Why do leaves FALL? (not change color)

More specifically, tell me if this sentence is correct:

(caption to a photo of a leafless ash tree, leaves strewn on the grass below)

Now, I know changes in sunlight trigger the whole color-change/sealing-off-the-leaf-so-it-can-fall-away process. But would falling temperatures be a proximate cause for the final detachment of the leaves?

Going way back to the botany class I had to take as a biochem major. Yes, the falling temps and shorter daylight are signals (don’t recall which is the stronger signal) to the tree that winter is coming. If you look at the underside of a leaf in a microscope, you’'ll see LOTS of pores. This is where gases are exchanged, but also where water escapes from the plant. Therefore, the tree would die (or at least be harmed) if it kept its leaves through the winter when water to replace that lost through the leaves is scarce.

Of course, IANABotanist.

Plus the leaves act like a ‘sail’ - high winter winds combined with extra weight of water on the leaves could cause the tree to be uprooted, (this is [part of] the same reason why most evergreens have samll waxy leaves)

Thanks. I know the ULTIMATE causes of falling leaves (i.e., why deciduous trees shed their leaves). That is not my question. I’m concerned more with PROXIMATE causes and, specifically, whether or not the sentence I listed is accurate or inaccurate.

Here ya go: http://www.forestry.uga.edu/warnell/service/library/index.php3?docID=177&docHistory[]=2

Trees lose thier leaves for any number of reasons, and the shortening daylight is one reason, not all the reasons.

High temps and drought will cause some trees to drop some leaves (yes green ones). There is a whole explanation about food supply, and the tree moving towards dormancy in drought & heat to protect itself.

Also, insects will gnaw (twig eaters? among others) on weakend trees. Squirrels, brids…insects all contribute…and more frequently when the tree is stressed.

Autumn arrives with less daylight AND combined with dropping soil tems AND dropping air temps, trees are sginaled into dormancy. T

My big white oaks are currently dropping many a green leaf right now, because in the Notheast USA there is a drought and a heat wave. The trees will do this as they remain victims of insects (and weaker to defend)…and they do it for survival. The cut off parts of themselves as preservation in lean times.

Use Google to look upo info on trees. I used key words like “trees, drought, dormancy”.

Trees are incredibly sensitive to many factors…and the older trees usually have a ton of signals about what they are doing.

My 150+ year-old Oaks didn’t make it this long by accident. They know how to beat insects, critters, drought, heat, cold and frost.

Um, to answer the OP directly: Yes. Falling Temps alone could trigger a tree into dormancy.

Shortening days play a role - they are not THE reason.

Well, the most proximate cause is gravity. When the leaves become detached from the tree, there is nothing counteracting the force of gravity, which is one of the fundamental forces in the universe, causing objects to be attracted to each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. At the surface of the earth, this force is equivalent to 9.8m/s[sup]2[/sup] times the mass of the object in kilograms. Unless there is a force holding an object up, gravity will cause everything on earth to fall downwards, i.e. towards the center of the earth.

Gravity is responsible for many other phenomena you may have noticed, such as water flowing downhill and the difficulty of flying.

Or was this not what you were looking for?

“Abscission” is the technical term for the process leaves undergo that makes them fall, for those wishing to more accurately direct their search for further info.


Thanks all, esp. Philster and Shiva.