We want to fill in about half of the area around this maple extending maybe 10 ft from the base for use as an easily accessible level area for putting plants on the bench. Landscape timbers will be used and the fill may be as deep as 18 inches at the lowest part. If I fill with 2 -year old pine chips will that endanger the tree? As they rot I would replenish them. (I have put a ring of soil around the tree to level the benches but am reluctant to fill the whole area with it.)
Bob
(https://forestryforum.com/gallery/albums/userpics/13036/P1020175.JPG?easyrotate_cache=1464260844)
The pine chips may lower the ph some, but to what extent I not sure, I know pine bark and needles will lower the ph. You can always ammend the soil with lime to raise the ph.
It is never recommended to fill above ground level other than normal mulching Too much dirt or more than a few inches of mulch will cause a plethora of problems
So, adding a few inches of chips a year is OK?
I agree with Buck. Filling soil much more than 6" over existing root systems will almost surely cause problems and result in eventual decline. A properly constructed tree well might help, specially on the downhill side. Do a net search. This will kinda give you the idea. http://aggie-horticulture.tamu.edu/earthkind/landscape/protecting-trees/. Too much mulch is a problem too.
https://pender.ces.ncsu.edu/2012/10/can-too-much-mulch-killl-plants/. If you want to get serious :), get rid of the fill you put in to level the bench and start a well three feet from the trunk. Put down the minimum 4-6"layer of coarse gravel or use Stalite (an expanded shale product made right there in NC), add a layer of filter fabric and then cover with a structural soil mix to get the level area you are looking for. http://www.permatill.com/commercial-applications.php?cat=39. Just my 2 cents YMMV.
You should tell the landscapers around here that, I've seen the mulch 2' over the curb height on flower beds from years of mulching.
This is more dire than I expected. Suppose I change the question to ask how to prevent erosion under this very shady sloped terrain. I planted the maple 20 years ago and its shade is preventing growth under the tree. Suppose I just put landscape timbers in place just to stop erosion and replenish just the soil washes away?
Maybe you could get the dirt back to it's regular level, put down some weed barrier fabric ( which is porous) and put ¾ chushed stone on top of that.
Unless you have a raging stream when it rains, the gravel should stop soil movement.