Fig. 1. The location of the four sub-areas (E2, N2, Q2 and R2) on a background map of the Nordic countries (©ESRI).

Table 1. Data collection year for sub-areas E2, N2, Q2 and R2.
E2 N2 Q2 R2
NFI plots 2012–2013 2012–2013 2012–2013 2013–2014
Company stands 2014 - 2015 2015
Biotope stands 2012–2014 - 2012–2015 2012–2015
Aerial images leaf-on 2013 leaf-on 2014 leaf-off 2014 leaf-on 2015
LiDAR data 2010–2012 2010–2012 2009–2011 2009–2012
Table 2. Summary of basal area weighted mean height (Hbw), basal area weighted mean diameter (Dbw), stem volume (Vol), and basal area (BA) of the training data for sub-areas E2, N2, Q2 and R2. Species proportion is proportion of basal area.
Characteristic Minimum Maximum Mean Standard
deviation
E2, n = 216 plots
Hbw, m 3.0 28.2 14.3 6.2
Dbw, cm 1.3 43.4 18.2 9.4
BA, m2 ha–1 0.7 61.3 21.3 12.4
Vol, m3 ha–1 2.4 669.1 168.6 131.1
Pine proportion (%) 0 100 21 31
Spruce proportion (%) 0 100 49 36
Deciduous proportion (%) 0 100 30 35
N2, n = 167 plots
Hbw, m 3.0 30.8 15.9 6.1
Dbw, cm 3.5 42.8 21.0 9.1
BA, m2 ha–1 1.0 50.8 22.3 10.4
Vol, m3 ha–1 2.7 627.1 181.4 117.7
Pine proportion (%) 0 100 34 36
Spruce proportion (%) 0 100 38 32
Deciduous proportion (%) 0 100 28 32
Q2, n = 223 plots
Hbw, m 3.1 28.4 15.0 5.9
Dbw, cm 2.8 41.2 19.0 8.4
BA, m2 ha–1 2.7 50.5 22.0 9.9
Vol, m3 ha–1 6.6 547.9 174.9 119.6
Pine proportion (%) 0 100 49 37
Spruce proportion (%) 0 100 34 32
Deciduous proportion (%) 0 100 17 25
R2,n = 233 plots
Hbw, m 3.0 29.8 17.0 6.1
Dbw, cm 1.0 45.4 22.5 9.3
BA, m2 ha–1 0.1 59.8 23.8 12.1
Vol, m3 ha–1 0.46 701.5 207.9 144.3
Pine proportion (%) 0 100 37 36
Spruce proportion (%) 0 100 39 34
Deciduous proportion (%) 0 100 24 32
Table 3. Summary of basal area weighted mean height (Hbw), basal area weighted mean diameter (Dbw), stem volume (Vol), and basal area (BA) of the validation data for sub-areas E2, Q2 and R2. Species proportion is proportion of basal area.
Company stands Biotope stands
Characteristic Min Max Mean Std Min Max Mean Std
E2, Company stands: n = 25; Biotope stands: n = 10
Hbw, m - - - - - - -
Dbw, cm 13 31 21.9 4.9 - - - -
BA, m2 ha–1 12.6 42.7 25.9 7.3 - - - -
Vol, m3 ha–1 91 380 211 75.6 144.4 357.0 262.5 66.3
Pine prop. (%) - - - - - - - -
Spruce prop. (%) - - - - - - - -
Deciduous prop. (%) - - - - 30 100 87 21
Q2, n = 37/11
Hbw, m 10.5 24.8 17.8 3.7 - - - -
Dbw, cm 12.2 30.6 22.7 5.2 - - - -
BA, m2 ha–1 12.0 47.0 21.8 8.8 - - - -
Vol, m3 ha–1 65 540 219.0 106.4 179.3 508.8 341.0 95.1
Pine prop. (%) 0 100 57 32 - - - 0
Spruce prop. (%) 0 98 35 28 - - - 0
Deciduous prop. (%) 0 69 8 13 1 79 23 27
R2, n = 82/18
Hbw, m 9.1 24.9 18.9 3.7 - - - -
Dbw, cm 8.4 40.0 25.4 6.1 - - - -
BA, m2 ha–1 12.0 50.0 27.4 8.4 - - - -
Vol, m3 ha–1 61.0 509.0 243.3 101.3 254 481 349 67.0
Pine prop. (%) 0 100 46 33 - - - -
Spruce prop. (%) 0 99 42 30 - - - -
Deciduous prop. (%) 0 98 12.4 16 2 78 22 21
Table 4. Description of strata and number of plots (N.) used in performance assessment of predictions. Hbw = basal area weighted mean height and BA = basal area.
Strata Description N. image N. LiDAR
All All plots 839 856
Deciduous Proportion of deciduous trees > 50% 169 157
Conifer Proportion of conifer trees > 50% 670 699
Pine dom. Proportion of pine > proportion of spruce or proportion of deciduous trees 318 349
Spruce dom. Proportion of spruce > proportion of pine or proportion of deciduous trees 336 334
Young Hbw < 15 meter 363 367
Mature Hbw > = 15 meter 476 489
Mature sparse Hbw > = 15 meter and BA < 20 101 106
Mature dense Hbw > = 15 meter and BA > 35 103 106
Table 5. Image based model statistics and plot level performance for basal area weighted mean height (Hbw), basal area weighted mean diameter (Dbw), stem volume (Vol), and basal area (BA) in different sub-areas. VRx is the vegetation ratio, i.e. the proportion of points over a threshold x divided by the total number of points. mCHM0 = mean CHM with Nodata set to zero. Other metrics are calculated by FUSION (McGaughey 2015). Bias was less than 0.1% in each case. * Square root transformation of Y-variable was used and back-transformed.
Dataset Stand attribute Metrics R2 adjusted RMSE RMSE (%)
E2 Hbw Elev.P95 0.90 1.9 13.5
Dbw Elev.LCV, Elev.P95 0.86 3.6 19.6
Vol* Elev.minimum, Elev.CURT.mean.CUBE 0.88 55.0 32.6
BA Elev.mean 0.73 6.4 29.8
N2 Hbw Elev.P95 0.90 1.9 12.0
Dbw Elev.P95 0.79 4.1 19.7
Vol CanopyReliefRatio, Elev.CURT.mean.CUBE 0.74 59.9 32.9
BA Elev.maximum, VR70%.Elev.P95 0.62 6.4 28.7
Q2 Hbw Elev.P95 0.92 1.7 11.1
Dbw Elev.stdev, Elev.P95 0.82 3.5 18.5
Vol* Elev.CURT.mean.CUBE 0.84 55.3 31.6
BA* Elev.P99, mCHM0 0.69 5.7 26.0
R2 Hbw Elev.P95 0.93 1.6 9.5
Dbw Elev.P95, Elev.LCV 0.84 3.7 16.6
Vol* Elev.SQRT.mean.SQ, VR5 0.83 60.0 28.8
BA Elev.minimum, Elev.CURT.mean.CUBE 0.69 6.7 28.1

Fig. 2. RMSE in percent of mean and bias in percent of mean for mean height (Hbw), mean diameter (Dbw), stem volume (Vol) and basal area (BA) in different strata based on all plots for image and LiDAR models.

Table 6. Stand level accuracies for basal area weighted mean height (Hbw), basal area weighted mean diameter (Dbw), stem volume (Vol), and basal area (BA) in different sub-areas (all stands).
Dataset Stand attribute RMSE RMSE (%) Bias (%)
E2 Hbw, m - - -
Dbw, cm 3.9 17.8 –6.8
Vol, m3 ha–1 51.3 22.7 0.4
BA, m2 ha–1 4.6 17.7 –3.6
Q2 Hbw, m 1.9 10.5 –9.1
Dbw, cm 2.9 12.7 –9.6
Vol, m3 ha–1 55.4 22.4 –9.3
BA, m2 ha–1 4.6 18.6 –5.0
R2 Hbw, m 1.5 7.7 –0.4
Dbw, cm 3.1 12.0 –5.3
Vol, m3 ha–1 57.4 22.0 –6.7
BA, m2 ha–1 5.8 21.1 –6.1

Fig. 3. Stand level stem volumes (m3 ha–1) in sub-areas; E2, Q2 and R2, based on field measurements and image data.

Table 7. Stand level accuracies of forest attributes in combined datasets. Hbw = basal area weighted mean height and Dbw = basal area weighted mean diameter.
Characteristic RMSE RMSE (%) Bias (%)
Company stands, n = 144
Hbw, m 1.6 8.6 –5.6
Dbw, cm 2.7 11.0 –5.6
Basal area, m2 ha–1 5.3 20.2 –5.4
Stem volume, m3 ha–1 54.8 23.7 –7.8
Biotope stands, n = 39
Stem volume, m3 ha–1 61.2 18.9 –2.2
All stands, n = 183
Stem volume, m3 ha–1 55.7 22.2 –6.1
Table 8. RMSE% and Bias% of stem volume using models from different sub-areas to company stand of Q2 and R2.
Model-area RMSE (%) Bias (%)
Q2-Q2 23.0 –11.5
E2-Q2 19.2 –8.3
R2-Q2 25.3 –17.3
R2-R2 24.1 5.3
E2-R2 22.4 0.9
Q2-R2 24.6 2.0