Trees seem to have some extreme growth conditions in the 1.15 snapshots. What I've found with some experimenting is this:

• Every tree type (except for trees that grow with 2x2 saplings) needs sky access in order to grow, even a glass block at sky limit blocks it from growing. (Not sure about this being a bug, but I recall growing trees underground) In addition to this, I found a very small 'huge' spruce tree which grew after I put glass blocks at sky limit. I don't think this is intended behaviour either (See pictures)
• Birch, jungle and oak trees need a 7x7 area of air centered around all the logs that do not have leaves. For example: If a tree wants to grow 6 blocks high, 3 of which are surrounded by leaves, it would need a 7x7x3 area centered on the sapling for it to grow. See the attached screenshot for a visual representation. This means that if there is some grass within a 7x7 area of the sapling it never grows, which is quite annoying. This also breaks most tree farms, since they have a 5x5 chamber. This doesn't apply to acacia, spruce, large oak, large spruce, large jungle and dark oak trees.