Birch saplings. 2 blocks between them, 10 open blocks above them. light level is at least 10 above all saplings. I have 6 planted and nothing. I've grown grass to y level 37 (working on getting it to this area at 20) and I started that project AFTER I planted the trees.
"Saplings can be planted on any dirt or grass block, and will grow into a new tree if they are exposed to a light level of at least 9 in the block above the sapling, and have at least a certain number of empty blocks, depending on sapling type, above the sapling "block" itself. Normal saplings need a 1x6x1 area, birch saplings need a 5x7x5 area, and pine saplings need a 7x10x7 area."