A civilization is living on an ancient super structure, they don't know much about it, they just live there and have lived there for as long as they know. For our current situation we can ignore most of this structure. The interior of it is hollow with the center hanging around an artificial gravity well/power source, for this case just imagine it as a star. Everything living on the inside is literally hanging over the void above this.
The outside of the structure is unsuitable for life, and the old working have a tendency to periodically get exposed to open space and the old machines can take a bit to fix things. Sure many people live here, but it's a dangerous life. So unless they're pretty bad off most live on the interior of the structure overhanging the void.
To sustain atmosphere and life on the interior we have a soil that can stick to the interior of the surface with minimal loss to the central mass. On this we have plant life, part of it being trees.
Details that may matter
- Gravity at the surface (pull toward the central structure), is approximately 1 G. Important to remember that down, is toward the central gravity well.
- I haven't figured out what the soil would need yet...that's going to be in a different question. Figure it's something that will be sticking to the surface between composition and plants preventing erosion, maybe magical machine tech making sure it stays in place.
- Root structures can dig deeper into the superstructure and can wrap around some of the super tech metals that are making it up. They won't break easily under any weight said plants and civ can put on them.
- Trees are growing both toward the center of gravity and their light source. Figure they have plentiful water from the super-structure they're growing on.
- Been running through tree questions and learning things, but figured this was a bit different since these trees aren't going to be fighting anything but their own weight from what I can assess.
- Not much happening to cause a change in seasons, assume we have an eternal summer going. Maybe no more than slight temperature variations of ~5C, so 21-26C.
- The super structure has in place its own super hand-waving tech in place that we don't understand that can recycle lost matter going to the center. So don't be concerned about some matter loss. It IS sustainable as long as it's slow enough that things grow and people can actually make decently long term cities in places (200-300 years.) This includes allow air near the "top" to stay at a level that can sustain life.
- Central mass is far enough away things aren't going to randomly hit it. It's not going to cook everything, it's designed to sustain the life not kill everything.
- It's less important of how this environment is possible, and more about how things would be or may need to be if it was possible. For this particular question the concern is the large plant life (trees.) How would they react if light and gravity worked from the same direction, unlike on a normal planet where they are basically fighting against gravity to get to the light.
The people are building structures utilizing these trees as supports and structures. How large and strong can I realistically expect these inverted trees to grow?