So I guess, actually most tree roots don't actually go very deep underground at all. There are a few exceptions and we've got parameters now that let you kind of set what the roots do for your trees. I don't remember if it's oaks, one of them has a big tap root or something, but even then the tap root doesn't kind of go down through, you know, fifty tiles of granite or whatever. *laughs* At the same time it's a cool image, right? To have roots coming down into the cavern.
I really don't know, that's from the May 23, 2013 DF Talk. I recall something more recent about it but I'm not sure if it was on this thread or what. I believe there are going to be roots in this release, out of a hunch based on a possible fake/wrong memory. I guess we need to wait for our master librarian to answer this.
Oh! Also I think there's something about roots (or naming parts of the tree like X trunk, X root's.. and such).
Edit after serious search:
Yes there is, or it seems. Regarding the question, made by Caldfir on February 17, 2013
How deep do you plan on having tree roots get? Are they planned for this release? If not, do trees (for now) just end at the surface with nothing beneath but soil?
Toady answered:
Fortunately my father has a background in botany, and we talked that out before I even checked that stuff out online, since he was curious about what I was working on. I don't remember if it was in any of my older notes, but I know it is for this version. They might not spread far enough depending on memory concerns, and they might not intermingle right, and any number of interesting things might not happen.
So it seems we indeed are getting roots, or that was the plan at that moment. The
image here is from 2012 and you can see from the sideview that they don't have roots, but as that is from before the above quote I'm confident we will have them.
2nd edit:
In fact, on the raws for trees Toady put on the devlog on April 6th we find:
[ROOT_DENSITY:5]
[ROOT_RADIUS:3]
So yes, it seems roots are 100% in.