BigMam's possible Devil Fruit power.. Tree Tree Fruit
1. on her jolly roger it shows a tree to the side of the skull.
2. "BiG" could either mean fat, or Thick which would also explain the tree's size comparison to Big Mam.
3. It is shown to have "hair" on the jolly roger, but it could possibly be leaves/bushes.
Now this doesnt mean her power consists of turning into a huge ugly tree, perhaps her body is as hard and tough as a tree, and she can extend her arms/legs like branches do.
People tend to call huge trees Big Mama.
Here is a link to the article: http://travel.webshots.com/photo/1422714356061928784gCYECi
Theres another here: http://www.tommeersman.com/blog/
You heard it here first at Arlong Park!