common alder vs alder tree

common alder and alder tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
common alder	Yes	No	No	No
alder tree	Yes	No	No	No

As nouns, alder tree is a hypernym of common alder; that is, alder tree is a word with a broader meaning than common alder:

common alder: medium-sized tree with brown-black bark and woody fruiting catkins; leaves are hairy beneath
alder tree: north temperate shrubs or trees having toothed leaves and conelike fruit; bark is used in tanning and dyeing and the wood is rot-resistant

Other hypernyms of common alder include alder.

common alder (noun)	alder tree (noun)
medium-sized tree with brown-black bark and woody fruiting catkins; leaves are hairy beneath	north temperate shrubs or trees having toothed leaves and conelike fruit; bark is used in tanning and dyeing and the wood is rot-resistant