I'd been putting off trying to make an automated tree farm for a long time, not able to work out how it could be done without horribly complex redstone, but then I saw this beautiful vanilla design from Stormweaver. So I got to work on tweaking it and adapting it for BTW harvesting, but the one problem I had with it was that the height of the growth area meant that a lot of the pine varieties wouldn't grow (I was only getting about 1 out of 8 growths when trying with bonemeal). Since pine is the main block I want to automate, I pushed and pulled at the design till I stretched it to an 11 high growth area, which I think supports all but the tall variants of pine (and all birch and most jungle).
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If people are interested in any of the details I could put up more of an explanation for them, or maybe a world download.