The Forest of Drunken Trees

Black spruce forests in northern regions are tipping over. A new study seeks to understand why, and uncovers the role climate change plays in tilting trees. Featured image source:

Reference: Citation: Fujii, K., Yasue, K., & Matsuura, Y. (2022). Tree ring evidence of rapid development of drunken forest induced by permafrost warming. Global Change Biology, 00, 1– 9.

Humans have a long social connection to alcohol, one that dates at least as far back as our use of the written word: a Mesopotamian recipe for beer is, in fact, one of our oldest pieces of text. We’re far from the only species to have developed a taste for the drink, too: elephants have been witnessed getting drunk off of fermented marala fruit, and monkeys in some cities have wreaked havoc to get their fix. One of the most enjoyably titled articles in recent Nature history describes how fruit-eating bats crave sugary foods when they drink naturally-occurring alcohol in their food.

You might now be wondering how a tree can imbibe, let alone a whole forest. Taken literally, a “drunk forest” might remain an idea that stays on the shelves of Studio Ghibli executives for now. In the forests of the circumpolar region, a “drunk” tree is one that is slanted like in the featured image above. The cause of this tilt is only somewhat understood, but a recent paper in Global Change Biology sheds new insight into how a normal forest becomes a drunk one, and the role climate change plays in the phenomenon in the far north.

Bah, hummock

Two related theories exist to explain how trees tilt, and they both relate to the yearly freezing and thawing of the topmost permafrost. The first theory posits that trees tilt because the soil foundation loosens in warmer months as soil thaws, creating an unstable base for the tree. The second theory is a yin to the first’s yang: it posits that the winter freezing of the same soil layer causes the earth to push up in uneven patterns, since ice expands as it freezes. This freezing forms small mounds in the earth called hummocks, which appear underneath trees and can cause them to tilt.

Permafrost expansion in colder months causes the mounds of earth in the image on the right, which may cause trees to tilt. Image sourced from article.
Lean on me

To understand which of these theories was the real cause of drunken trees, scientists turned to an often utilized tool in forest science: tree rings. Trees in colder regions grow in the warm months and remain mostly dormant during the long winter, which causes them to grow a distinct new ring each year. Individual rings can look dark, light, thin, or thick depending on the conditions in which the tree grew at that time.

When a tree begins to tilt it develops what is known as “reaction wood,” or growth that occurs in response to a stimulus. Drunken trees grow a large amount of reaction wood to provide more rigid support for the side on which they lean, and the result is obvious when comparing cross sections (see image below).

Scientists looked at the formation of this reaction wood over time and found that it synchronized with the formation in the winter of the hummock underneath each tree. Across the landscape, more hummocks were also found in areas that had a greater amount of drunken trees. This relationship supports the second theory of drunken tree formation: that winter freezing creates hummocks which tilt trees as they grow.

Tree ring from a normal tree (left) and from a “drunken” tree (right). The dark semicircles are the reaction wood. Images sourced from article.
Drunken trees in a warmer world

The rise in drunken forests over the past few decades is likely due to climate change, and the phenomenon will almost certainly be more common in the coming years. Warmer global temperatures increase the depth at which permafrost thaws each year, which leads to a greater amount of hummock formation when it freezes back over in the winter. The ecological role of drunken forests is not well understood, but the tilt does not seem to kill the trees. Hummock formation may increase soil carbon sequestration, but the forests may also be more susceptible to fire due to the dry lichen-covered mounds that form on hummocks. Fires, too, are likely to increase due to climate change.

In the end, these tilted trees may be nothing more than a symptom of a changing world, one with rising temperatures, less permafrost, and more phenomena that could take years to fully understand. Nature responds in ways that we cannot anticipate in a human-dominated world, but as the case of drunken trees shows us, nothing is off the table.

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