Aerial survey LiDAR can process multiple returns from a single laser pulse. So, some energy might be reflected back from a leaf, but some energy will pass through (or around) the leaf, hit the ground, then reflect back to the sensor. Some systems can record 5+ points from a single laser pulse.

With this information, you can filter the point cloud to only include points from the final return, which is likely to be the ground/a solid surface unless the vegetation is very dense.