I think Dave sets up a new forum once it's clear that it will have a lot of "action".
Some temporary gathering place helps to make that obvious, and if it collected a bunch of links to other threads and articles, it helps. And even if a separate forum doesn't occur right away, that temporary gathering place IS still a gathering place.
For example, ask a question in Ask A Question Forum ... then add a link to that thread in the gathering place.
Find articles about how-to-photo, as has started here on this thread, then collect those links into one post or article or blog entry.
To me, finding those links seems hard, since what would you search on?
Another possible "gathering place" would be an article. Add a comment under an existing article, and start collecting links and tips in that thread.
That's not a good final solution, as Lynn said: "There are so many topics pertaining to photographing plants that it might be difficult to find the information you need if it is all in one thread. ".
But once some "meeting place" stimulates enough interest, Dave will probably set up a forum.
I think that the main reason NOT to create forums too freely is that when there are too many forums, each forum gets very little traffic. Then the whole website feels empty and dull. Kind of like what happened to cable TV after there were 200 channels. None of them had enough watchers, and now all we have is "reality TV" and re-runs of old shows.
DG started creating forums at the drop of a hat, and then I had a list of dozens of forums, none of which had more than one post per month.
