Thanks for the kind words and for taking the time to clean up my mess

The line was curved (this was actually a big circle) but I don't rule out lens distortion as well. My Nikon 18-200 (which I've just sold after two great years of fun) tended to distort towards the wider end.
All of the above, while appreciated, is rocket science to me. Would you mind if I sent you the RAW file for you to pull this same magic with it?
Oh, I'm sure you can get it! It's actually pretty easy to do. Here's what I did step by step. (I'd rather pass on the tools than do it all myself!)
1. Copy the whole image to a new one.
2. Use the ruler tool between where the curve meets the edge of the picture on both sides.
3. Image> Rotate> Arbitrary, and accept the prefilled value.
4. Copy the original image again, into a third file.
5. Use the ruler to make a line down the center of the tree
6. Rotate the same way.
7. Take the first rotated image and paste it into the original.
8. Select that layer, and erase everything except the path line (metal siver part)
9. Take the second one, and paste it over both layers
10. Set its opacity to 50% and line it up as best you can with the background layer.
11. Erase all but the tree with some space around it to cover up the original tree.
12. Set opacity back to 100%
13. Erase where necessary to blend the layers together. Use a larger eraser size, with the hardness set very low.
These kinds of edits are intimidating at first, but well worth learning. You can download a tiff with all the layers I used for the edit
here.