## 4 Results and Analysis
### LAI Retrieval in Saihanba National Forest Park
#### 4.2.2 GLAS VFP Inversion  
We selected two typical inversion results of the VFP for display and analysis to verify our method, and the results are shown in Figure 10, including those for site A and site B. Site A corresponds to a birch forest, in which the foliage is sparse, and there is almost no understory vegetation (Figure 10). Site B corresponds to a dense larch forest with lush understory vegetation (Figure 10). Here, we used the LAD profile to represent the VFP. The vertical resolution of the VFP was 0.15 m, which was consistent with the resolution of the echoes recorded by GLAS lidar.  
At site A, the LAD gradually increases as the canopy height decreased and exhibited peak leaf density at a height of approximately 7 m; then, the LAD shows a downward trend and gradually increases when approaching the ground. The LAD distribution in the vertical direction was nearly consistent with the foliage distribution density shown in the photo. From the ground measurement side, the integration of LAD from the canopy top to the ground is almost consistent with TRAC measurement, which is 3.36 vs. 3.09.  
At site B, the LAD distribution exhibited the same trend as site A except for the part close to the ground; this is because site B had denser understory vegetation than site A (Figure 10), which caused the LAD for site B to be larger than that for site A when approaching the ground. In addition, we can note that the LAD value of site B performed larger than that of site A in the middle parts of the canopy (e.g., at canopy heights equal to 8 m); this result was roughly consistent with the density of foliage shown in the photos. The LAI from the TRAC measurement for site B shows a bias with  
Figure 8: An example of GLAS observations with complex terrain. (**a**) High-resolution remote-sensing image for the GLAS observation area. The yellow circle is the footprint of the GLAS observation, with a diameter of 65 m. (**b**) Schematic diagram of the cross-section along the line segment “AB” shown in (**a**). It can be noted that the ground echo signal (Rg) was mixed with the canopy echo signal (Rv) because of the complex terrain. In addition, the obvious scattering phenomenon (S) on the slope area caused the echo energy of the last Gaussian component to be very small.  
Figure 9: Two typical GLAS received waveforms with different SNR values. (**a**) is a received waveform with a high SNR value. Gaussian component 1 is the canopy return signal, Gaussian component 2 is the ground signal, and the red line marks the position of the ground. (**b**) is the received waveform with a low SNR value. It can be noted that it is hard to recognize useful information from the decomposed Gaussian components.  
the integration of LAD from the canopy top to the ground (i.e., 3.28 vs. 4.50). Recalling the ground validation analysis in Section 4.2.1, the analysis result explains the understory vegetation caused the positive bias between the integration of LAD from the canopy top to the ground and ground measurement. Here, our validation result fits this reason, which also indirectly proves the rationality of LAD inversed based on our method.