I noticed this question from a post in 2009 that was never answered.

I wanted to filter the TreeDropdownfield to only return a subtree of the whole tree and can't seem to pull it off.

SiteTree
--- SubTree1
------- Subtree1.1
------- Subtree1.2
--- SubTree2
------- Subtree2.1
------- Subtree2.2

Depending on where the users is navigating I will only want to show Subtree1.2 or Subtree2.2 or both.

I appreciate any help here.

Thanks,

Sean

I realize this reviving a very old thread, but I've been looking for an answer to this problem as well and this seems to be the only topic that shows up on a Google search.

The solution is to set setTreeBaseID on the TreeDropdownfield.
Example:

$select = TreeMultiselectField::create('Related', 'Related pages', 'SiteTree', 'ID', 'Title');
$select->setTreeBaseID($parent->ID);

Another thing to note: Even if you want to target specific subclasses of SiteTree, you have to use SiteTree as the source-object parameter. Something like this won't work, even if all your sub-pages are of type SubPage (for it to work, all the parent pages would have to be of that page-type as well):

//                                                           ↓ NOPE!
TreeMultiselectField::create('Related', 'Related pages', 'SubPage', 'ID', 'Title');