We would like to collect a generic purpose topographic LiDAR dataset over a large region in Belgium. Our minimum requirement is to achieve at least 1 point/m² on the ground. Ideally, we would like to have images from spring or summer because it would then be possible to get some extra information about shrubs AND more chances to have good flight conditions, but this is not our priority (the density of soil return must be sufficient). Alternatively, we could work during winter in order to have less vegetation cover in crop fields and deciduous forest, but then we would have less information on understory shrubs. This decision is difficult to take without knowing how much "ground return" we would lose when the vegetation cover is at its maximum.
Therefore, I am wondering if there are some shared experience about the proportion of ground returns under deciduous forest with or without leaves. I guess that this proportion is also depends on the LiDAR footprint, but I could not find any comparative studies.