It does not have to be. The English language has a process where phrases become hyphenated compounds which then become single words. It's permissible to be partway along that path, and for people to disagree where something is on that path.
Pick any point in the past few centuries, and there's going to be something, possibly nowadays always a single word, but not necessarily so even now, that was in a state of flux at the time. The same goes for today.
“Old growth forest” is incorrect in any formulation of English grammar that I'm familiar with. It's not about fixed phrases. It's about using an adjective+noun pair (in this case, old+growth) as an adjective modifying another noun (in this case, forest). This is a general rule that applies across the board, not an isolated phrase example.
The poster was correct in asking what a “growth forest” is, because without the hyphen, the phrase parses as an adjective (old) modifying a compound noun.
Hyphenation is useful in phrasal adjectives, like "heavy-metal shield" (to distinguish it from a shield that is of metal and is heavy, but is not or a for example Pb) or in something like "four-day trips" (the trips last four days; they are not necessarily four in number).
I like it too, and you're talking to someone who has been trying, mostly in vain, to teach his kids the difference between "I wish I was there" and "I wish I were there", or how you can get eggs from a coop or a coöp, and they're different things, but eventually the hyphenation dies out and the phrase remains.
Mandarin written in pinyin comes to mind as another example. Do you discount that because pinyin is not the primary writing system used for that language?
It does not have to be. The English language has a process where phrases become hyphenated compounds which then become single words. It's permissible to be partway along that path, and for people to disagree where something is on that path.
Pick any point in the past few centuries, and there's going to be something, possibly nowadays always a single word, but not necessarily so even now, that was in a state of flux at the time. The same goes for today.
“Old growth forest” is incorrect in any formulation of English grammar that I'm familiar with. It's not about fixed phrases. It's about using an adjective+noun pair (in this case, old+growth) as an adjective modifying another noun (in this case, forest). This is a general rule that applies across the board, not an isolated phrase example.
The poster was correct in asking what a “growth forest” is, because without the hyphen, the phrase parses as an adjective (old) modifying a compound noun.
And "nowadays" is an example of that process https://books.google.com/ngrams/graph?content=nowadays,%20no...
Have a nice week-end! I fondly recall the days of my youth as a teen-ager.
Though I still lament the loss of the subjunctive form. And diaeresis.
Hyphenation is useful in phrasal adjectives, like "heavy-metal shield" (to distinguish it from a shield that is of metal and is heavy, but is not or a for example Pb) or in something like "four-day trips" (the trips last four days; they are not necessarily four in number).
I like it too, and you're talking to someone who has been trying, mostly in vain, to teach his kids the difference between "I wish I was there" and "I wish I were there", or how you can get eggs from a coop or a coöp, and they're different things, but eventually the hyphenation dies out and the phrase remains.
Tangential, how do you hyphenate (((very old) growth) tree)?
In English, we're making a compound adjective so it would be very-old-growth tree.
It's one step short of the German compound noun, and we make it easier to find the fragments...
English is the only language I know of that allows spaces in compound words at all. It's a very peculiar feature of English orthography.
Mandarin written in pinyin comes to mind as another example. Do you discount that because pinyin is not the primary writing system used for that language?
ancient-growth tree
This sort of thing comes up often for me. I use extended hyphenation to declare precedence: very-old--growth tree.