How Trees Get Naked – Leaf Senescence And Why Leaves Fall Off A Tree

Fall TreesWe have gotten our first week of real frost and the trees are undeniably changing their fashion sense faster than a model on a Paris catwalk. There is no denying that we are full into Fall. And while I know how a tree changes the color of its leaves, I have been wondering why it happens at all. Why do the leaves fall off a tree? I am fairly certain they don’t all just adopt a nudist attitude, but who knows?

So it was off to the Great WWW of Oz to find out the answer.

It turns out that the death of a tree’s leaves actually has a name. It is called Leaf Senescence. Senescence is the process by which a biological organism crosses into old age. Everything that lives reaches senescence. Senescence it what causes us to get old and get those wonderful laugh lines that we very rarely laugh about. The same process that leads us into old age and eventually death is the same process that kills the leaves on a tree. Basically, with senescence, the cells stop functioning as they should. Their functions shut down slowly until they no longer function at all.

In a tree, leaf senescence starts when the Fall photoperiodism is triggered. I know, I keep using those $10 words, but bear with me.

Photoperiodism is the ability of a tree to measure the amount of sun that the tree is getting each day. In the Fall, photoperiodism triggers the leaves to enter senescence. Amazingly enough, there is no universal amount of sunlight that will trigger senescence in trees. Even trees within the same species will be triggered at slightly different amounts. Which is a good thing for Fall foliage watchers, because this is what causes the lovely collage of colors. The trees are at different points in their senescence cycle, resulting in different colors.

All that is known is that at around 12 hours of daily sunlight, photoperiodism triggers leaf senescence in trees. Kind of like sometime around 18, teenagers turn into adults. Kinda. I know you parents out there with college age children are probably disagreeing with that, but in theory…

Anyhoo, so we have established that trees don’t go on some kind of arbortorial bender in the fall and get naked. It is actually a process that is rather sad or romantic, if you think about it. Once a year part of the tree dies. Once a year, the tree kills some part of itself so that it can survive until next year.

12 thoughts on “How Trees Get Naked – Leaf Senescence And Why Leaves Fall Off A Tree
  1. I always thought that senescence was also related to temperature. The first frost seems to work together with day length. What do you think? I’m glad that I’m not responsible!

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  2. I have to tell you I never cease to be entertained and educated by your blog, thank you.

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  3. Hello

    I followed the “what flower are you” link and found this blog. Nice. I find nothing sad about the circle of life. It’s just the way nature is. :)

    BTW, I’m a daffodil.

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  4. …wow, that was a really interesting read, thanks – i was just pondering that myself the other day! now i have the sollution! love your blog, by the way!

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  5. Hanna on

    Village Green – How odd that your maple is still green. maybe she is just shy? 😉

    Jane – That would make sense. Less sun to warm up the earth.

    gus – Thank You for reading!

    Bong – I suppose you are right. I suppose I should think of the flip side when the tree bursts back into life in the Spring.

    nà – Thanks for stopping by. Glad you learned something new. Hope to see you around again. :)

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  6. Hello there, found your blog by following the ‘What flower are you?’ link and I’m so glad I did. It’s fascinating reading, very informative and you write with a lovely sense of humour.
    Now I know why the leaves in autumn blush like they do, I shall be able to astound my friends with the knowledge especially as this year the trees on Mull have been stunning. Thank you :-)

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  7. Thanks alot for this blog, it was really helpful! I’m doing a biology paper on why leaves change colour and this gave me a great base to work from:)

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  8. Cool post!

    Those are some big words… lol!

    That is true that part of the tree dies

    But Oh! how they come back to life next spring!

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  9. Captain Comma on

    Fascinating article!

    When I was reading I kept asking myself “but is this a built-in cycle, or is it a side-effect of cyclical changes in the environment?” If you were to transplant a maple tree from Toronto, Canada to Aruba, or someplace where it’s always summer, would it still turn colour and shed its leaves?

    It’s an important philosophical question, too! Because it mirrors the difference between “human old age is an intrinsic part of human life” and “human old age occurs as a side-effect of predictable, but essentially arbitrary, changes in cells as they accumulate time”. It’s the difference between built-in senescence and senescence as a regular but again essentially arbitrary side-effect of cyclical environmental changes.

    Thanks for the post!

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  10. mike porter on

    I have a lot of trees that are still holding on to there leaves in mid Dec even after a real cool fall . I have noticed it happening to other of my neighbors trees and was wondering what is causing this. The trees are usually bare by this time of the year.

    Thanks

    Mike Porter

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