By selectively viewing the treasures you get by shaking a tree together than by yourself , i have noticed that you get literally 1 thing.Besides all the awesome stuff i got from fighting a bunch of pker wannabees and group shaking trees, is literally the only way to get it. I recently fond and shook a whole tree by myself, no achievement, nothing besides 1 normal acorn. With this, hope that you get lucky when rampaging around yanking poor trees.Because nothing happens when you solo yank them.