Why All the Colors of Fall?

Oh no, they're dying! They're all slowly dying! Fresh from the lush and tropical Philippines, I was alarmed that at the end of September and through October, in my first year in Canada, the leaves of the plants and some trees in our garden and neighborhood were starting to turn yellow or yellowish brown. Some were starting to turn red and orangey. Then the leaves started to fall off. Soon most plants and trees in our garden and the neighbors' were reduced to leafless branches and limbs and twigs. They're slowly dying, I feared. What's wrong?

As Fall marches in, the leaves of the trees turn on their fiery reds and yellows and Halloweeny orange

As I have more years in my new residence in this cold part of the world, I slowly realized and learned the whys and hows of all these leafless trees and plants and bushes. This short article talks about these whys and hows.

Fall is indeed known for reds, oranges, and yellows. The leaves of trees, shrubs, and most other plants trade their greenness for those vibrant colors. Just look around you and you’ll see what I mean. But why do leaves change colors as they beautifully do in Fall?

The colors of Fall at our nearby Rocky Point marina park: oranges, yellows, and reds

It’s all about adaptations and survival, as it turns out.

Trees and shrubs change leaf colors as they prepare to become dormant. Plants sense signals to get ready for less friendly plant-world weather that’s coming.

You know: shorter days and longer nights. Colder daytime temperature, and frigid night temperature. Cold rain and blustery wind. If you’re a plant you won’t like these either.

Plants temporarily shut down many of their functions to protect themselves from altogether running out of energy and nutrients when a long winter might plunge the plant world into a frigid wasteland. Otherwise, they might not bounce back in Spring. The color change is simply part of their road to dormancy and a way to survival.

Plants throughout the world undergo dormancy to get through bad times – going dormant is an effective tool for survival.

Early Fall still gives us delight in our garden, courtesy of the fall foliage of the reds and oranges of the maples and the golds of the hosta

It’s like when we people run low on food and other basic supplies for whatever bad reason, and we fear things are not going to get better soon.

Think: powerful and disastrous typhoons or hurricanes, widespread fire, long droughts, pestilence, Covid-19 pandemic, or worse, war.

What do we do in times like those? We stock up on meat, eggs, canned goods, and other food supplies and drinking water and candles. We hoard toilet paper. We slow down, and even try to rest and sleep more to conserve what little resources we have left. If need be, we remain indoors and some even hide in bunkers. Other members of the Animal Kingdom, like bears, squirrels and chipmunks, snakes and many other animals big and small, hibernate. And it’s all about adaptations and ultimately survival.

Plants do the same. They’re even in a worse situation than humans, because plants can’t move, can’t drive, can’t gather food, and can't get water whenever they want to. They simply rely on Nature and on what they already have, not on what they could have.

With less chlorophyll l (green pigment) due to less sunshine and lower temperature, leaves turn yellow as xanthophyll (yellow pigment) becomes the dominant pigment. Over time, leaves fall off to reduce water evaporation.

As daylight shortens and temperatures plummet, plants reduce making chlorophyll. Chlorophyll is the green pigment or chemical that powers photosynthesis. Photosynthesis provides plants with their energy (sugar). But they need sunlight to do that. Without much sunlight, photosynthesis drains more energy than it creates. So plants reduce, then even stop production of chlorophyll in Fall and into Winter.

As the green chlorophyll drains away from each leaf, other pigments in the leaves start to show their colors: reds, oranges, and yellows.

In response to the shorter days and less intense sunlight, leaves begin the processes leading up to their falling off right off by early Fall. The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf. Much like humans creating a scab when we get a cut. These clogged veins of leaves trap sugars in the leaf and promote the production of anthocyanin, a red pigment in leaves. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall off. And most shed off their leaves in abandon

When chlorophyll (green pigment) is produced less in leaves at the end of summer, xanthophyll (yellow pigment), carotenoid (orange pigment), and anthocyanin (red pigment) start to show up more prominently. Thus leaves turn yellow, orange, or red at the start of Fall.

Warm, sunny days and cool, crisp but not freezing nights at the start of Fall seem to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual clogging or closing of veins going into the leaf prevent these sugars from moving out.

These conditions – lots of sugar and light – set the motion for making brilliant anthocyanin pigments. Anthocyanins tint reds, purples, and crimson.

The yellow pigments, the carotenoids, on the other hand, are always present in leaves. So the yellow and gold colors remain fairly constant from year to year. You see this when leaves start to die out and turn green to yellow before the leaf dies, even in Summer. Even in the lush and tropical Philippines, plants' leaves turn yellow, then brown, then blacken when they're dying off.

Fallen leaves feed the cycle of life of trees as the leaves later decompose and become part of the plants’ nutrients the following growing season. For science-inclined gardeners, the colors of the fallen leaves indicate the timing when the leaves had fallen off.

Leaf abscission (the fancy technical term for the dropping of leaves) begins with the degradation of chlorophyll. As the green fades, yellow and orange pigments known as carotenoids are revealed in the leaves of many species. After showing off their glorious Fall colors, the leaves fall off. Fallen leaves eventually decay and become part of the plants’ nutrients the following growing season. The Cycle of Life in Nature. Nature supports itself.

More importantly, the shedding of their leaves conserves whatever little water is stored by the trees and plants. Plants lose water mostly through their leaves. No leaves, minimal loss of precious water. Smart move.

The weather all through the year can affect the Fall color season, too. Temperature affects the rate of chemical reactions in leaves. Sunny autumn days are needed for the brightest color displays since anthocyanins require light. Overcast days will lead to more yellows and browns.

This year, for example, a warm spring in Vancouver, BC was followed by a hot and rainy July but cooler than normal August. Some trees reacted to the variable stress of prolonged heat by starting dormancy early, dropping their leaves over the summer. Yet August was warmer than usual so most trees in our area changed their colors a bit late.

Yellow-green (combined carotenoids pigment) creates artful foliar displays

In general, the Fall foliage color show is most grand when late September and early October bring adequate rain, sunny days, and cool nights. A hot or dry September can make the Fall colors early but muted.

The red pigment, anthocyanin, that typifies fall color is produced only by some tree and shrub species. The amount they produce varies by species and depends both on that year’s weather and the circumstances in which the tree is growing. The more sun a tree gets, the more anthocyanin it makes, giving a fiery red and brilliant reddish-orange display.

Red and orange leaves indicate more production of anthocyanin, good sun exposure, and longer warm temperatures

I’m sure you are keenly aware that at the start of Fall, leaves transform from lush green into fiery reds and resplendent yellows and shades of oranges, offering us a kaleidoscope of colorful masterpieces. It’s as if the plant world is staging an impassioned, dramatic performance or creating masterful strokes of painterly colors. Their sort of Swan Lake performance, their last hurrah.

Unfortunately, a big Fall storm can blow most leaves down and end the show early. So, this Fall, we'll wait and see. At any rate, we shall still be grateful that hurricanes like Fiona and Ian won't blow us and things away. There's always something to be thankful for, regardless of how Nature behaves and blesses and delights us, or sends us down on our knees.