what kind of trees ? certain maple trees are a horrible mess spreading a dense matlike root system . cutting them down wont work ... they will grow back unless thr stump is ground out

Most people in this situation will build the bed up. Putting at least 12 inches of 1/2 topsoil and 1/2 compost mixed together will solve the problem instantly. Just be aware that the raised bed will settle to 1/2 to 2/3 it's original depth, and that you will likely need some sort of edging so the sides of the bed don't melt away. You'll also have to adjust your planting depths so that the crowns don't become exposed as the bed settles down. I usually leave a saucer-shaped dent in the soil to catch water that can be filled in later to compensate for settling.

Also, if you have a year, search for instructions on how to create a lasagne bed. That would work pretty well in this situation. This will shrink down to an even smaller fraction of its depth, so really pile the organic matter on there.

My maple tree roots grow up into my raised beds a little bit.