Q: Will the trees ever melt or burn in magma? The magma depth has reached 2-3-4 on some tiles close to the pump assembly (see setup) yet the trees remain. Will we only know when the first level is entirely filled(7)? Do you need to fully submerge a tree in magma (including all its branches on all its levels) for it to go away? Please tell me at least that new trees will not grow out magma or that the current ones won't grow further, something?
Context
Love trees but in the game, they've been messing with my zen. Because cutting down trees can leave a hole in the floor if the layer below has been mined, it's really not a good idea to build a giant stockpile on the level below the ground. If you forget this, and then designate a bunch of trees to be cut, you may end up with a bunch of holes on which you'll have to build flooring to avoid intruders (and constructing close to trees can be inherently dangerous in 40.24; so you better save often). For the same reason, you can't build a moat around the fortress using channels if you've used the level below (in that case you'll have a taste of the cave-in experience!).
Over time, trees can also grow too close to the fortress walls, so I built a 3-tile wide paved road around it to make sure nothing grows there. Nevertheless, growth dynamics means some branches expand outward over z-levels and can come pretty close; this weakens our defenses. In one instance vile enemies attempted (and were smashed) to climb on the second story where we station our marksdwarves. In so many words, when I build my fortress I don't ask for the trees permission and now I devised a plan which (I thought) really ups the ante on the annoying trees…
Setup
We built walls and are now filling this magma reservoir surrounding our fortress, and the trees are welcome. Bottom right is a 2-pump head-to-tail stack with one 3×1 horizontal reservoir, each with a 40 hp windmill for power. Pumping is not managed per se; a mere magma-safe steel floodgate controls the assembly flow on output (using all-steel components and exceptional mechanisms). The fortress first level walls are all made from magma-safe material (olivine and basalt) but the second floor is surrounded by… fortifications (couldn't withstand 7+1 fill). At some point branches caught on fire and I thought that was it, but trunks remain in their entirety in the aftermath, as do branches and even flowers. With their trunks, the trees on the right are some 5 levels high! We set up a few bridges (raised) along the external wall, for a warm welcome…
Best Answer
My testing in the object testing arena shows that parts of a tree above the first z-level will burn easily in magma of any depth, and the fire will burn everything above the first z-level, but there will be a healthy tree trunk left on that first z-level, even if it is entirely submerged in lava to several z-levels. If the tree trunk appears in a tile that already has magma the first level will also die, but this is probably never going to happen in fortress mode.
So trees cannot be entirely killed by magma, just as you have observed (and even the parts that can, well the dead wood can stick around). However, if you keep two z-levels with magma in them then you can just ignore the stumps, for fortress defence. Of course it could lead to fun if mining root bundles also can leave a hole in the floor to let the magma in.
Sorry that this isn't the answer you wanted, but zealous clearing and perhaps larger paved road building are probably a more efficient tactic for achieving your stated goals.
However new trees should never grow on submerged tiles, although water doesn't kill saplings it merely prevents their growth. I can only assume that this holds for magma as well as it does for water (or that saplings burn). On a side note, farm plots will block tree growth just as well as paved roads and are cheaper to produce. I don't know about their resistance to building destroyers though.