## 3 Applications of L-Band SAR Data in Forestry
### Forest Stocks  
Forest stocks include stand volume, above-ground biomass (AGB) and carbon stocks. [PERSON] et al. estimated changes [24] in stand volume in Thetford, United Kingdom from 1910 to 1997 using Seasat and JELLS-1 satellite data. [PERSON] et al. used JELLS-1 L-band SAR data to study volume of forests in Sweden, Finland and Siberia, and achieved an estimation accuracy of greater than 75% [25]. [PERSON] et al. used ALOS L-band data to estimate forest volume in Siberia and obtained an R\({}^{2}\) of about 0.60 between the estimated and observed values, with an accuracy greater than 70% [26]. [PERSON] et al. conducted a comprehensive assessment of forest volume by using L-band ALOS data from 2006 to 2011; they found that HH-polarized SAR data had a good estimation effect, and obtained an error of less than 30% when the area was larger than 20 hectares [27; 28]. [PERSON] et al. used ALOS data to estimate forest volume in central Siberia; the R\({}^{2}\) between the estimated and measured values reached 0.58, and the estimation accuracy was greater than 70% [29]. [PERSON] et al. employed ALOS PALSAR L-band to estimate forest volume in central Siberia, and demonstrated that HV backscattering achieved a slightly higher accuracy than HH. They also found that the simple inversion method, coupled with multi-temporal SAR images, performed well in feature correction, providing a feasibility study for forest resource estimation in this region. [PERSON] et al. compared the forest volume-estimation potential of SAR data in X-, C- and L-bands, and demonstrated that L-band led to the highest accuracy, with a relative error of 31.3% [30]. [PERSON] et al. used ALOS-2 (PALSAR2) data to estimate forest volume in Huangfengqiao Forest Farm, Hunan Province, and obtained an R\({}^{2}\) of 0.61 [31]. [PERSON] et al. used X-L band SAR data to estimate forest AGB in Indonesian Borneo, and the results showed that the X-L band was suitable for the estimation when AGB was low, with an R\({}^{2}\) of 0.53 [32]. [PERSON] et al. used ALOS PALSAR data to carry out regional scale mapping of forest biomass in northeastern United States. They combined SAR data and optical remote-sensing calibration models, and the results showed that the accuracy and performance of the method was superior to the results from using SAR data alone, which are dependent on imaging conditions [33]. [PERSON] and [PERSON] used L-band SAR data to carry out AGB estimation on deciduous forests in Western Siberia, and obtained an R\({}^{2}\) of 0.72 with an estimation accuracy of 85% [34]. [PERSON] et al. analyzed different observation models of ALOS PALSAR data, and conducted a regression analysis and estimation of natural forests in southeast Bangladesh [35]. [PERSON] et al. studied the use of C- and X-band vegetation optical depth to estimate forest biomass and carbon balance, and concluded that vegetation optical depth from the L-band provided more accurate information because the penetration of microwaves through the canopy is higher at longer wavelengths and lower frequencies [36]. [PERSON] et al. used L-band and C-band SAR data for large-scale AGB monitoring, providing important information for accurately portraying forest loss [37; 38]. [PERSON] et al. compared airborne P-band and L-band TomoSAR measurements of the canopy-height model (CHM) and AGB over a tropical forest in Lopez, Gabon, and found that the results of the CHM did not significantly differ, while the P-band was more sensitive than the L-band in the estimation. [PERSON] et al. used P-band SAR data to estimate AGB in tropical forests and obtained an accuracy of 80%, based on the field measurements from 141 plots [39; 40]. [PERSON] et al. studied the sensitivity of L-band SAR backscattering with respect to forests with conditions of different mean canopy densities, different mean tree height, and different mean diameter at breast height (DBH), and found that the way of backscattering affected the improvement in biomass estimation accuracy. [PERSON] et al.