about the word of only

Dear Sir / Ma'am,

English dictionaries and grammar books say the word 'only' in the followig sentence is an adverb.

Only God can make a tree.

In the above sentence, I can fully understand that the word of only modifies, or limits the word of God which is a noun. But I can not understand that they call it an adverb instead of an adjective.

I would appreciate it if you kindly explain the reason why the word of only in the above sentence is called an adverb instead of an adjective.

Sincerely yours,
Sariputra

In one of the on-line dictionaries I have found this definition:

only (adj.): Standing alone by reason of superiority or excellence

which fully applies to the example you quoted in your post.

So an adjective or an adverb? We seem to have conflicting explanations here. :-? :?: ;-)