Birch or beach, with white bark, maintain markings pretty clearly. They don't really distort all that much for very long periods of time. I'm certain if you took a mircometer to one when it was first marked and came back 30 years later you'd find a difference. But I think if you took photos and compared by eye you would have a hard time seeing it. It certainly doesn't become hard to read. I know places where people carved their initials and dates into trees as much as 40 years ago and it's perfectly readable.