php - CakePHP $actsAs having no effect except in AppModel?

Question

I am trying to implement a Tree of Categories in CakePHP.

If I add this line in AppModel:

    public $actsAs = array('Tree');

and output this from CategoriesController:

    $behaviors = $this->Category->Behaviors->attached();
    debug($behaviors); die;

The result is:

    array(
            (int) 0 => 'Tree'
    )

But I don't want all my models to behave like Tree. So if I add this in the Category model (Category.php):

    public $actsAs = array('Tree');

And output this from CategoriesController:

    $behaviors = $this->Category->Behaviors->attached();
    debug($behaviors); die;

The result is:

    array()

Any idea why? The cookbook here: http://book.cakephp.org/2.0/en/models/behaviors.html#using-behaviors says that I can use the $actsAs just in the models I need a specific behavior.

Thank you.