Hmm, perhaps I am missing something Elanor, but how exactly are the Ents and Beorn related? Sure they have things to do with nature... but... then you could say that pebble that moved over there has something related to do with Beorn... . I would appreciate it if you could explain/expand on that, I'm always looking for more knowledge!
The ents and Beorn do have similarities...(bearing in mind Tolkiens extreme dislike of allegorical comparisons).
Both Beorn and the Ents represented in the story, a concept and a very powerful plot device. The concept they represent is the power of nature that tolkien loved, Both Beorn and Treebeard were powerful lords of the natural world, and were representatives of and provided a voice to things that could not speak for themselves.
Beorn and Treebeard also represented the plot device of rest and rejuvenation and unlooked for help. Unlooked for but not unknown (In both instances Gandalf knew of them). In both instances, characters gained insight through interaction with them. It was a place for characters to regroup and refresh (in treebeards case it was just the hobbits Merry and Pippin).
I am sure that they have other similarities. But thats all I can think of at the moment.