How do I work around a tree in my fence line?
Build a wrought iron ring either around the back or depending on how big the tree is or around the whole thing.

Make a half circle with two horizontal 1" strips and 1/2" iron posts. 1 post every 8 - 12" would be enough. The two outside posts can be longer and set in concrete. You can easily make the diameter of the ring big enough to leave 6-8" of space for future growth. If you want to still block it off more you can string it with vinyl straps used on balconies or something that fits your style. Tabs welded to the outside posts would allow you to connect the boards coming from the wood posts to support them.

It would be a fairly simple build for someone with a welder.