Edit the /etc/aliases file. There is an comment example at the end of the file for forwarding mail for 'root'; you can simply edit this line. You MUST be logged in as root (or use su or sudo) to edit the aliases file. Root can add a forward for any local user to the aliases file. After the changes are made to the aliases file, they are actuated with the command:

In the scheme of things, the /etc/aliases file is processed before the ~/.forward file.