Ask Dr. Universe – Leaves Fall

Dr. Universe: What makes leaves fall? – Kaitlyn, 13, Moncks Corner, SC;
Why do leaves fall in the fall?
– Aiden, California, 11
Dear Kaitlyn and Aiden,
You’re right, each year during the fall, we often see a lot of trees dropping their leaves. To find out exactly what happens when leaves fall, I talked to my friend Henry Adams, a researcher at Washington State University.
Adams is very curious about the lives of trees and how they can survive harsh conditions. He reminded me that all year long trees make their own chemicals that help control how they grow.
It turns out there are two main chemicals that play a part in whether the leaves stay on the tree or fall to the ground. 
The chemical that helps a tree keep its leaves is called auxin. Meanwhile, a chemical called ethylene helps flowers open, fruit ripen and leaves fall.
“There are these two chemicals, and they are sort of fighting it out throughout the year,” Adams said.
In summer, the tree makes a lot of auxin. But when fall comes around, the tree starts making less auxin and more ethylene at the place on the plant where the stem attaches to the leaf. Scientists call this the abscission zone.
When there is too much ethylene in the tree, the building blocks that make up the leaf will start to die. The leaves detach from the tree and fall to the ground—with help from gravity and the wind, of course. 
I started to wonder why we see these kinds of changes in chemicals. Adams said one thing we know is that plants can respond to different kinds of signals from the environment. Some of these signals include the amount of daylight, temperature and overall climate where the plant lives.
When the days get shorter and colder as they do during fall in the Northern Hemisphere, that’s a signal to the plant to stop making the leaf-attaching chemical, auxin.  
It’s actually a good thing the tree doesn’t want to keep its leaves, though. Big flat leaves are a problem if they freeze and die, Adams said.
Plants put a lot of important stuff called nutrients, which is kind of like their food source, into their leaves. These nutrients include things like magnesium and nitrogen that help the trees grow.
“Those nutrients are hard to come by. It’s really valuable, and the plant would rather keep that stuff around,” Adams said.
Instead of continuing to make leaves that would otherwise just die during a frost, the trees store up some nutrients from their leaves in other parts of the tree before the winter. Then they drop their leaves. For some trees, it’s better just to have bare branches during the cold seasons. It will help them survive.
As fallen leaves break down, or decompose, they can also provide some nutrients to the soil and play a part in helping a new generation of trees to grow. That means more new tree leaves to watch bud in the spring and more leaves to jump in when fall comes around again.  
Dr. Universe

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