you could use a php or html redirect to have the default page in your web root forward to the forum subdomain.
PHP Redirect (index.php):
<?php
header('Location: http://forum.yoursite.com/');
exit;
?>
HTML Redirect (index.html/index.htm/index.shtml):
<html>
<head>
<title>Redirecting</title>
<meta http-equiv="Refresh" content="5; url=http://forum.yoursite.com/" />
</head>
<body>
<center><h1>We're now redirecting you to the portal</h1></center>
</body>