When trees are young if they’re close enough to touch they will fuse together as they grow and the resulting wood will be solid. But at some point they’re too old to do that any more and if they grow together the seam where they joined will retain traces of the bark that originally separated them. How far along in the trees development does this change occur?
My credentials to help answer this question: I’ve been pruning young street trees for nearly 9 years with Friends of the Urban Forest in San Francisco. I have no academic credentials, just some training and that much experience.
First, I think it depends heavily on the species. But of the trees that we plant around here, they all already have bark when we get them (not sure how old they are when we plant, generally they are 6-8’ tall). Sometimes new branches grow too close to the trunk and they get conjoined in the way you describe, and that always seems to include bark between the trunk and branch, so we try to remove the branch while we still can.
I think this might be possible with very young trees without getting bark between them, I’ve seen pictures of it with bonsai, I’ll try to see if I can find any. I hope these remarks help with your query.
The term is inosculation:
from here
I’ve seen it a lot with trees that would originally have been a hedge* - whether or not the pieces can completely fuse is probably mainly limited by whether the touching parts have developed thicker bark containing cork or cork-like material - younger branches where the bark is thin and green underneath a relatively short outer layer, probably join together quite easily if they are pressed together for a few season’s growth.
But older pieces can join together in the parts that are new growth - in the same way that you sometimes see a mature tree growing around, and absorbing a non-living object like a rock or a metal fence stake, the parts of two trees, even if mature and well grown, will ‘grow around’ each other if they are pressed together (it looks sort of like a messy kiss), and the parts that are actively growing can fuse in this way. When such a tree is felled and the joint is cut apart, you’ll see something like a perimeter of fused wood, surrounding a patch of dead, non-fused material.
It does depend on the species - some will not do this at all; others seem almost eager to join up with their neighbours.
*Hedge Laying is a thing we have a lot of around where I live - living hedges are created by partly cutting through the stems of sapling trees then bending them over and laying them down into the hedge line - the uncut stem material keeps the tree alive and the now-horizontal trunk typically creates a load of new growth that shoots upwards. A few years later, some of that growth is treated in the same way - partial cutting and laying down. After a decade or two, there is a sort of coarse basket or lattice of living tree branches, which are beginning to fuse. After a century of growth, especially if it is neglected, it looks like a line of full grown trees that are trying to be a wall.