That's the drivers doing that. I know this because when the game first came out I did a lot of image tweaking as part of a group investigating image quality on ATI cards with different drivers and third party apps, such as ATI Tray tools which gives you access to higher functions than the CCC does.
I'm sorry to say that I don't remember exactly what it was I was playing with driver wise when I managed to turn my trees square the same as you seem to be experiencing.
Have you tried using a different form of AA ? There are a few different options in the CCC to chose from.
Then obviously there are the drivers themselves. have you updated recently ? Maybe just before you played the game and found the issue ?