My world is essentially a Neptune-sized gas planet that got seeded with life and its surface layer filled with oxygen, therefore gaining a sizeable layer with Earthlike conditions.
Within this layer, life is diverse, but the largest lifeforms would be colonies of treelike plants that have connected branches and a translucent, photosynthetic canopy stretched between those branches. Think an organic version of the tensegrity sphere. The branches would be connected by non-rigid "ligaments". The life cycle would start with a single instance, which would clone itself, forming a mushroom cap shape with a flat, ring-shaped floor on the inside bottom with a circular hole in the middle. Gradually, all instances would grow in size, preserving the general shape but the center would die, decompose, and form a "soil" layer in the inner flat ring.
The young colonies would be buoyed up by updrafts as they fell, in clear skies the sun would warm the air inside the canopy enough to slow the fall, allowing it to heat up even more and maybe rise. The early stages are the most difficult. The majority of instances sink to the depths. As the branches, canopy, and connective structures are all on the outside, the volume of heated air also grows and it becomes easier to stay aloft as the colony grows larger. The colony is adaptive and can regulate the inside air temperature and pressure.
Could such a forest grow as large as a city? A continent? Could it actually support an entire ecosystem within itself of creatures that don't drift, fly, or float their entire lifecycle? Would it be strong enough to withstand turbulence and storms, or would it need to stay within a jet stream to survive? Would extremely large ones generally be stable?
I thought of this through pondering how a floating island would actually be possible. In the lore of my world, human colonists discovered this world and created aerostat colonies before a disaster forced a migration to these forests.