They are almost certainly new trees - the originals having been cut down in the decades after the battle. I'm not sure when the programs started, but I know a

They are almost certainly new trees - the originals having been cut down in the decades after the battle. I'm not sure when the programs started, but I know a lot of the battlefields have been intentionally restoring tree lines to their original appearance at many of the battlefields - although this is of course going to be a decades-long process.