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

We 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.