Hanging World - Trees

Question

Scenario
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.

Inquiry
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?

Answer 1

A lot will depend on the "surface" weather, especially regarding winds.

In the absense of winds, the most successful plants will be those that can grow the longest leaf-bearing stalks down into the abyss (though somehow "abyss" seems wrong for an endlessly sunlit void). Those with longer stalks will of course be able to get more sun than those with shorter stalks, because there will be fewer competing leaves in the way.

Immediately, this suggests that there will be no trees of any sort, because there's no benefit to having big chunky trunks; you don't have to support a huge canopy waaaay up in the sky against gravity, as gravity is now on your side. Instead you'll get lots of vine and creeper-like things... stuff like ivy, for example.

Instead of the tallest, strongest trees, you'll end up with the longest stems with sufficient tensile strength to hold up their weight plus whatever loads might be imposed upon them by epiphytes, wildlife, other creepers, debris, whatever.

A possible figure of merit is "breaking length", or perhaps specific strength:

the maximum length of a vertical column of the material (assuming a fixed cross-section) that could suspend its own weight when supported only at the top

This can be surprisingly long! natural oak has a breaking length of over 12 kilometres, if those linked figures are to be believed, and lighter balsa could manage an amazing 53 kilometres. Obviously your plants will be unlikely to grow out to their own breaking lengths... something will break them, perhaps an accident, maybe the weather, maybe grazing, whatever, but clearly your upside-down canopy could be very, very deep indeed.

Your problem will be more how this aerial canopy is anchored to the body of the megastructure above it. A huge thick mat of vegetation might one day reach a critical mass, and thanks to the enormous tensile strengths of its components a huge scab of the world could peel off and fall to its eventual doom. You might want a lot of decorative knobbly bits on the underside of the structure for the vegetation to cling too, and plenty of grazers and weather to stop things dangerously overgrowing.

Answer 2

You may turn this into a greenhouse structure instead. However, the "glass roof" will be inside the "floor" structure, which is a giant tube or a dyson sphere. The roofing will be valuable in keeping all things within, including the precious atmosphere. The system, however, looks like an inverted greenhouse to its dwellers.

Plants hang from a hard-material roof which does not allow roots in, so roots evolve as a gripping organ, not unlike that of sea weeds. Water and nutrients intake would take place through aerial roots and foliage feeding, similar to what epiphytic plants do. Non-flying animals swing among the vines (Think Tarzan...). Tree seeds and spores are lighter-than-air thanks to gas bladders. They don't get rain, so they germinate as a response to humidity in the air. Water does not obstruct the sunlight as much as debris and is allowed to become the bottom of a lake. For the dwellers, it's like a swimming roof with glass floor and lighting underneath it, and then an opaque roof with hanging vines above it. Aquatic life would recycle the fallen debris and not allow them to pile-up until they obstruct the sunlight. Hunters feed on creatures in this glass-bottomed lake, fly back to the hanging "trees" and "feed" them with their guano...