The Science of Inosculation
So what are we looking at in the photos above as well as the image of the sugar maples below? This somewhat common forest phenomenon happens when 2 different parts of either 1 or 2 different individual trees initially grow separately, but at some point in their life they came into contact with one another. As the trees rub and knock against each other, the trunks/branches are damaged, and eventually “fuse” together. This can happen on a single individual (as with the beech on the top right), but is more common between two individuals growing adjacent to each other, the latter referred to by botanists as inosculation (from Latin: in- = “into” + osculus = “kiss”). Humans have long taken advantage of this process to graft apple scions onto rootstock or even for art (called tree shaping or arborsculpture).
Why it’s happening
At the core of inosculation, there’s some process of wounding or mechanical damage that brings the living tissue or two trunks/branches into direct contact with one other. Often its wind that blows the two abutting trunks of adjacent trees against each other day after day for many years, inflicting the paired wounds. The rubbing initially scours away the protective outer layers of bark until the living bark cambium tissue (or if it goes deeper, the vascular cambium between the wood and the bark) is exposed. In response to the wound, the living cambial tissue starts to rapidly lay down callus – or scar – tissue. This growth is produced much quicker than normal growth and so the branch or trunk starts to swell with new cells around the wound. The trunks/branches on both trees grow closer and closer to the other tree and both wind up “healing” or fusing into each other.