Why trees turn colors before dropping their leaves

HostI was looking at that big maple tree in the yard this morning and it's just glowing right now. It's such a deep, bright red that it almost looks fake against the grass. But it also seems like a lot of work for a tree that's about to go to sleep for the winter. Why do trees go through this whole big show of changing colors instead of just letting the green leaves fall off?

GuestIt does look like a big party before the end of the year, but it's actually a very smart way for the tree to save its strength. Think of each leaf like a tiny solar power plant. All summer long, they use the sun to make food. To do that, they use a special green stuff called chlorophyll. That green stuff is amazing at catching light, but it's also very expensive to make. It's full of nitrogen and other bits of fuel that are hard for the tree to find in the dirt. If the tree just dropped its leaves while they were still green, it would be like throwing away all the expensive machinery in the factory every single year. The tree can't afford that kind of waste.

HostSo it's not just dying. It's more like the tree is cleaning out the office before it moves out for the season?

GuestThat's a great way to put it. As the days get shorter and the air gets crisp, the tree gets the signal to start packing up. It begins to break down that green chlorophyll and suck all those good bits of nitrogen and phosphorus back into the trunk and the roots. It stores them there so it has a head start when it needs to grow new leaves next spring. As the green fades away, it reveals other colors that were actually there the whole time. The yellows and oranges you see in trees like birches or poplars are always inside the leaf, but the green is so strong all summer that it hides them. It's like taking a green coat off a person who was wearing a yellow shirt underneath.

HostOkay, that makes sense for the yellows. But what about that bright red maple I saw this morning? If the tree is just uncovering what's already there, where does that deep red come from?

GuestThe red is actually a bit of a mystery that scientists are still figuring out. Unlike the yellow, the red isn't hidden. The tree has to spend energy to make that red color right as the leaf is starting to die. For a long time, people thought that was weird. Why buy a new red coat if you're about to throw it in the trash? One of the best ideas we have now is that the red acts like a kind of sunscreen. See, as the tree is trying to pull all that good food out of the leaf, the leaf becomes very fragile. If the sun is too bright on a cold day, it can actually damage the leaf before the tree is finished sucking the nutrients out. The red color acts as a shield to keep the sun from messing up the recycling process.

HostWait, I don't get why a dying leaf needs a shield. If the tree is done with it, why does it matter if the sun beats down on it for a few days?

GuestBecause the plumbing has to stay open for the move. If the sun fries the leaf while it's still full of nitrogen, the tree can't get that food back. It gets stuck out in the leaf and then hits the ground where other plants or bugs might get it. The red shield buys the tree a few more days to finish the job. There's also a chance the red is a warning sign. Some bugs, like aphids, love to lay their eggs on trees in the fall. Some people think the red color tells the bugs that this tree is strong and has a lot of chemical defenses, so they should go lay their eggs somewhere else. It's like a neon sign saying don't mess with me.

HostThat's a lot of effort just to go dormant. It makes me wonder about the trees that don't do this. I have a few pines nearby that stay green all year. If saving food is so important, why aren't they going through this whole recycling dance too?

GuestThose trees are playing a different game. Their needles are built like little tanks. They have a thick, waxy skin that protects them from the cold and keeps water from escaping. Because they're so tough, they can keep working a little bit even in the winter, and they don't have to grow a whole new set of leaves every spring. But for a maple or an oak, those broad, thin leaves would be a disaster in the winter. They would catch too much wind and heavy snow, and the water inside them would freeze and pop the cells like a soda can left in the freezer. So the broad-leaf trees decide it's better to just tear the whole factory down and start over later.

HostWhen the time finally comes, how does the leaf actually fall? It seems like they all drop at once the moment a big wind hits.

GuestIt's a very clean break. Right where the leaf stem meets the branch, the tree grows a special layer of cells that act like a pair of scissors. We call it the cutting layer. It's made of a sort of corky material. As that layer grows, it slowly cuts off the flow of water to the leaf and seals the branch at the same time. It's like the tree is scabbing over before the wound even happens. Once that seal is finished, the leaf is only hanging on by a few tiny threads. The next breeze that comes along snaps those threads, and the leaf drifts down. By then, it's just a dry husk because the tree has already taken everything of value back inside.

GuestEven the brown leaves you see on some oaks have a purpose, as they stay on the tree to protect the new buds from the wind until the very last moment in spring.

HostThat big maple in my yard isn't just showing off then; it's busy locking the doors and bringing the valuables inside before the snow flies.