Unless the surrounding grade near a particular tree is being lowered due to nearby construction or grade change, there is no benefit in having a raised planter box or flower bed around trees. The necessary additions of soil required to meet the new retaining wall grade adds fill over the roots and usually buries the tree’s natural root flare.
This is generally detrimental by likely introducing decay to above ground tissues not naturally fortified against it. Also, trees in the above environment can be encouraged to root into the added materials. This also happens quite often in an erroneous process called “volcano mulching” when excess mulch is piled up against the trunk of the tree. The tree is then stressed when hot temperatures and/or drought conditions occur. This can lead to a general “decline spiral”.
When trees have had raised planter/flower boxes for some time, upon wall removal, the grade should be sloped away gradually from the exposed root flare with good topsoil to meet the now lower existing grade. Grass seed, mulch or edging can then be added.