We would not recommend altering the grade (soil level) over any tree roots as this can cause them to either die out or infiltrate the new bed and compete with the desired plantings. Tree roots are sensitive to soil depth changes and adding too much atop their existing location can deprive them of enough oxygen and cause dieback, which would then risk causing a corresponding dieback of the canopy. While we agree this gardening style of encircling tree trunks with raised beds appears to be widespread, it's not in the best health interest of the tree, even though any resulting tree decline may take years to manifest afterwards. Some species might also tolerate it better than others, and some individuals may tolerate it better if most of their roots are already well beyond the bed, so it's hard to predict with certainty what the impacts will be in every situation.

Raised beds are primarily used for the ease of growing vegetables or herbs, either for improved drainage over soil difficult to cultivate or to make harvests and weeds easier to reach to minimize bending over. Since vegetables and herbs would not receive enough sun under a tree to grow well and produce a harvest, is this intended for a decorative flower bed instead? If so, you shouldn't need to raise the planting level for the health of any annuals, perennials, or small shrubs that will cohabitate with the tree roots, though simply getting them planted might be a challenge depending on how many shallow tree roots are currently in the way when digging. (Avoid injuring or trimming them if so.) Small pots of plants are younger and will have smaller root balls and thus will be easier to plant among the tree's roots, though not all species are available in small pot sizes.

Miri

Thank you very much for this information, Miri! A significant part of the intent was to have beautiful flowers around the tree where there is currently dirt due to the dog chasing squirrels around it. The raised bed was to protect the flowers against that same activity :) We'll work out something to keep all parties alive and well.

You're welcome.