stiffening vs natural action

stiffening and natural action both are nouns.

Word	Noun	Adjective	Verb	Adverb
stiffening	Yes	No	No	No
natural action	Yes	No	No	No

As nouns, natural action is a hypernym of stiffening; that is, natural action is a word with a broader meaning than stiffening:

stiffening: the process of becoming stiff or rigid
natural action: a process existing in or produced by nature (rather than by the intent of human beings)

Other hypernyms of stiffening include action, activity, natural process.

stiffening (noun)	natural action (noun)
the act of becoming stiff	a process existing in or produced by nature (rather than by the intent of human beings)
the process of becoming stiff or rigid