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 3x1 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...