Currently, the sapling requires that there be NO blocks directly above it, all the way to world height, in order to grow. I feel it may be a bit too restrictive, and rules out growing rubber trees or using TechReborn entirely should someone be in a cave world, either through dimensions or a modpack centered on that gameplay (e.g. Banished).

Another consideration to keep in mind is that most other trees do not require sky access, only a certain light level and amount of empty space. So it's inconsistent with behavior that players have come to expect and may leave them wondering why their saplings aren't growing.

Should you be inclined to change how it works, possible changes include allowing transparent/glass blocks at a certain height above the sapling so people can at least have some kind of roof, or even checking only a certain number of blocks above the sapling, rather than all the way to world height, so that people can have an opaque block if it's high enough as well.

Of course these are just some considerations. If you're content with the current requirements, that's perfectly understandable. It does make some level of sense in terms of realism for this requirement.

Thank you for your time.