This Is Why Leaves Really Fall Off Trees

We learn very early on in life that each fall is marked by the leaves changing colors and eventually falling to the ground (where we need to rake them up).  But why?  Hint – it’s not the drop in temperature or the wind that causes the leaves to fall.  The truth is actually much more interesting.

As the seasons turn to fall, the days become shorter and colder.  Those changes force the leaves to stop producing chlorophyll, causing them to change colors.  They also cause a hormone change in leaf-dropping trees that sends a chemical message to every leaf on the tree.  Once that message is received, tiny cells begin to appear at the spot where the stem of the leaf meets the branch.  They are called “abscission” cells.  The root of the word is the same as the the root of the word scissors, which gives you an idea of what these cells do.


The scissor cells are stained red, marking the boundary | University of Wisconsin Plant Image Teaching Collection


Days to weeks after receiving the message, the leaf develops a full line of the cells that push the leaf away from the tree, bit by bit.  If you grabbed a microscope and took a look, you would be able to view the abscission cells lined up against the stem.  The cells keep pushing and eventually a strong enough breeze comes along to complete the detachment.  Ultimately, it’s not the leaf falling off, it’s the tree telling it to get lost.  But why?



Well, outside of adding color to our lives, leaves serve a real purpose – to help make the food that allows the tree to grow and thrive.  As the sunlight reduces each day, the food production lessens, and the tree has an option.  It can shed the leaves and prepare for winter.  Or it could retain the leaves, which would lose color but stay living.  But when a rare sunny winter day pops up, the leaves would again begin drawing water and photosynthesizing.  It would soon turn cold again, and the leaves would be caught with water in their veins, which would freeze and kill the leaves completely.  When spring returned, the dead leaves would be stuck and would not be able to produce food for the tree.  So, eons of evolution have given us trees that shed the leaves and re-grow them in the spring.

That’s why we see deciduous trees changing color and dropping leaves every fall.  It’s their way of prepping for the cold, dark winter ahead.