i have created a Hallow Montain with level and on one of the level that is 16 Block height with bottom in rows of 2 wide of grass block and water separated the row  also 2 wide just under the water glowstone  ( light level 12 at sapling level).

I planted the 2x2 Dark oak sapling on the grass  the separation between 2 sapling are of 10 block in all direction , and no growth even when using bone meal… not a single one have growth at all

The level of the dark oak level is 81

now here something i never expect i was working on the 187 level part of my building creating a dome like roof and using the dirt block as a start poing for my glass done after making the round and returning to my start a Dark oak tree had appear out of nowhere and created 2 dirt block for it's own base and i check below and beneath it all there was the sapling i've planted but there is about 106 block between the 2 place the space between is almost all stone and 5 levels between floor and ceiling x6.. so your thought on both subject are apreciated

Srg