How it begun:
In my world, the surface has become inhospitable due to the ozone layer depleting and excessive levels of air pollution. Life migrated to a giant cave (roughly the size of north America) constituted of many "zones" of varying size connected by tunnels. Most of the zones have access to light through holes on the ceiling filled with glass (filters UVA and UVB). Many biomes are present throughout the cave from volcanic ones to "oceans" (giant aquifers) to plains (with grass and everything).
Where we are now:
It's been a few thousand years since the surface has been abandoned and populations' genes pool have strongly derived. They are not exactly different species (they can mate and give birth) but they are very different physically.
One of these populations are the Sub-sylvans (think elves basically) and they migrated to a forest biome. Initially, the soil was just hard rock and the ceiling was dirt from which trees were growing upside down. The Sub-sylvans made their homes in those trees to avoid predators hunting on the ground and not live in obscurity because the leaves were blocking most of the light coming through the ceiling.
First of, fruit creation takes a big toll on a tree so it wouldn't make sense that those tree would make any especially because you don't want the seeds to drop on the ground but on the ceiling.
Second, the only reason trees grow tall is to take the most light in real life which wouldn't be the case in my setting. My answer would be that the trees get their water from a ground fog. The lower the moister.
Third, because of the leafs blocking the light, agriculture isn't an option.
Fourth, hunting could be an option but it'd be very dangerous and they'd not be able to properly cook the meat because of the ambient moisture.
Those are the problems I am having with keeping the tribe alive and in that particular biome. How do the Sub-sylvans survive and sustain themselves in this environment?
The level of technology is what you'd see from medieval civilization. They have gunpowder if needs be.
There are a few remains of advanced technology from the surfaceans (Nanotechnology) so if it can help them I am willing to accept the answer.
They are in very bad terms with the other 2 principal tribes and trading with them is not a viable option. The other 2 factions would rather let them starve and take control of the zone afterwards.
I would rather they did not simply cut some trees to make room because they basically worship the forest. It could be a last resort solution and I am hoping to use something else. They also know that the whole wasteland surface is due to the surfaceans cutting trees all over the place without any concerns.