.91 | |
300 | 1.81 | 1.50 | 0.15 | 0.14 | 0.16 | 0.26 | 2.05 | 50.46 | 5.48 | |
400 | 1.34 | 1.39 | 0.14 | 0.13 | 0.15 | 0.23 | 1.88 | 45.66 | 5.12 | |
600 | 1.31 | 1.36 | 0.14 | 0.13 | 0.15 | 0.23 | 1.81 | 44.73 | 5.01 | |
900 | 1.29 | 1.34 | 0.14 | 0.14 | 0.16 | 0.23 | 1.81 | 43.86 | 4.89 | |
Pine | 100 | 5.08 | 3.88 | 0.19 | 0.17 | 0.22 | 0.45 | 3.67 | 90.63 | 10.43 |
200 | 4.46 | 3.80 | 0.17 | 0.15 | 0.21 | 0.36 | 3.55 | 79.54 | 8.48 | |
300 | 4.34 | 3.80 | 0.16 | 0.14 | 0.20 | 0.33 | 3.55 | 78.11 | 8.26 | |
400 | 4.40 | 3.82 | 0.15 | 0.13 | 0.21 | 0.31 | 3.65 | 75.79 | 7.98 | |
600 | 4.38 | 3.80 | 0.15 | 0.13 | 0.20 | 0.30 | 3.54 | 74.80 | 7.75 | |
900 | 4.36 | 3.78 | 0.14 | 0.12 | 0.20 | 0.29 | 3.56 | 74.45 | 7.61 | |
Eucalyptus | 100 | 5.95 | 3.73 | 0.13 | 0.22 | 0.16 | 0.62 | 3.57 | 76.16 | 7.05 |
200 | 5.42 | 3.63 | 0.13 | 0.22 | 0.15 | 0.55 | 3.42 | 74.06 | 6.62 | |
300 | 5.51 | 3.59 | 0.13 | 0.22 | 0.15 | 0.50 | 3.41 | 73.73 | 6.52 | |
400 | 5.24 | 3.59 | 0.13 | 0.21 | 0.15 | 0.46 | 3.56 | 71.80 | 6.22 | |
600 | 5.04 | 3.49 | 0.12 | 0.21 | 0.14 | 0.47 | 3.43 | 72.15 | 6.25 | |
900 | 4.77 | 3.37 | 0.12 | 0.21 | 0.14 | 0.46 | 3.41 | 72.09 | 6.23 | |
Broad-leaved | 100 | 5.85 | 5.47 | 0.21 | 0.19 | 0.26 | 0.34 | 4.04 | 93.61 | 9.90 |
200 | 5.47 | 5.34 | 0.20 | 0.19 | 0.26 | 0.27 | 3.92 | 83.39 | 8.90 | |
300 | 5.50 | 5.36 | 0.20 | 0.19 | 0.27 | 0.25 | 3.85 | 81.65 | 8.44 | |
400 | 5.52 | 5.37 | 0.21 | 0.19 | 0.27 | 0.24 | 3.79 | 79.89 | 8.34 | |
600 | 5.51 | 5.35 | 0.20 | 0.19 | 0.27 | 0.22 | 3.72 | 77.72 | 8.12 | |
900 | 5.51 | 5.34 | 0.20 | 0.18 | 0.28 | 0.22 | 3.66 | 76.25 | 8.03 |
Stratum | Plot Size (m2) | VOL | BA | ||||
---|---|---|---|---|---|---|---|
R2 | rRMSE (%) | MPE (%) | R2 | rRMSE (%) | MPE (%) | ||
Fir | 100 | 0.390 | 29.31 | 13.93 | 0.313 | 25.00 | 11.88 |
200 | 0.433 | 22.38 | 10.64 | 0.310 | 19.77 | 9.40 | |
300 | 0.354 | 21.56 | 10.25 | 0.211 | 19.21 | 9.13 | |
400 | 0.424 | 19.07 | 9.07 | 0.327 | 17.10 | 8.13 | |
600 | 0.467 | 18.11 | 8.61 | 0.337 | 16.55 | 7.87 | |
900 | 0.554 | 16.28 | 7.74 | 0.378 | 15.58 | 7.41 | |
Pine | 100 | 0.327 | 43.69 | 17.48 | 0.098 | 37.88 | 15.15 |
200 | 0.445 | 34.13 | 13.66 | 0.172 | 29.34 | 11.74 | |
300 | 0.527 | 30.73 | 12.29 | 0.247 | 27.13 | 10.86 | |
400 | 0.517 | 30.41 | 12.17 | 0.235 | 26.53 | 10.61 | |
600 | 0.572 | 28.06 | 11.23 | 0.302 | 24.51 | 9.81 | |
900 | 0.596 | 26.93 | 10.77 | 0.331 | 23.46 | 9.39 | |
Eucalyptus | 100 | 0.669 | 30.75 | 13.48 | 0.569 | 26.96 | 11.81 |
200 | 0.772 | 24.48 | 10.73 | 0.710 | 20.42 | 8.95 | |
300 | 0.812 | 22.05 | 9.66 | 0.770 | 17.90 | 7.85 | |
400 | 0.864 | 18.26 | 8.00 | 0.823 | 15.03 | 6.59 | |
600 | 0.877 | 17.37 | 7.61 | 0.835 | 14.45 | 6.33 | |
900 | 0.905 | 15.18 | 6.65 | 0.876 | 12.46 | 5.46 | |
Broad-leaved | 100 | 0.698 | 38.73 | 15.83 | 0.560 | 31.45 | 12.85 |
200 | 0.779 | 30.84 | 12.60 | 0.657 | 25.68 | 10.49 | |
300 | 0.788 | 28.89 | 11.81 | 0.668 | 23.46 | 9.59 | |
400 | 0.802 | 27.43 | 11.21 | 0.665 | 23.73 | 9.70 | |
600 | 0.821 | 25.37 | 10.37 | 0.668 | 22.94 | 9.37 | |
900 | 0.847 | 23.13 | 9.45 | 0.690 | 21.89 | 8.94 |
Attribute | Forest Type | a0 | a1 | R2 | rRMSE (%) |
---|---|---|---|---|---|
VOL | Fir | 6.4570 | −0.3574 | 0.890 | 11.40 |
Pine | 13.7107 | −0.2637 | 0.772 | 11.02 | |
Eucalyptus | 7.0049 | −0.3422 | 0.828 | 13.79 | |
Broad-leaved | 10.2562 | −0.3360 | 0.879 | 13.19 | |
BA | Fir | 6.7816 | −0.3109 | 0.811 | 11.87 |
Pine | 11.3946 | −0.2706 | 0.608 | 9.64 | |
Eucalyptus | 5.8844 | −0.3186 | 0.842 | 11.34 | |
Broad-leaved | 11.4020 | −0.2745 | 0.735 | 15.85 |
Plot Size (m2) | rRMSE Difference (%) in VOL Estimation | rRMSE Difference (%) in BA Estimation | ||||||
---|---|---|---|---|---|---|---|---|
Fir | Pine | Eucalyptus | Broad-Leaved | Fir | Pine | Eucalyptus | Broad-Leaved | |
100 | 119.3 | 78.5 | 112.1 | 109.2 | 98.0 | 83.4 | 101.4 | 82.8 |
200 | 71.2 | 48.7 | 67.3 | 65.8 | 59.6 | 51.5 | 61.5 | 51.1 |
300 | 48.1 | 33.6 | 45.6 | 44.6 | 40.7 | 35.4 | 41.9 | 35.2 |
400 | 33.6 | 23.8 | 32.0 | 31.3 | 28.7 | 25.1 | 29.5 | 24.9 |
500 | 23.4 | 16.8 | 22.3 | 21.8 | 20.1 | 17.6 | 20.6 | 17.5 |
600 | 15.6 | 11.3 | 14.9 | 14.6 | 13.4 | 11.8 | 13.8 | 11.8 |
700 | 9.4 | 6.9 | 9.0 | 8.8 | 8.1 | 7.2 | 8.3 | 7.1 |
800 | 4.3 | 3.2 | 4.1 | 4.0 | 3.7 | 3.3 | 3.8 | 3.3 |
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Li, C.; Lin, X.; Dai, H.; Li, Z.; Zhou, M. Effects of Plot Size on Airborne LiDAR-Derived Metrics and Predicted Model Performances of Subtropical Planted Forest Attributes. Forests 2022, 13, 2124. https://doi.org/10.3390/f13122124
Li C, Lin X, Dai H, Li Z, Zhou M. Effects of Plot Size on Airborne LiDAR-Derived Metrics and Predicted Model Performances of Subtropical Planted Forest Attributes. Forests. 2022; 13(12):2124. https://doi.org/10.3390/f13122124
Chicago/Turabian StyleLi, Chungan, Xin Lin, Huabing Dai, Zhen Li, and Mei Zhou. 2022. "Effects of Plot Size on Airborne LiDAR-Derived Metrics and Predicted Model Performances of Subtropical Planted Forest Attributes" Forests 13, no. 12: 2124. https://doi.org/10.3390/f13122124