CakePHP select default value in SELECT input

Using CakePHP:

I have a many-to-one relationship, let's pretend it's many Leafs to Trees. Of course, I baked a form to add a Leaf to a Tree, and you can specify which Tree it is with a drop-down box ( tag) created by the form helper.

The only thing is, the SELECT box always defaults to Tree #1, but I would like it to default to the Tree it's being added to:

For example, calling would bring up the interface to add a new Leaf to Tree #5. The dropdown box for Leaf.tree_id would default to "Tree 5", instead of "Tree 1" that it currently defaults to.

What do I need to put in my Leaf controller and Leaf view/add.ctp to do this?