You should use onchange function. When you are referring below the tree, does it mean below the one2many field or new line in tree view?

Using onchange function you can achieve, you should know what you have to return.

Thank you Atchuthan, 

But this is is not the required behavior. Let me say it in another way:

If I select a line in the One2Many tree widget I need to perform some action and display some data below the tree  based on the selected line. Then when I select another line update the result below the tree.

Regards,  

In tree view, if you add editable="bottom", it will make the tree view as default and editable instead of using FORM view.

<tree string="YOUR STRING" editable="bottom">
..
</tree>

You can check sale module as a reference https://github.com/odoo/odoo/blob/8.0/addons/sale/sale_view.xml#L164-L185