In the way I am using them, there is a process in which imaginary trees reproduce, you know as trees typically do. There are three ways this can happen.
First is through bearing fruit, similar to how an apple tree would, budding, blossoming, cross-pollination, etc.
This has drastic consequences on the universes that are living on the branches the fruit is being nurtured from as the fruit will absorb the essence from that universe and surrounding ones to grow and mature.
An imaginary trees can only support around 1-3 fruits at a time. And the cycles of developing fruits never overlap as the tree needs time to recover from them.
The universes that are used to nurture the fruit will experience apocalyptic events and eventually cease to exist as their essence is absorbed and broken down.
The fruits are called Conceptual Clerevellas and hold the seed that will sprout into a new imaginary tree. The forest tenders will pick these fruits when they are mature, and either let them fall amongst the trees and sprout in the forest, toss it into one of the currents within the Sea of Quanta and let it take root elsewhere, or consume the fruit to sustain their existence, similar to how the Norse Gods consumed Golden apples to remain youthful and healthy.
The second option of how they reproduce is through self-induced pollination, where multiple branches of the tree to converge and develop a fruit. Imaginary Trees can only support one fruit this way and take googols of years to recover. However the fruit and the seed produced by this method typically have more vitality and a variety in the concepts they contain compared to other fruits. The forest keepers are forbidden from eating the fruits born from this method.
The final method that can cause the birth of a new(ish) Imaginary Tree is by slicing off a branch of a preexisting tree, planting it, and nurturing it until it has grown. Trees born this way are very weak and are likely to be subsumed by the Sea of Quanta. There is only one case where a tree born from this method was able to survive onto maturity.
Moving on from reproduction, to how they actually live. Imaginary Trees are embodiments of certain concepts their seeds absorbed during their maturation. So a tree that holds dnd universes would have been exposed to fantasy and imagination concepts while being conceived. These concepts take the chaotic energy from the Sea of Quanta that is constantly attacking them and trying to dissolve the tree and basically suck out the elements in that chaos that orientate/resonate with their concepts. The flesh of the fruit acts as a shield and initall fuel source for the seed before it's primarily source of energy becomes the sea that is constantly trying to devour it.
The trees are conscious, and can move their conceptual energy around to further shield budding trees and each other from being overwhelmed by the surrounding Quanta, that is why the Forest keepers stay and work in the forest, as outside of it they would likely dissolve into the sea without the trees dampening the effects.