white alder vs alder tree

white alder and alder tree both are nouns.

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

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

white alder: tree of western United States
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 white alder include alder.

white alder (noun)	alder tree (noun)
tree of western United States	north temperate shrubs or trees having toothed leaves and conelike fruit; bark is used in tanning and dyeing and the wood is rot-resistant
shrub of eastern and southern coastal United States having beautiful racemes of spice-scented white flowers