Function not recognizing page id?

missing “else” in the 3rd elseif.

 <?php 
/*
Which logo do we use?
*/
if (is_tree('33') ) { // if is conway
  $logoClass="logo-conway "; 
  $link = get_bloginfo('url') . '/conway';

} elseif (is_tree(35) ) { // if is cullowhee
  $logoClass="logo-cullowhee ";
  $link = get_bloginfo('url') . '/cullowhee';

} elseif (is_tree(31) ) { // if is Johnson City
  $logoClass="logo-johnson-city ";
  $link = get_bloginfo('url') . '/johnson-city';

} else { // if is something else
  $logoClass="logo-home";
  $link = get_bloginfo('url') . '/home';
}

?>