This is hypothetical. I’m not actually going to do it.
Every so often I look out at the back yard. It slopes from left to right, and it slopes from the house for about half of the property before dropping off sharply to a little creek. ‘Wouldn’t it be nice,’ I think, ‘if I could put a retaining wall at the drop-off and level the rest to make it more usable?’ The thing is, there are several nice cedars that provide shade (the sun is often low here) and shield us from high winds. Suppose I put in a retaining wall and levelled the ground. The trunks on some of them would be buried six or eight feet. Would the trees survive?
It would eventually kill the trees. The shallow feeder roots need access to oxygen and nutrients, and the additional depth would hinder that. Some trees will put out new roots from the now-buried trunks, but these can strangle the tree as they get bigger.
Probably not. While many can survive some additional soil added to the top, the roots need air to operate correctly and that much soil would smother them. What you could do though is to build circular retaining walls around their canopy perimeter that leave them in a sort of pit and that might work. I’ve seen that done with oaks, probably need to investigate further if it’s been done successfully for cedars as well.
Another consideration, do like the Incas and terrace the slope while maintaining the original elevation where the trees are.
Some years ago when we lived in a neighborhood, one of our neighbors built a flower bed, maybe 18" deep, 8’ diameter, around a big old tree. It killed it within a few months.
You can let a tree grow in a well, but 8 feet is pretty deep. Just snake the wall around the trees at the bottom of the slope and maybe you could use wells for some half way up that wouldn’t be so deep.
I just woke up from a nap and as usual headed for SDMB GQ and parsed this thread title as “Buying tree trunks”.
I chuckled to myself, “What is Johnny L.A. up to now?”
I even had a factual answer for said question.
