This forum is for turfgrass, but I'll answer your question for you. Trees should never have their roots disturbed by adding several inches of soil, especially right around the trunk. Tree roots are very shallow and easily damaged. Changing the grade can smother those essential roots. The more mature the tree, the more difficult it is for it to withstand such damage or to recover from it.

Dear Sue Kend

Thank you very much for your inquiry. I agree with the previous personÂs answer, and so does my garden expert Susan Gruber. See her suggestion below.

It is never advisable to raise the soil level over the root system of established trees. This reduces the oxygen available to the roots and can quickly kill many of the fine feeder roots. It is particularly risky with older oaks which have fungal associates (micorrhizae) that help the tree take up nutrients. They are very sensitive to compaction and grade changes.
Depending on the amount of soil added, the amount of the root system covered and the initial health of the tree, the resulting stress may be minimal or fatal. Trees often look fin at first, but die over a period of many years. Others will respond better and survive by rooting up into the added soil.
As an alternative, mulching the area with 3-4" of shredded bark and using plants in containers (even if they are many and large) will be neutral or even beneficial.

Again, thanks for the question and let me know if you have more.

Respectfully

Trey Rogers, The Briggs & Stratton Yard Doctor, YardDoctor.com