grassland vs piece of ground

grassland and piece of ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
grassland	Yes	No	No	No
piece of ground	Yes	No	No	No

As nouns, piece of ground is a hypernym of grassland; that is, piece of ground is a word with a broader meaning than grassland:

grassland: land where grass or grasslike vegetation grows and is the dominant form of plant life
piece of ground: an extended area of land

Other hypernyms of grassland include biome, parcel, parcel of land, piece of land, tract.

grassland (noun)	piece of ground (noun)
land where grass or grasslike vegetation grows and is the dominant form of plant life	an extended area of land