Adding soil to bury the roots is very bad for your trees.

It is natural for there to be a limited amount of grass right under your tree. Here in Missouri I have a battle keeping grass and weeds (uck, honeysuckle) from growing at the base of limbed up or smaller trees so in some ways it is a blessing if little grows there.

I am wondering if it is a climate thing keeping the grass from growing THAT far from the trees. Where in Nebraska are you?

I'm in far NE Nebraska. The area receives some late afternoon sun and became much worse after the 2012 drought when I cut back on water use so we could utilize it for crops. My whole lawn suffered and I'm still paying the price.

Yes, it's not the trees. Turf grass species are very thirsty plants. That's the reason that in drought-prone areas, the use of large areas of turf is being discouraged. You need to dig up, level, and re-seed, followed by mulching, much watering, then weed control if you want decent turf in that area. It's easy enough to do, but highly demanding of water and other resources. Trees had little to nothing to do with this.

+oM