tread on vs tread

tread on is not a noun while tread is a noun.

Word	Noun	Adjective	Verb	Adverb
tread on	No	No	Yes	No
tread	Yes	No	Yes	No

As verbs, tread is a hypernym of tread on; that is, tread is a word with a broader meaning than tread on:

Other hypernyms of tread on include step.

tread on (noun)	tread (noun)
	a step in walking or running
	structural member consisting of the horizontal part of a stair or step
	the part (as of a wheel or shoe) that makes contact with the ground
	the grooved surface of a pneumatic tire

tread on (verb)	tread (verb)
place or press the foot on	crush as if by treading on
	brace (an archer's bow) by pressing the foot against the center
	mate with (used of male birds)
	tread or stomp heavily or roughly
	put down or press the foot, place the foot
	apply (the tread) to a tire