When the berm was originally planted they put a black plastic ground sheet down on top of the soil to keep the weeds and grass from competing. 15 years later this black ground sheet is still there mostly burried but poking up out of the soil here and there. I removed some of this material where it was visible. It seems to have received some holes over the years but was still a material barrier for the plant roots as every hole had roots poking through itin search of nutrients and water. At this point I think the ground sheet may be hurting the circulation of nutrients back to the trees. According to the text books the tree roots will reach into the humus made from the leaves they shed each fall seaking their lost nutrients. It also means that the humus is likely to dry out more quickly as the rain water runs off the berm and the water can't wick up from below.
I don't know how continuous the ground sheet is, certainly there are many gaps in it where trees once were planted and over the years various animals have probably chewed through it. Perhaps it is not materially impacting the nutrient or water flow. In some places where the ground sheet is visible it looks like it is fairly intact but in other areas it dives down under the forest litter and humus and it is impossible to remove without tearing it to shreds. The humus layer can be quite thick and looks like a rich organic soil full of roots.