Question on 'especially'

See below a sentence containing an adverbial phrase headed by 'especially'.

It was very difficult for boats, especially large boats, to cross the river.

I am aware that the adjunct containing 'especially' is supplementary, as indicated by the commas surrounding it. Its meaning, too, I can understand: 'especially' emphasises that it was more difficult for large boats to cross the river than it was for any other kind of boat.

What I do not understand, however, is what 'especially' modifies. Is it the NP in 'large boats' that it modifies, or some other element? Thanks.

Solution 1:

John Lawler in a comment wrote:

Especially modifies the same thing as very, namely the predicate adjective difficult. The full sentence would be something like It was very difficult for boats to cross the river, and it was especially difficult for large boats to cross the river. Many repeated words have been deleted, including the conjunction, making the interpolation into an apposition.