I just had a revelation: trees have roots.
I'm realising this falls into the domain of many 'multi-tile tree' ideas.
I was thinking about how a *bonsai* would be an excellent craft for elves and might even be artifacts. Then I realised fortresses are poepled by dwarves, why treat trees like they treat ores (hack at it untill usable chunks fall off).
This lead to thoughts of "heh dwarves would probably cut a tree down by undermining it anyhow".
so.. right now trees have very superficial roots that are rooted only in the floor-part of the tile.
how about roots extending below to the soil layer underneath? No extra wood, but some interesting results, such as an entire forest just dying off, because the local fortress-lord decided that the soil layer makes for excellent storage space.
Other things that might make soil-burrows less amiable could be
-percolation of rainwater.
wetting any underground structures and maybe causing slime and rot to 'soil' stuff.
-digging critters like worms and ants infesting your soil-dug foodstores. (rats etc should be able to move through ~2 tiles of soil too)
-poor support capability. more risk of caveins, maybe only when wet.
Yeah, this would also lead to soil-dug trenches and canals collapsing into the water they carry.
Finnally a reason to line the canals with stone.