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Why Do Trees Have Knots?

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Created: 2/6/2017      Updated: 3/3/2017

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In this series, we'll be addressing some common questions from visitors and readers. Do you have a science question that has you stumped? Ask our museum scientists via our form here and we'll answer it on our blog!

Today's Question:

Why do trees have knots?

Answer:

Tree knots are also known as "burls". Burls form on the outside of trees as a reaction to stress. Trees don't have strict parents or follow politics, so what stress could trees possibly have?

Dozens of burls on a centuries-old cypress tree at the Beijing Temple of Confucius in China [1]

Stress, to trees, is life-threatening. Stress is injury from an axe or chainsaw. Stress is infection from insects or bacteria. Stress is extreme flooding or drought.[2] A burl is formed as a last-ditch effort to save the tree's life after an injury or infection. A burl protects a tree like a scar.

A burl can do more than seal off a wound like a scar. Redwoods use burls for self-defense like other trees, but redwood burls can actually sprout and create new redwood trees.[3] This is an important part of the redwood life cycle: if a redwood thinks that it may die from injury or disease, the redwood creates burls that can sprout many more redwoods in its place.

It's knot not all smooth sailing once the tree has protected itself with a burl. Wood from a burl is prized by woodworkers for its intricate design, and some will pay top dollar for it. Burl hunters use saws to hack the burl off, giving the tree a fresh wound. Poachers will even steal redwood burls from national forests, resorting, at times, to killing the tree.[4]

A park ranger in Redwood National Park inspects a redwood tree attacked by poachers for its burl [5]

Burls aren't the only reaction that trees have as a response to stress. Nature can get creative. Acacia trees in Africa get stressed when antelope or giraffes start eating the tree's leaves. The acacia tree, in defense, releases bitter chemicals called tannins that make the leaves too bitter to eat. That's not all: the tannins are released into the air and alert other acacia trees nearby to begin producing tannins as well. Smart giraffes know to eat trees that are downwind first, so as not to alert other trees nearby![6]

All living things undergo stress--it's just part of being alive. Every living organism deals with stress in its own way. That being said, if your stress is giving you knots, see a doctor. I'm a blog author, not a physician. Seek professional advice.

Kyle Schiber
Nature Museum Volunteer

[1] https://commons.wikimedia.org/wiki/File%3ABroussins_sur_un_cypr%C3%A8s.jpg, By Laurent Bélanger (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

[2] Feeley, Tivon. Iowa State University. (23 October, 2007) Retrieved January 31, 2017, from http://www.extension.iastate.edu/news/2007/oct/071901.htm

[3] Redwood National and State Parks. Retrieved February 1, 2017 from  https://www.nps.gov/redw/planyourvisit/upload/Redwood_Burl_Final.pdf

[4] Brown, Patricia Leigh. Poachers Attack Beloved Elders of California, Its Redwoods. (8 April, 2014) Retrieved January 30, 2017 from  https://www.nytimes.com/2014/04/09/us/poachers-attack-beloved-elders-of-california-its-redwoods.html

[5] Bomke, Jeff. National Park Service. (28 February, 2014) Retrieved February 1, 2017, from https://www.nps.gov/redw/learn/news/newton-drury-parkway-will-be-closed-at-night-due-to-increased-wood-poaching.htm

[6] Hughes, Sylvia. Antelope activate the acacia’s alarm system. (29 September, 1990) Retrieved February 3, 2017, from  https://www.newscientist.com/article/mg12717361-200-antelope-activate-the-acacias-alarm-system/



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