5.1. Relationship between SAR Backscatter and Stem Volume
The SAR backscatter increased for increasing stem volume, with a rapid ascent for low stem volumes (<100 m
3/ha or less) followed by a significant decrease of sensitivity. The relationship between the SAR backscatter and stem volume depended upon environmental conditions and polarization (see examples in
Figure 3), as well as on look angle. At Krycklan, we observed strongest sensitivity of the backscatter to stem volume under unfrozen conditions and at HV-polarization (
Figure 3a and
Table 5). The backscatter increased rapidly for increasing stem volume; the sensitivity of the backscatter to stem volume became extremely weak in the densest forests. Observations taken during winter-time (frozen conditions or freeze/thaw events) were much less correlated with stem volume than under unfrozen conditions (
Figure 3a,b and
Table 5). At Remningstorp, we observed a slightly different trend, with SAR backscatter from data acquired under frozen conditions being better correlated with stem volume than in case of data acquired under unfrozen conditions (
Figure 3c,d and
Table 5).
An almost linear trend between SAR backscatter and stem volume was observed in several cases when images were acquired under frozen conditions. There did not seem to be any apparent difference between statistics for images acquired under unfrozen moist (
i.e., <2 mm of recorded precipitation) and unfrozen wet
(i.e., >2 mm of recorded precipitation) condition. Overall, the observations at the two test sites and the temporal consistency for a given environmental condition agreed with trends of the SAR backscatter at L-band with respect to stem volume and above-ground biomass in boreal as well as in other forest environments [
5,
6,
15,
16,
17,
18,
19,
20,
22,
23,
28,
39,
45,
46]. At both test sites, the spread of the SAR backscatter for a given stem volume was considerable, thus confirming that L-band backscatter captures only part of the information on structural properties of a forest and the signal recorded by the radar contains additional contributions [
28].
Table 5.
Distribution of the Pearson's correlation coefficient between stem volume and SAR backscatter for a given combination of look angle, polarization and environmental condition. Combinations are listed consisting of at least three PALSAR datasets. The minimum (Min), three quartiles (Q1, Q2 and Q3) and the maximum (Max) are listed. For combinations with three datasets, only Q2 is given. For combinations including four or five datasets, only Min, Q2 and Max are given. Transparent cells refer to Krycklan and shaded cells to Remningstorp.
Table 5.
Distribution of the Pearson's correlation coefficient between stem volume and SAR backscatter for a given combination of look angle, polarization and environmental condition. Combinations are listed consisting of at least three PALSAR datasets. The minimum (Min), three quartiles (Q1, Q2 and Q3) and the maximum (Max) are listed. For combinations with three datasets, only Q2 is given. For combinations including four or five datasets, only Min, Q2 and Max are given. Transparent cells refer to Krycklan and shaded cells to Remningstorp.
Look Angle | Polarization | Environmental Condition | Correlation Coefficient |
---|
Min | Q1 | Q2 | Q3 | Max |
---|
21.5° | HH | Unfrozen dry | 0.14 | 0.17 | 0.25 | 0.32 | 0.39 |
Unfrozen wet | 0.12 | 0.14 | 0.15 | 0.16 | 0.24 |
HV | Unfrozen dry | 0.08 | 0.18 | 0.30 | 0.43 | 0.48 |
Unfrozen wet | 0.12 | 0.13 | 0.24 | 0.28 | 0.36 |
VV | Unfrozen dry | 0.04 | 0.06 | 0.10 | 0.23 | 0.28 |
Unfrozen wet | −0.19 | −0.07 | 0.03 | 0.12 | 0.14 |
34.3° | HH | Unfrozen dry | 0.41 | 0.47 | 0.52 | 0.55 | 0.58 |
−0.21 | 0.16 | 0.32 | 0.37 | 0.41 |
Unfrozen moist | -- | -- | 0.46 | -- | -- |
−0.06 | -- | 0.24 | -- | 0.27 |
Unfrozen wet | 0.33 | 0.44 | 0.48 | 0.51 | 0.55 |
−0.21 | 0.01 | 0.18 | 0.25 | 0.43 |
Freeze | 0.34 | -- | 0.49 | -- | 0.50 |
0.09 | -- | 0.42 | -- | 0.63 |
Frozen | 0.14 | 0.18 | 0.26 | 0.33 | 0.43 |
0.46 | 0.52 | 0.66 | 0.73 | 0.77 |
Thaw | 0.36 | -- | 0.50 | -- | 0.56 |
-- | -- | 0.76 | -- | -- |
HV | Unfrozen dry | 0.50 | 0.56 | 0.62 | 0.65 | 0.66 |
−0.23 | −0.16 | −0.01 | 0.15 | 0.19 |
-- | -- | 0.01 | -- | -- |
Unfrozen wet | 0.48 | 0.48 | 0.55 | 0.61 | 0.62 |
−0.26 | −0.21 | −0.09 | 0.02 | 0.18 |
41.5° | HH | Unfrozen dry | 0.50 | -- | 0.53 | -- | 0.60 |
0.36 | -- | 0.36 | -- | 0.36 |
Figure 3.
Panels (
a) and (
b) illustrate data from Krycklan. Panels (
c) and
(d) illustrate data from Remningstorp. SAR backscatter with respect to stem volume for unfrozen (Unfr.) conditions (panels (a) and (c)) and frozen (Fr.) conditions (panels (b) and (d)) with among the highest correlation coefficients (see
Table 5). Look angle: 34.3°.
Figure 3.
Panels (
a) and (
b) illustrate data from Krycklan. Panels (
c) and
(d) illustrate data from Remningstorp. SAR backscatter with respect to stem volume for unfrozen (Unfr.) conditions (panels (a) and (c)) and frozen (Fr.) conditions (panels (b) and (d)) with among the highest correlation coefficients (see
Table 5). Look angle: 34.3°.
5.2. Forest Backscatter Modeling
To illustrate the performance of the modeling approach with respect to the measurements of SAR backscatter and stem volume, we focus on the Krycklan test site because of the availability of stem volumes throughout all growth stages. The lack of stands with low stem volumes in Remningstorp hindered the assessment of the performance of the backscatter model in Equation (1) (as in [
5]).
Figure 4 shows one example of modeled and measured backscatter with respect to stem volume for each type of PALSAR dataset available at Krycklan. Taking into account previous investigation where it was shown that the backscatter is highly consistent over time for a given polarization, look angle and environmental condition [
35], the examples in
Figure 4 can be considered general enough to represent the behavior of the backscatter for all images available in this study. The modeled backscatter followed well the trend in the measurements (
Figure 4). The strongest sensitivity of the backscatter to stem volume was found for look angles of 34.3° and 41.5° under unfrozen conditions with an increase of approximately 3 dB at HH-polarization and 4 dB at HV-polarization. Under frozen conditions, the HH-polarized backscatter increased by only 1 dB. For the 21.5° look angle, the co-polarized backscatter increased by less than 2 dB with an almost linear trend both in the FBS mode (left panel in
Figure 4) and in the PLR mode (right panel in
Figure 4). The HV-backscatter of the PLR mode increased by slightly less than 3 dB, thus less than the observations at shallower look angles.
Figure 4.
Measured and modeled PALSAR backscatter as a function of stem volume for Krycklan. The model curves are based on Equation (1). The crosses and the vertical bars represent the median backscatter and the interquartile range in 25 m3/ha large intervals of stem volume. All data acquired under unfrozen conditions unless specified in the legend (fr = frozen).
Figure 4.
Measured and modeled PALSAR backscatter as a function of stem volume for Krycklan. The model curves are based on Equation (1). The crosses and the vertical bars represent the median backscatter and the interquartile range in 25 m3/ha large intervals of stem volume. All data acquired under unfrozen conditions unless specified in the legend (fr = frozen).
The modeled backscatter in
Figure 4 was obtained by considering three unknowns in Equation (1) and showed a large range of slopes,
i.e., a wide range of the values estimated for the parameter β. To get further understanding on the behavior of the coefficient β, we looked at the statistical distribution of the estimates of β with respect to environmental conditions and look angle, which was possible only for HH-polarized data. Polarization did not seem to have an effect on the estimates of the coefficient β.
To understand the dependency of β upon environmental conditions, we selected the largest dataset for a given look angle and polarization, covering all seasons (34.3° look angle and HH-polarization). The estimates of β were more consistent under unfrozen conditions than under frozen conditions or during periods of freeze/thaw transitions (
Figure 5). Under unfrozen conditions, the estimates of the coefficient β were mostly between 0.005 and 0.009 (
Figure 5), being in line with a previous investigation in Swedish boreal forest using JERS-1 data [
5]. The estimates did not show any significant difference between dry, moist and wet environmental conditions except for one observation acquired when 12 mm of precipitation were recorded during the day. On such date, the backscatter did not show any sensitivity to the stem volume and the model curve flattened after a rapid increase for the lowest stem volumes. As a consequence, the estimate of β should not be interpreted as having a physical meaning. The same explanation applies to the observations for the case with the highest overall β estimate (0.0189), in correspondence with an acquisition under thawing conditions (temperature around the freezing point, diminishing snow cover, and precipitation). The environmental conditions affected the relationship between backscatter and stem volume to such extent that they masked out the true dependency between the two variables. Frozen conditions were characterized by the largest range of β estimates. From the weather records we could not infer dependencies between the estimates of β and weather parameters (e.g., temperature, snow depth, precipitation). We interpret the results as a consequence of the limited sensitivity of the backscatter to stem volume under frozen conditions; given the non-negligible spread of the backscatter measurements with respect to stem volume, the confidence interval of the β estimate was rather large.
Figure 5.
Estimates of the model parameter β at Krycklan with respect to environmental conditions for PALSAR data acquired with 34.3° look angle and HH-polarization. “Unfr.” refers to unfrozen conditions. Frozen conditions refer to images acquired under dry conditions as well as cases with snow fall. If precipitation was recorded at <2 mm, the unfrozen conditions were moist; otherwise, the conditions were wet.
Figure 5.
Estimates of the model parameter β at Krycklan with respect to environmental conditions for PALSAR data acquired with 34.3° look angle and HH-polarization. “Unfr.” refers to unfrozen conditions. Frozen conditions refer to images acquired under dry conditions as well as cases with snow fall. If precipitation was recorded at <2 mm, the unfrozen conditions were moist; otherwise, the conditions were wet.
The dependency of the estimates of β upon the look angle was limited (
Figure 6). There did not seem to be any relevant difference between estimates corresponding to look angles of 34.3° and 41.5°. For both look angles, the histogram had a peak between 0.006 and 0.007. For the very few acquisitions at 21.5°, the estimates were somewhat lower (<0.004), which agrees with the understanding that at steeper look angles the forest transmissivity is higher because of larger gaps and less vegetation along the path travelled by the microwaves.
Based on the outcome of these analyses, we compared the modeled backscatter assuming β unknown
a priori (
i.e., model with three unknowns) and for a predefined β value (
i.e., model with two unknowns). In the latter case, β was set equal to 0.006, which was considered the most reasonable value over all acquisition geometries, polarizations and environmental conditions.
Figure 7 shows the modeled backscatter assuming two and three unknowns in Equation (1) for three extreme β values. The black curves correspond to the dataset for which the highest estimate of β was obtained (see
Figure 7). The red and blue curves correspond to the acquisition with the highest and lowest β estimate for dry conditions, respectively; both images were acquired under frozen conditions. The modeled backscatter for such extreme cases differed only for the lowest (below 50 m
3/ha) and highest (above 250 m
3/ha) stem volumes. However, except for the dataset acquired under thawing conditions, the difference between the model realizations based on three and two unknowns is minimal, suggesting that a retrieval based on modeling solution where the β coefficient is set
a priori equal to a constant would perform equally well as compared to a more rigorous approach where β is unknown.
Figure 6.
Histograms of the estimates of the model parameter β at Krycklan as a function of look angle.
Figure 6.
Histograms of the estimates of the model parameter β at Krycklan as a function of look angle.
Figure 7.
Three examples of modeled backscatter as a function of stem volume assuming β unknown (solid curves) and set a priori (dashed curves). The measurements of backscatter are represented by crosses and vertical bars (median and interquartile range) for groups of stem volume, each being 25 m3/ha wide. Test site: Krycklan.
Figure 7.
Three examples of modeled backscatter as a function of stem volume assuming β unknown (solid curves) and set a priori (dashed curves). The measurements of backscatter are represented by crosses and vertical bars (median and interquartile range) for groups of stem volume, each being 25 m3/ha wide. Test site: Krycklan.
5.3. Retrieval of Stem Volume
To verify that the retrieval of stem volume based on a model containing only two unknowns would perform similarly to the case of three unknowns, the RMSEs for each image acquired over Krycklan were compared (
Figure 8). The scatter plot shows that the error was lower when using β = 0.006 in most cases. Only for some images acquired under frozen conditions, the model training with three unknowns performed better. Nonetheless, these images were characterized by low correlation, a consequence of the weak sensitivity of the backscatter to stem volume. At Remningstorp, the model training with a constant β = 0.006 performed in general worse than when assumed unknown
a priori (
Figure 9). This is a consequence of the distribution of stem volumes in the dataset available to this study. The dataset included mostly mature forest being characterized by weaker sensitivity of the backscatter to stem volume than at low stem volumes (
Figure 3). The lack of stands with low stem volumes caused the estimate of the ground backscatter in the model training with two unknowns to be more imprecise and the modeled backscatter only partially fitted the observations. This effect was most prominent in images showing the highest correlation between stem volume and backscatter. It was indeed negligible for all other images where the stem volume and the backscatter were almost uncorrelated.
Figure 8.
Scatter plot of single-image RMSEs for a model with three unknowns (horizontal axis) and a model with two unknowns where the parameter β was set a priori equal to 0.006 (vertical axis). Test site: Krycklan.
Figure 8.
Scatter plot of single-image RMSEs for a model with three unknowns (horizontal axis) and a model with two unknowns where the parameter β was set a priori equal to 0.006 (vertical axis). Test site: Krycklan.
Figure 9.
Scatter plot of single-image RMSEs for a model with three unknowns (horizontal axis) and a model with two unknowns where the parameter β was set a priori equal to 0.006 (vertical axis). Test site: Remningstorp.
Figure 9.
Scatter plot of single-image RMSEs for a model with three unknowns (horizontal axis) and a model with two unknowns where the parameter β was set a priori equal to 0.006 (vertical axis). Test site: Remningstorp.
The evaluation of the retrieval is done at both test sites based on the model with two unknowns and constant β = 0.006. The RMSE for single images differed depending on polarization and environmental conditions. We evaluate the error at Krycklan in
Figure 10 and
Table 6; the RMSE was smaller for HV- than for HH-polarized data for similar environmental conditions. For a given polarization, slightly lower errors were obtained under unfrozen dry conditions compared to unfrozen wet and thaw conditions; much larger errors were obtained for frozen conditions (HH-polarization only) because of the much weaker sensitivity of the backscatter to stem volume (
Figure 3 and
Figure 4). At Remningstorp, the retrieval error was smallest under frozen conditions, at HH-polarization (see in
Table 6, column of “
Single-Image” retrieval for 34.3°, HH and FBS). Under unfrozen conditions, the retrieval performed poorly because of the frequent wet and moist ground conditions, which almost entirely suppressed the sensitivity of the backscatter to stem volume and caused large variability of the backscatter for similar stem volume.
The extensive dataset of PALSAR images acquired under different look angles, polarizations and environmental conditions allowed different groupings to assess the role of each on the multi-temporal combination of stem volume estimates. All retrieval statistics from the multi-temporal combination are reported in
Table 6; results are grouped according to look angle and then for different combinations of polarizations. To appreciate the performance of the multi-temporal combination, the best and the worst relative RMSE for the retrieval based on a single image are also included in
Table 6. Yearly retrievals have been considered to allow the multi-temporal dataset to include a fairly large number of stem volume estimates per stand while avoiding that growth and/or disturbances would distort the values of the
in situ stem volumes used as reference.
Table 6.
Retrieval statistics for multi-temporal combinations available in the PALSAR datasets. For each combination, the best and worst retrieval statistics for a single-image retrieval are also reported. Transparent cells refer to Krycklan and shaded cells to Remningstorp.
Table 6.
Retrieval statistics for multi-temporal combinations available in the PALSAR datasets. For each combination, the best and worst retrieval statistics for a single-image retrieval are also reported. Transparent cells refer to Krycklan and shaded cells to Remningstorp.
Look Angle | Polarization | Year | Images | Multi-Temporal Retrieval Statistics | Single-Image Rel. RMSE |
---|
RMSE [m3/ha] | Rel. RMSE [%] | R2 | Bias [m3/ha] | Best [%] | Worst [%] |
---|
21.5° (FBS) | HH | 2006 | 2 | 105.2 | 79.2 | 0.14 | 4.9 | 80.6 | 89.8 |
2006 | 2 | 217.6 | 68.5 | 0.09 | −80.3 | 72.8 | 77.0 |
21.5° (PLR) | HH | 2006 | 9 | 198.6 | 62.2 | 0.09 | −1.1 | 70.0 | 110.5 |
2007 | 3 | 190.7 | 57.2 | 0.09 | −51.3 | 60.3 | 89.6 |
HV | 2006 | 9 | 173.1 | 54.2 | 0.12 | −29 | 67.7 | 105.6 |
2007 | 3 | 175.6 | 52.6 | 0.17 | 14.2 | 65.4 | 76.7 |
VV | 2006 | 9 | 213.4 | 66.9 | 0.02 | −35.7 | 76.0 | 110.6 |
2007 | 3 | 217.8 | 65.3 | 0.03 | −48.6 | 77.8 | 108.2 |
HH, HV, VH, VV | 2006 | 36 | 176.3 | 55.2 | 0.09 | −22.5 | 67.7 | 110.6 |
2007 | 12 | 168.4 | 50.5 | 0.14 | −16.2 | 59.2 | 108.2 |
34.3° | HH | 2006 | 8 | 72.6 | 54.6 | 0.34 | 16.3 | 60.4 | 87.1 |
2007 | 15 | 68.5 | 51.5 | 0.32 | 8.4 | 58.0 | 90.7 |
2008 | 14 | 68.1 | 51.2 | 0.33 | 7.4 | 55.4 | 102.2 |
2009 | 8 | 72.3 | 54.4 | 0.28 | 7.4 | 64.8 | 102.3 |
2010 | 11 | 78.7 | 59.2 | 0.25 | 11.9 | 68.1 | 107.4 |
2006 | 5 | 201.1 | 62.8 | 0.16 | −0.7 | 62.7 | 92.2 |
2007 | 11 | 154.3 | 46.4 | 0.34 | −19.6 | 40.5 | 106.9 |
2008 | 10 | 184.0 | 53.7 | 0.19 | −35.9 | 45.3 | 100.9 |
2009 | 7 | 121.6 | 35.1 | 0.37 | −42.1 | 37.8 | 95.4 |
2010 | 6 | 168.1 | 47.2 | 0.18 | −54.6 | 49.4 | 97.6 |
HV | 2007 | 7 | 60.3 | 45.7 | 0.44 | 7.9 | 46.4 | 56.1 |
2008 | 7 | 58.4 | 44.0 | 0.46 | 9.2 | 48.3 | 52.4 |
2009 | 4 | 68.5 | 51.6 | 0.35 | 12.8 | 55.9 | 59.7 |
2010 | 7 | 77.7 | 58.5 | 0.29 | 15.4 | 57.6 | 69.9 |
2007 | 6 | 299.0 | 90.7 | 0.02 | −43.7 | 96.5 | 117.2 |
2008 | 5 | 268.8 | 78.7 | 0.01 | −47.2 | 80.4 | 109.0 |
2009 | 3 | 331.1 | 96.1 | 0.00 | −27.6 | 96.1 | 96.1 |
2010 | 4 | 235.7 | 66.2 | 0.00 | −60.4 | 80.7 | 103.5 |
34.3° | HH, HV (FBD) | 2007 | 14 | 60.7 | 45.7 | 0.42 | 7.8 | 46.4 | 67.5 |
2008 | 14 | 58.8 | 44.2 | 0.44 | 8.5 | 48.3 | 76.7 |
2009 | 8 | 68.6 | 51.6 | 0.34 | 12.0 | 55.9 | 70.4 |
2010 | 14 | 76.1 | 57.3 | 0.29 | 14.6 | 57.6 | 81.5 |
2007 | 12 | 237.5 | 72.1 | 0.01 | −54.1 | 62.7 | 117.2 |
2008 | 10 | 225.5 | 66.1 | 0.03 | −53.7 | 61.1 | 109.0 |
2009 | 6 | 234.5 | 68.1 | 0.01 | −67.1 | 88.9 | 96.2 |
2010 | 8 | 234.4 | 65.8 | 0.01 | −78.5 | 73.9 | 103.5 |
34.3° | HH (FBS) | 2007 | 5 | 77.9 | 58.6 | 0.22 | 13.7 | 66.7 | 90.7 |
2008 | 10 | 74.3 | 55.9 | 0.28 | 10.1 | 60.4 | 102.2 |
2009 | 3 | 93.1 | 70.1 | 0.18 | −6.1 | 71.7 | 101.0 |
2010 | 4 | 94.7 | 71.3 | 0.16 | 4.8 | 76.6 | 102.3 |
2011 | 2 | 115.6 | 87.0 | 0.08 | 0.4 | 87.5 | 107.4 |
2007 | 4 | 128.1 | 39.2 | 0.55 | 21.1 | 40.5 | 92.2 |
2008 | 7 | 164.4 | 48.2 | 0.32 | −8.7 | 45.3 | 100.8 |
2009 | 4 | 171.1 | 48.4 | 0.24 | −57.0 | 37.8 | 75.8 |
2010 | 3 | 158.8 | 44.6 | 0.31 | −21.0 | 49.4 | 54.7 |
HH, HV (FBS+FBD) | 2006 | 8 | 72.6 | 54.6 | 0.34 | 16.3 | 60.4 | 87.1 |
2007 | 22 | 61.1 | 46.0 | 0.41 | 8.2 | 46.4 | 90.7 |
2008 | 21 | 59.1 | 44.5 | 0.43 | 8.4 | 48.3 | 102.2 |
2009 | 12 | 66.0 | 49.7 | 0.35 | 10.4 | 55.9 | 102.3 |
2010 | 18 | 74.1 | 55.7 | 0.29 | 13.9 | 57.6 | 107.4 |
2006 | 5 | 201.1 | 62.8 | 0.16 | −0.7 | 62.7 | 92.2 |
2007 | 17 | 166.8 | 50.2 | 0.21 | −23.6 | 40.5 | 117.2 |
2008 | 15 | 188.2 | 55.0 | 0.13 | −38.6 | 45.3 | 109.0 |
2009 | 10 | 120.7 | 35.2 | 0.35 | −40.9 | 37.8 | 96.2 |
2010 | 10 | 171.7 | 48.2 | 0.13 | −56.0 | 49.4 | 103.5 |
41.5° | HH | 2006 | 9 | 70.9 | 53.4 | 0.32 | 8.3 | 54.2 | 73.0 |
2006 | 6 | 238.9 | 74.7 | 0.10 | −24.8 | 74.9 | 86.0 |
HV | 2006 | 3 | 72.8 | 54.8 | 0.34 | 12.0 | 55.0 | 62.9 |
2006 | 3 | 280.8 | 88.3 | 0.01 | 69.9 | 91.2 | 95.1 |
HH, HV | 2006 | 12 | 67.5 | 50.8 | 0.35 | 9.3 | 54.2 | 73.0 |
2006 | 9 | 221.0 | 69.2 | 0.06 | 19.7 | 74.9 | 95.1 |
50.8° | HH | 2006 | 2 | 227.2 | 68.9 | 0.21 | 54.5 | 70.8 | 85.8 |
HH, HV | 2006 | 4 | 213.1 | 64.6 | 0.23 | 71.2 | 70.6 | 85.8 |
The retrieval error was never below 35%, being mostly between 40% and 70% and occasionally even in the 90% range. With respect to single-image retrieval, the stem volume estimates from the multi-temporal combination were closer to the
in situ stem volumes.
Table 6 shows substantial differences between the two test sites. At Krycklan, the agreement between the retrieved stem volumes with the multi-temporal combination and the
in situ stem volumes was strongest for the 34.3°, HV-polarized dataset (only unfrozen conditions). The smallest relative RMSE was 44.0% from data acquired during 2008 (
Table 6).
Figure 11 shows that the estimated stem volume agreed well with the
in situ data; nonetheless, the scatter plot did not match the 1:1 line indicating some deficiencies in either the modeling approach or the model training. The loose agreement between retrieved and
in situ stem volumes is then attributed to the large scatter of the SAR backscatter for a given stem volume (see
Figure 3). At Remningstorp, the best agreement between retrieved and
in situ stem volumes was obtained with the 34.3°, HH-polarized dataset; the contribution of stem volumes estimated from winter-time data was predominant. The smallest relative RMSE was 35.1% from data acquired during 2009 (
Table 6). As in Krycklan, the levels of retrieved and
in situ stem volumes agreed well; nonetheless, the scatter between the two datasets was large (
Figure 12). Remarkably, stem volume could be retrieved for the entire range of values represented at each test site (
Figure 11 and
Figure 12). At both test sites, the multi-temporal combination of estimates from the two polarizations of the Fine Beam modes did not perform better compared to using the best result obtained with a single polarization (
i.e., HV for Krycklan and HH for Remningstorp) (
Table 6). At Remningstorp, in some cases the multi-temporal retrieval using all observations performed worse compared to the best single-image retrieval or to a combination based on the couple of images characterized by the lowest RMSEs (
Table 6).
Figure 10.
Distribution of single-image retrieval RMSE at Krycklan for combinations of look angle, polarization and environmental conditions for which multi-temporal SAR backscatter observations (at least three) were available.
Figure 10.
Distribution of single-image retrieval RMSE at Krycklan for combinations of look angle, polarization and environmental conditions for which multi-temporal SAR backscatter observations (at least three) were available.
The multi-temporal combination performed similarly across the different years, except when the RMSE was high for each of the images being combined (
Table 6). In such cases, the retrieval statistics presented fluctuations, which are however of minor importance given that the retrieval performed poorly. The multi-temporal retrieval did not seem to be affected by the look angle nor could we notice an advantage of using full polarimetric data with respect to single- or dual-polarized data (
Table 6). In PLR mode, the best retrieval (in a multi-temporal sense) was obtained with HV-polarized data only (
Table 6); the contribution of stem volume estimates from other polarizations to the multi-temporal retrieval using all polarizations was minimal.
The retrieval error was finally investigated with respect to stand size. This investigation was possible at Krycklan only, because of the large range of stand sizes and number of stands (
Table 1). The relative RMSE for the multi-temporal combinations of stem volumes estimated from the 34.3°, HH- and HV-polarized datasets decreased for increasing minimum stand size (
Figure 13), thus confirming results in the Northeast U.S. [
12]. For the retrieval based only on HV-polarized backscatter, the relative RMSE was below 30% for a minimum stand size of approximately 20 ha. The lack of a number of forest stands larger than 20 ha sufficient to compute a reliable value of the relative RMSE did not allow clarifying whether the retrieval error would further improve or reach saturation as in the case of HH-polarized data where the relative RMSE was consistently between 37% and 42% for stands with a minimum size between 10 ha and 20 ha.
Figure 11.
Scatter plot of retrieved stem volume with respect to in situ stem volume in the case of all HV-polarized images acquired during 2008 over Krycklan with a look angle of 34.3°.
Figure 11.
Scatter plot of retrieved stem volume with respect to in situ stem volume in the case of all HV-polarized images acquired during 2008 over Krycklan with a look angle of 34.3°.
Figure 12.
Scatter plot of retrieved stem volume with respect to in situ stem volume in the case of all HH-polarized images acquired during 2009 over Remningstorp with a look angle of 34.3°.
Figure 12.
Scatter plot of retrieved stem volume with respect to in situ stem volume in the case of all HH-polarized images acquired during 2009 over Remningstorp with a look angle of 34.3°.
Figure 13.
Relative RMSE with respect to minimum stand size for the multi-temporal combination of stem volumes estimated from HH- and HV-polarized images acquired during 2008 over Krycklan with a look angle of 34.3°.
Figure 13.
Relative RMSE with respect to minimum stand size for the multi-temporal combination of stem volumes estimated from HH- and HV-polarized images acquired during 2008 over Krycklan with a look angle of 34.3°.