I'm curious, though, what type of obsidian farm you're actually using - the common 3-layer bridge model (lava at bottom, air in middle, water at top held up by bridges) produces no waste water at all and thus doesn't create any mud on which trees can start to grow.
I like that 3-layer bridge model. What I have been using is not as clean as that. I have 2 rooms, 2-Z tall each, separated in the middle by 1-Tile wide drawbridges, where the magma enters on the bottom, and the water on the top.
|| B || <- Water
|| B || <- Magma
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I just let the lower right quadrant fill with magma, then close off the source when it gets at least 4/7 full. Then open the lower bridge to allow that to disperse over the lower left quadrant with at least 2/7 depth everywhere since the rooms are the same size. Then I open the water source and as it casts obsidian in the lower level, it fills the area above with water. Then with at least 4/7 full, I open the upper bridge and as it disperses, it casts the other side too.
With this setup, I can forget I opened the magma source, come back a good while later, and at most, use 3.5/7 depth of magma times the area of my casting zone, rather than twice that amount without the bridge separator. Due to a rather large demand for obsidian blocks in this fort, I have a total casting zone of 26 x 13, and even using minimal magma, the magma sea has been drained too low for the magma forges to operate. It does refill, but not as fast as it can be turned into obsidian at the farm.
The recent pause in production leading to tree growth was due to the miners being tasked with too much outdoor digging work to also keep up with the obsidian work.