You are correct. I'm not sure what the sheath is made out of (possibly metal) but it is designed to stop the zoo's 3 tigers from damaging the tree. There are a trio of young tiger brothers (one-and-a-half year-olds) and they can be rambunctious.
The sheath is probably meant to keep them from climbing the tree. Since the bottom is exposed they can still damage it. However, tigers are not great climbers (unlike other cats), so I am not sure it is necessary?
Thanks lads, I thought the same as you AD as I assumed the tree was actually only a trunk the same ,as is the other one and that the lower section was still usable as scratch post or scent marking.
