shrub vs ligneous plant

shrub and ligneous plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
shrub	Yes	No	No	No
ligneous plant	Yes	No	No	No

As nouns, ligneous plant is a hypernym of shrub; that is, ligneous plant is a word with a broader meaning than shrub:

shrub: a low woody perennial plant usually having several major stems
ligneous plant: a plant having hard lignified tissues or woody parts especially stems

Other hypernyms of shrub include woody plant.

shrub (noun)	ligneous plant (noun)
a low woody perennial plant usually having several major stems	a plant having hard lignified tissues or woody parts especially stems