05/11/2024
Every year, many trees change their leaf color from green to gold, crimson, amber, and even jewel-toned purple. For those who live in areas that don’t experience a major color shift or live on treeless plains, deserts, or deserted islands, (is that a cactus?) we see you. We be-leaf in you. But let’s talk leaves.
Why do some leaves fall off?
It’s because they’ve had enough of the tree’s drama. We said it. They’ve been putting up with all the photosynthesis and chlorophyll shenanigans for months, and frankly, they’re over it.
Sorry, checking notes. The supervisor is requesting a sidebar.
And we’re back.
Broadleaf trees are responsible for the colorful annual display that defines autumn. For most of the year, a bright green chemical called chlorophyll supports photosynthesis in leaves. When the days grow shorter, chlorophyll dies off and other chemicals become the main source of color. At the same time, the tree starts to grow a layer of scar tissue at the bottom of each leaf’s stem. Over time, the connection between the leaf and the tree weakens enough that the wind or gravity pulls the leaf down.
Why do some leaves stick around?
Conifer trees are like that coworker that never takes a sick day. While other trees are dropping leaves left and right, these evergreens soldier on, year after year. They show up to work, never complain (outwardly), and always look green and fresh. But seriously, let's focus on the trees. (The supervisor is right behind us.) The tree's needles, compared to broadleaf greenery, lose less water to the air. Most precipitation often comes in the form of snow during the winter, which needs to melt and turn into water before the tree can use it. That means trees are waiting all winter for a drink of water! While conifer needles can do a little photosynthesis on warm days in the winter, the main reason that they keep their needles is to save energy. Just like you might reuse water bottles to save money, conifers reuse their needles for several years before replacing them with new ones. That means they can’t just throw away their needles during the winter.
We hope this has been helpful, enlightening...well, we hope it's been something.
Image: Trees with striking golden leaves along Skyline Drive at Shenandoah National Park last fall. NPS/ Peter Forister
