From a writer’s perspective, it’s even more interesting to find out why they exist on a horticultural level, because it opens up a whole realm of fictional possibilities. Science doesn’t have to invalidate mythology or fiction, no more than mythology or fiction invalidates science.
For example, doesn’t that just essentially make this a tree grave? And if folklore has taught us anything, it’s that “fairies” and other spirits usually occupy trees, or have them as their life force. And that’s to say nothing of the folklore of trees being spirits in and of themselves, or kitsunes that live in tree hollows, or dryads, etc., etc.. So, if it’s disrespectful or feels like a slight to step on human graves, wouldn’t that logic transfer to stepping inside the Fairy Circle, AKA, the tree’s grave? It’s essentially giving more fuel to the story, not detracting from it, in my humble opinion!