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?

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    $\begingroup$ I get the feeling that in this world everything is "upside-down". If I shed a tear it will not hit my foot, but soar away into the sky. A fallen leaf will likewise wind up in the central star. Is that right? $\endgroup$
    – Willk
    Nov 5 '19 at 14:30
  • $\begingroup$ Well when walking along your tears would still go "down" but floors and everything would instead point toward the sky, and the forest "floor" would be above you. The whole forest and the cities are hanging from the ceiling. Leaves and stuff would indeed fall into the sky. Though, that does remind me of another point I should add! There wouldn't really be seasons, it would always be the same one I'd assume. $\endgroup$
    – Nymn
    Nov 5 '19 at 14:39
  • $\begingroup$ How are you keeping the air 'up'? $\endgroup$
    – Separatrix
    Nov 5 '19 at 14:41
  • $\begingroup$ I'm not entirely sure on that, handwaving that the super-structure is doing something to recycle materials from the center mass back to itself. It would deal with keeping air at a quality that is good to sustain life, as well as other materials like water and soil that over time would fall into it. My people aren't really techy, they're just utilizing the environment that exists. $\endgroup$
    – Nymn
    Nov 5 '19 at 14:49
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    $\begingroup$ The webcomic GrappleSeed is set in a very similar world to what you describe - might give you some inspiration $\endgroup$
    – Nicolai
    Nov 6 '19 at 14:28

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.

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    $\begingroup$ "longest stems with sufficient tensile strength to hold up their weight plus whatever loads might be imposed upon them" That's starting to sound to me like a tree, or at the very least overlapping with a tree ecological niche. $\endgroup$
    – kleer001
    Nov 5 '19 at 18:00
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    $\begingroup$ @kleer001 only it will have such a vastly different structure and appearance to a tree, it barely seems worth calling it one. Hence my references to vines and creepers. More giant kelp than giant redwood, that's for sure. $\endgroup$ Nov 5 '19 at 18:06
  • $\begingroup$ I see lots of things to work out: falling leaves will fall to the star and eventually deplete the system of organic matter, sticky soil may crumble under the pulling weights of the trunk, leaving the bare structure. Rain will fall to the center. Non-flying animals will be isolated and falling may still happen. Does your plot allow a rotating structure like in Ringworld instead? $\endgroup$ Nov 5 '19 at 19:55
  • $\begingroup$ @ChristmasSnow these are questions for the OP, not me. It is their world. They are handwaving some kind of mass-recycling system. $\endgroup$ Nov 5 '19 at 20:17
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    $\begingroup$ You could have a periodic recycling of material from the core to the outer world with the "soil" being a kind of metal foam that allows for the hydroponics so that roots may be fed with the recycled material from the core. $\endgroup$
    – Borgh
    Nov 6 '19 at 10:59

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...

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    $\begingroup$ I figured most the life would be like that lives high in a rain-forest canopy, since there is plenty of non-flyers that live in those environments that may never come to the ground. Which is exactly as you described. The greenhouse does give some interesting concepts to how the central structure could be constructed. I've been playing with the idea of some rotating shield/shutter on it that could disrupt light and give a semblance of night/day which would help with keeping regions from overheating. $\endgroup$
    – Nymn
    Nov 6 '19 at 13:49
  • $\begingroup$ Yes, adding the light shutter and rotation, will give you the ringworld. $\endgroup$ Nov 6 '19 at 14:01
  • $\begingroup$ Though, it's not actually the exterior shell structure rotating. It's just a shutter setup on the central structure. Not going for trying to actually create any type of artificial gravity on the outer shell, more of a hollow world concept I figured. $\endgroup$
    – Nymn
    Nov 6 '19 at 14:11

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