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Fig. 1. Map of the location of the study area and field plot positions.

Table 1. Summary of the ground reference biophysical forest attributes by stratum (after season correction).
Young forest Mature forest SI ≤ 11 m Mature forest SI > 11 m
Range mean Range mean Range mean
Training plots 2001 n 35 37 37  
Ho 10.3–25.1 17.6 10.7–25.5 17.5 18.8–28.9 23.4
HL 8.2–20.5 13.8 9.9–23.5 15.7 15.8–26.8 20.6
N 386–3693 1809 172–1632 629 301–1503 819
G 6.4–62.6 25.1 5.3–43.1 22.7 15.5–56.9 34.2
V 29.8–642.9 187.8 28.7–420.3 178.9 121.2–681.6 337.5
AGB 19.1–368.6 117.2 17.7–241.6 102.9 76.6–355.0 192.9
2016 n 40 34 42  
Ho 11.9–28.8 18.2 11.5–25.7 18.2 17.5–30.3 22.9
HL 9.0–25.2 14.9 11.0–24.3 16.6 14.8–25.9 19.8
N 429–3220 1545 129–1331 609 301–3521 958
G 6.2–72.6 27.9 5.3–58.4 26.6 14.2–72.2 34.6
V 26.4–904.3 214.6 30.3–532.6 220.9 100.9–819.2 328.6
AGB 20.3–499.7 136.5 19.0–286.2 127.1 73.6–404.1 185.3
Forecasted 2016 n 12   33   42  
Ho 14.0–27.2 18.9 11.8–26. 5 18.5 15.9–30 .0 22.1
HL 11.6–22.9 15.7 11.0–24.4 16.9 13.0–28.0  19.2
N 429–2319 1417 172–1202 599 301–3306 1114
G 15.8–86.8 34.8 7.4–60.1 28.4 12.7–73.3 38.9
V 97.7–975.2 292.2 41.9–582.5 237.0 104.1–903.3 357.6
AGB 62.0–552.3 178.4 26.1–311.8 135.6 73.5–439.8 204.0
Validation plots 2016 n 11 14   24  
Ho 16.1–20.7 18.3 13.7–24.6 17.9 17.6–26.4 23.1
HL 13.5–17.2 15.4 12.1–20.9 15.9 14.9–25.1 20.4
N 452–1712 1254 201–1147 632 337–1221 762
G 13.9–30.7 25.7 10.4–40.8 22.9 17.9–46.5 30.6
V 99.1–241.6 194.1 68.8–359.8 178.6 124.5–525.2 298.7
AGB 64.6–163.0 125.7 41.2–221.1 108.6 87.2–294.8 170.0
SI = site index, n = Number of plots. Biophysical forest attributes: Ho = Dominant height (m), HL = Lorey’s mean height (m), N = Stem number (ha–1), G = Basal area (m2 ha–1), V = Volume (m3 ha–1), AGB = Aboveground biomass (Mg ha–1).
Table 2. Airborne laser scanning acquisition parameters.
Year of acquisition 2001 (T1) 2016 (T2)
Time period 23 June – 1 August 7 June – 31 July
Aircraft Piper PA-31 Piper PA-31
Laser scanning system Optech ALTM 1210 Riegl LMS Q-1560
Operating firm Fotonor AS Terratec AS
Average flying altitude (m) 650 1280
Average flying speed (m s–1) 75 69
Pulse repetition frequency PRF (kHz) 10 534
Scanning frequency (kHz) 30 115
Side overlap (%) 50 20
Maximum scan angle (degrees) 15 20
Average point density (pts m–2) 0.9 11.8
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Fig. 2. Conceptual framework of the three approaches. Top: The NEW approach represents concurrent ALS and field reference values from T2 used to construct models that later were used to make predictions on the validation plots. Middle: The REUSE approach applies temporally transferred models from T1 to predict the forest attributes at T2. Bottom: The FORECAST approach applies data projected from T1 to T2 together with ALS data from T2 used to construct models that were used for predictions on the validation plots.

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Fig. 3. Density plots showing the distributions of the ground references and the estimated biophysical forest attributes for the different approaches.

Table 3. Selected variables for the predictive nonlinear models of biophysical forest attributes (BFA) by approach and their respective model fit and error structure. View in new window/tab.
Table 4. Validation accuracy assessment of the biophysical forest attributes (BFA) estimates. View in new window/tab.