Increasing a unit/building's defence

Hey guys, I was woundering if it was possible to increase a units defence by triggering (FOR EX: A building roots, I want his defence to increase while he's rooting, but go back to normal when he's unrooting)

Make a + armor ability and add it when the trees root?

or there is another way, use thinker's robo mode or metamorphosis to change between moving mode and tree mode (just set the tree mode with more armor, and 0 movement speed)

well doing it by trigger might have some trouble when dealing with different condition @@