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.
There are basically two different ways the trees can “fuse,” and this is largely dependent on the species of trees involved:
- Grafting: where the living tissues of the two trees fuse and ultimately begin to functionally act as a single unit.
- Wounding: where two trunks/branches growing adjacent to one another rub against each other and form callus tissue. While these trees might eventually grow into each other and lock together so that they sway in the wind as one, the living tissue does not fuse into a single unit and the contact site is made up of dead tissue alone
Grafting is common in species with thin bark and where the two trees are the same species (or very closely related species). Bark is usually rather thin on younger individuals, regardless of how thick the bark might be at maturity, so you can regularly find inosculated trunks or branches on younger trees of thick-barked trees (as in the sugar maples above).
While we might not see it happening, root grafting is a rather common phenomenon that takes place underfoot (source). There’s no wind to blow the roots against each other, so root grafting happens when root pressure squeezes two roots together and crushes the soft cortex enwrapping the roots (roots have thinner bark than trunks and twigs). Grafted roots can share nutrients and sugars (though they can also facilitate the transmission of diseases). Root grafting across species lines is very rare (source).
Root grafting in hemlocks
As in most conifers, hemlocks cannot stump sprout. So if a trunk gets cut below the lowest branch with needles on it, the tree will die. However, it’s not uncommon to find hemlock stumps with a ring of rounded living callus at the top. Hemlocks commonly fuse roots and share nutrients and sugars with one another. A cut stump may remain alive for decades by slowly siphoning off sugars from its neighbors (and presumably sharing nutrients as well). You can count the lines on the callus tissue to get a sense of how long the tree has been parasitizing its neighbors (see image below, which has somewhere around 7-9 years of growth).
Not every tree’s seeds find their way into perfect little germination sites. Some trees just so happen to sprout just a touch too close to their neighbor (or even at the feet of their neighbor’s trunk). And often this neighbor is of an entirely different species. Though as these two trees grow quietly and separately, their fates become indelibly interwoven. The wind again bumps and jostles the two trees through the years, wearing thin their protective layer of bark. Wounds form, and in response, the tree produces scar (callus) tissue over the damaged area. If the two trees cannot graft or the damage is too significant, the tissues around the wound sites die back and the contact site is either a surface of dead bark or dead wood (see image below).
Wounds and fences
While inosculation refers to two different trees growing into one another, we encounter the same phenomenon more frequently when we find a tree growing adjacent to an immovable object. Again, wind scratches and damages the trunk/bark of the tree and in the same way as we saw above, the tree responds by producing scar tissue over the damaged area. If the object is close enough, eventually the tree will grow around the object (as in the image below where a beech tree has grown adjacent to a barbed wire fence and created a mass of scar tissue that grew faster than the rest of the trunk as it enveloped the wire).