September equinox: All you need to know

KidsPost

Ever wondered why trees ditch their leaves each fall?

What looks like pretty scenery to us is a matter of deadly serious survival for trees.

By Jason Bittel
September 20, 2020 at 8:00 a.m. EDT

Autumn arrives this week, and that means pumpkins, football and piles of fresh, crackly leaves. Did you ever wonder why trees throw away an important part of their anatomy each year? After all, wouldn’t it be similar to people losing all their hair — or even weirder, their skin — just as our part of the world gets colder? ... While it might seem strange from the point of view of a human, to a plant, losing leaves makes perfect sense.

Trees are solar-powered. Each leaf is loaded with a pigment called chlorophyll (pronounced CLORE-o-fill), which absorbs light and helps convert water and carbon dioxide into energy. The process is called photosynthesis (fo-toe-SYN-thuh-sis).

But there’s a problem. In parts of the world that experience seasons, winter means less and less sunlight each day. It also comes with biting cold that can freeze the liquids inside leaves. These two factors hamper the tree’s ability to make energy.

A full-grown oak tree might have more than 60,000 leaves, and each one requires valuable nutrients. So when fall turns into winter, trees discharge their leaves as a cost-cutting measure. If it had to spend resources on all those leaves through the winter, not only would the leaves freeze, but the tree would die.

{snip}