Question about Generic Statements for Plural Nouns

Solution 1:

There are three kinds of generic noun phrases. As I put it in this answer to Ask-A-Linguist in 1997,

1. Definite Generic: the + Singular Noun
   The tiger is in danger of becoming extinct.
2. Plural Generic: 0 + Plural Noun [0 = Zero, the number; i.e, no article]
   Tigers are in danger of becoming extinct.
3. Indefinite Generic: a + Singular Noun
   *A tiger is in danger of becoming extinct.

(Note that the third one is ungrammatical with becoming extinct. There is a reason for this.)
As for their usage:

1. The Definite Generic refers to the Prototype of a species, roughly the image we associate with tiger. The tiger, as a prototype, has all the properties of anything we would call a tiger, except that it doesn't exist in an individual physical sense, like all real tigers do. This is a very abstract concept, and its use signals that the speaker is theorizing.
   The tiger is big means the speaker believes that "bigness", in some comparative context, is a characteristic property of tigers, that we should expect this to be true of any tiger.

2. The Plural Generic refers to the Norm of a species over its individuals, as perceived, of course, by the speaker, who is unlikely to have conducted tiger surveys, so the "statistics" here are very vague and impressional.
   Tigers are big means the speaker believes that, on the average, any tiger is likely to be "big". This doesn't mean all tigers are big, though that's close. This is potentially a less abstract concept, since its use implies a generalization based on experience of several individuals.

3. The Indefinite Generic refers to the Definition of a species -- that is, those properties that are absolutely necessary for anything to be a member. It doesn't work as the subject of any predicate that isn't definitional. But with a definitional property, it's certainly true for any member.

And that's the reason I mentioned above, why the third example is ungrammatical:
being in danger of becoming extinct is not a definitional property for tigers.
A tiger would still be a tiger if there were no danger of extinction.

As for rainforests and guitars:

• The rainforest is in danger.
  Rainforests are in danger.
  *A rainforest is in danger. (ungrammatical because this is not definitional for rainforests)
• The guitar is a musical instrument.
  Guitars are musical instruments.
  A guitar is a musical instrument. (OK because this is definitional for guitars)

Added directly from comment:

In the non-generic sense, A rainforest is in danger is not ungrammatical. And The rainforest is in danger is ambiguous, too. Both can refer to an individual rainforest. The generic constructions are simply additional idiomatic uses for articles