This is a good summary. Some lidars also offer two distance estimates per point, a first distance and a secondary distance. On the lidar I used that had this

This is a good summary. Some lidars also offer two distance estimates per point, a first distance and a secondary distance. On the lidar I used that had this capability, the secondary distance was not always robust. But on permeable materials like vegetation, the secondary distance would typically indicate solid materials behind the initial curtain of material. Anyway, the presence of multiple distance returns in a small solid-angle area (whether from the above primary/secondary, or from nearby scan points) can indicate density of vegetation. You can certainly see "through" the small gaps in a hedge. Depending on the lidar, and the range, gaps as small as a few cm^2 could be enough (talking about vehicle-mounded lidar for autonomous navigation, not remote sensing of forest canopy).