A loop in a tree! How can this be?

August 23, 2023 1:17 PM

This is a tree (bigleaf maple I think) that fell down the hillside here on Whidbey Island and it has been sloshed around by the sea. But you can clearly see a branch has made a full sealed loop. How??

I mean obvs it happened when the tree was alive, but still. Is this rare?

posted by The otter lady to Science & Nature (3 answers total)

I think this is called inosculation.
posted by niicholas at 1:24 PM on August 23, 2023 [6 favorites]

Also known as natural grafting, and specifically autografting in this instance. Completely natural grafting is uncommon because the two branches need to move against each other (in order to wear away the surface) and then stop moving so the graft can form.

From the link above:

2. Criteria for natural grafting

a. Natural graft union formation requires that the vascular cambia of two branches or roots directly contact one another, or nearly so. There must be steady, continuous pressure between the two members. This requires that the outer bark of each must be worn away, primarily by the pressure created as both members increase in diameter over time, while being held firmly together. Hence, thin-barked species tend to form these natural graft unions more readily than species with thicker bark, and most natural grafting occurs with young branches while bark thickness is minimal.

b. Although branch-to-branch contact is common within and between canopies of tree(s), most are unlikely to become grafted together because of the frequent branch movement, due to wind, which would disrupt the continuous adhesion and pressure necessary for the early stages of graft union (i.e. callus) formation. The sequential stages of graft union formation are explained more fully in the section on Anatomy and Physiology of Graft Union Formation.

c. Natural grafting is only likely to occur between two branches that are held firmly in place. In the case of shoots this may occur when one branch is wedged within a narrow Y-shaped crotch formed by two other branches. In the case of roots, the surrounding soil matrix prevents disruptive movement of two roots which have come into contact, and this would explain why natural grafting of roots is more common than natural grafting of shoots.

d. In his fascinating old paper on natural root grafting, La Rue (1934), took issue with the prevailing view that in order for two roots or two shoots to naturally graft, they had to rub against each other until the bark was worn away. Even in the case of roots, previous authors speculated that wind-induced swaying, back and forth, of a tree's above ground trunk, would translate into sufficient subterranean movement of the root system to result in wearing away of the bark between adjecent roots. La Rue ( La Rue, 1934) however observed situations when he considered it impossible/improbable that any root motion was involved. As an alternative explanation, he suggested that the soil matrix prevented roots from moving, so that any two roots that happen to be in contact, would exert increasing pressure on each other as their diameters increased with age (seconday thickening). He asserted that the gradually developing pressure would be suffcient to disrupt the bark of both roots, eventually bringing their vascular cambia in contact, and precipitating natural grafting. By a similar process, pressure would also be generated between above ground shoots, if one branch is held firmly in the crotch of another branching limb. If you look closely at the red pine natural shoot graft, you will see a very narrow crotch between the large stem and the smaller one (near the upper cut) which is now partially fused. Increasing pressure, as all three limbs increased in diameter, caused bark disruption and eventually cambial contact.

For the intentional human version, see Tree shaping.
posted by zamboni at 12:12 PM on August 24, 2023

This looks like thigmomorphogenetic inosculation. Don't get to say that every day. But I work with trees every day; there is the combination of genetics and environment that causes environmental reaction growth like that; there may have been an event or, more likely, series of events— that caused a change in growth direction from light or from physical impact, where the branch was moved back within itself, because normally they are phototropically moving outward towards light. But as mentioned, this only really happens with a strongly captured branch, because constant motion from wind will break the bonds that form; when they rub together over years and create matching, almost sanded response growth, they can stop moving enough to glue vasculature together.
posted by Red Loop at 7:11 PM on August 24, 2023 [1 favorite]