A growth encircling a tree is a burl. It may have been caused by an insect or mold infestation.  When this occurs, the tree grows in a deformed manner around dormant buds, and a burl grows. I discovered this burl in upstate NY where there is plenty of humidity and organic material, so the tree was growing nicely with its burl.

Wood sculptors value discovering burls. They slice the burl wood to use its unique grainy-look for tables, etc. Trees have been chopped down for its burl. I hope this burl will not be found as I enjoyed the tree’s shade!