Why the Trees Turn Brilliant Red, Orange & Yellow in Autumn
Have you ever wondered why the trees go from green all summer to the bright and gorgeous reds, oranges and yellows that we see in the fall? Maybe your 5-year-old asked you why the trees change color and you were stumped.
It’s actually a fascinating, and somewhat complicated, physiological process that the trees go through to prepare themselves for winter, but I’ll break it down and give you the info you need to wow the guests at your next cocktail party (or your 5-year-old)!
What Environmental Cues Trigger Fall Leaf Color?
Both changing day length and falling temperatures begin the process that starts with brilliant color and ends with the leaves falling from the trees. However, day length is more responsible for what happens than temperatures are.
You might have already guessed this if you live somewhere the temps don’t drop a lot in the fall, yet your trees still change each year. Even in colder climates such as here in CT, we often have very warm autumns, yet our leaves change reliably each and every year. There is a bit of variation in timing and brightness of the display, which I’ll get to in a minute, but they always change because day length variations are consistent each and every year!
The variations in timing and brilliance of the fall display are often related to how much rain fell during the months prior. In summers with very little rain (like we had this year), the leaves often fall earlier, sometimes even before they have a chance to change color, due to stress on the trees.
The recipe for the very best fall foliage? Let’s put it this way: The trees love the same kind of weather you probably do. That means a warm summer with moderate rainfall, followed by a sunny, warm autumn with cool nights and a late first frost.
Why Do these Cues Trigger the Process?
Leaves are genetically programmed right from the beginning to eventually senesce, or die. They contain genes that are activated by the environmental cues we just talked about that cause them to complete this process.
However, this death isn’t a willy nilly type of thing. Rather, it’s an organized process that allows the tree to recover many of the nutrients normally stored in the leaves, including sugars, amino acids (proteins) and even certain minerals.
These nutrients are transported from the leaves back into the base of the tree via the tree’s phloem. This is akin to our system of veins and arteries. Phloem can transport nutrients both from the roots to the leaves and the opposite direction as well, based on time of year and the tree’s needs.
As these nutrients are returned to the tree, they help fortify it for the difficult winter to come, when it can’t get nutrients from the photosynthesis carried on in its leaves. If it weren’t for this orderly process, deciduous trees wouldn’t survive harsh winters.
I just have to interject a fun little fact here.
If you’ve ever seen a gingko in the fall, they’re spectacular! Their leaves are a bright, clear yellow that’s consistent throughout the whole tree, and so bright it’s almost fluorescent.
BUT, ginkgos have one really weird quirk. The second a frost hits, the leaves fall. Not over the course of several days. The leaves ALL FALL within less than 24 hours!
I used to drive past a gingko on my way to school every day. I would notice it because of its gorgeous color, but I always knew that the morning after the first frost, it would be bare. Totally bare, with a brilliant yellow carpet underneath it. Crazy!
So Why, exactly, do the Leaves Change Color?
Oranges & Yellows
Ok, this is the cool part (at least to a plant nerd like me!). Those oranges and yellows you see in the fall?
THEY WERE THERE IN THE LEAVES ALL SUMMER!
The first time I heard that, it just blew me away! But, like I said, I’m a total plant nerd so you might not be as impressed.
What happens is that, during the summer, the leaves are producing so much chlorophyll through photosynthesis that all we can see is the green of the chlorophyll.
As the temperatures and amount of sunlight go down in the fall, photosynthesis, and thus chlorophyll production, drop sharply, and the chlorophyll in the leaves begins to break down, allowing the yellows and oranges to shine through.
The yellows and oranges you see are carotenoids and flavonoids. You might recognize these terms (or maybe not!). One carotenoid you’ve probably heard of is beta-carotene. It’s what makes carrots orange.
Another carotenoid you may have heard of (it’s technically a xanthophyll, a subclass of carotenoids) is lutein, which makes egg yolks yellow. Cool, huh?
These nutrients also break down like the chlorophyll does, but they break down more slowly, so we get a lovely display before they disappear.
Reds & Purples
The reds and purples you see on some trees (maples are a prime example) work a little differently.
The red and purple colors are caused by anthocyanins, the same thing that makes blueberries blue.
Anthocyanin production starts with a greater concentration of sugar in the leaves during the fall and is triggered by sunlight. Scientists don’t know exactly why anthocyanins are produced, by hypothesize that it might be a protective mechanism, protecting the now chlorophyll-deficient leaves from light and allowing them to remain on the trees longer.
Have you ever noticed that the red and purple leaves look muted or dulled after a rainstorm? You’re not crazy! Anthocyanins are water-soluble, and the rain leaches some of them from the leaves, muting their colors.
If you’ve ever grown purple “green” beans and then boiled them for eating, you’ve seen this process in action. Once they’re done boiling, they’re (disappointingly, at least to me) plain old green. The anthocyanins have leached out of them during the boiling process.
You might not really look at brown as a fall leaf color, but all leaves do eventually turn brown. This is caused by the tannins in the leaves, and is the last pigment to remain, which is why we see brown leaves all winter.
I hope I’ve made this fascinating process interesting and simple for you, and that you’ve enjoyed reading! If you’d like to save it for later (or to refresh your memory when your 5-year-old asks you yet again what makes the leaves turn color), please pin the images below to your Plants or Autumn boards on Pinterest.
As always, thanks for reading and have a crazy organic day!