Question
Can one cut the knees out of yard, without killing the tree, which is about 70 feet in height?

Answer
Billie, You can and many people do but you will be weakening the structure and opening large wounds that are entry points for insects and disease. I would widen your mulch ring to encompass these strange wonders of nature and enjoy them. These tree's will get 100 ft plus and weakening there support structure at the base is not a good idea. Best of luck.

           Cheers, Gary Overton