The rainforest is usually countable, but can be both, and used as well as an adjective Rainforest destruction - adj Rainforest conservation - adj There is much rainforest in Brazil. There are many rainforests in the southernmost hemisphere.

It’s always countable. It’s just that we usually (but not always, just usually) use the singular form as a noun modifier. For example, a carton designed to hold 12 eggs is an “egg carton”, not an “*eggs carton”.

You've used the countable and uncountable variations perfectly in your examples; you have also kind of answered your own question too. The countable version is used after describing what it is you are talking about in reference to them, whereas the uncountable version is used before the topic (compound nouns or nouns that act as adjectives in this case).