Good question.Acid rain has a devastating effect on the forest.
I don't think we need posit an extreme <scenario> for an unusual (perhaps) article. The sentence could perfectly well fit in many contexts where it's clear that a particular forest is spoken of.If I were writing this sentence, I'd probably write "... effect on forests." One of the problems with online dictionaries is that they take any examples of the word they find on the web - and the web is full of language errors. For one reason, a lot of it is written by people for whom English is a second, third or fourth language.