ELI5: Why do trees lose their leaves in autumn?
Trees lose their leaves in autumn to save water and energy so they can survive the cold, harsh winter.
Think of leaves like little solar panels that make food for the tree using warm sunlight and lots of water. In the spring and summer, the tree has plenty of both. But when autumn arrives, two big problems happen:
- The days get shorter, so there is less sunlight.
- The weather gets freezing cold, turning the water in the ground into ice, which the tree cannot drink through its roots.
If a tree kept its big, soft leaves during the winter, those leaves would freeze, get frostbite, and let the tree's precious water evaporate into the dry air. Heavy snow could also pile up on the flat leaves, snapping the tree's branches like dry twigs.
To protect itself, the tree prepares for a long winter nap, just like a sleeping bear:
- Saving the goodies: The tree pulls all the leftover sugary nutrients and green energy back inside its trunk, which reveals the pretty yellow, orange, and red colors that were hiding underneath.
- Closing the doors: The tree grows a tiny scab, like a bandage, where each leaf connects to the stem. This snips off the leaf's water supply.
- Letting go: Because the leaf is cut off, the autumn wind easily blows it away into a crunchy pile on the ground.
By dropping its leaves, the tree closes up shop, stops wasting water, and stays safe until the warm spring sun wakes it up to grow brand new leaves.
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