I was doing some research for this thread and then lost track of things. Basically, the Dark Oak needs a 5x5x7 block of Air centered over the North West Sapling, as that is treated as the Tree's center. Kind of similar to Jungle Trees.
Acacia needs a 3x3x7 Block of Air centered over the Acacia Sapling. Other than that it doesn't need much.
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I've been noticing something about the Dark Oak's canopies - it appears that they always attempt to join to it's "nearest neighbour"; they tend to "shift" towards the nearest pre-existing Dark Oak canopy, with north being the default direction if there's more than 1 tree canopy pre-existing, equidistant.
Here's an example. The nearest 2 trees on the right were last to spawn. Their canopies are "shifted" towards the north. I keep coming across this...
OK so here's where I'm at now. Note that I'm not necessarily attempting to go for maximum space efficiency; I prefer to balance that out with sustainability, by ensuring that I always get enough saplings to replant + a few extra. I've found sustainability can be a problem with dark oak especially, if planted too close together. Need as much canopy as possible. But it's OK, as the rewards are worth it.
(I forgot to enter the left-right spacing for Dark Oak - it's 5 blocks)
Results are only from my first harvest. I threw in my jungle stats simply because it was in the screenshot. I planted the Acacia in the same manner as I do Spruce & Birch. Dark Oak is very easy to farm. Acacia is actually OK - while it looks like quite a jumble to chop thru, the canopy is thin (so you can easily see every wood block) and it's all reachable from ground level (same with Dark Oak).
Just a note that Acaia seems to take a while to grow in my setup. Not the first ones so much, but it takes longer as more develop. I think it's because of the weird shapes they can grow into; they can block others out, and with so many shape possibilities, it can take a lot of ticks for one to randomly "hit" a positive spawn. I think it may be best to add 1 more block of space between the Acacia saplings, to help counter that.
And Dark Oak is now my go-to wood for non-wood-colour-affected items. (like Charcoal, Sticks, Chests, etc.) The wood amount + sapling return + ease of harvest (never have to leave the ground or hack thru canopy to access the trunk) makes it a real godsend!
I was having trouble getting enough saplings with the dark oak, so I made a narrow, tall 2x2 giant fir farm. I think it's probably less efficient than your dark oak farm (or even a birch farm, at this point) but the sapling sustainability is huge which lets me replant a giant fir immediately after clearing the first.
hmm that's too bad. The dark oak farm (the layout is in that picture above) I'm using leaves me with 2-12 extra saplings after each harvest/re-planting. Very sustainable. Just need enough room to allow the canopies to grow as large as they can.