For nature lovers, autumn is the most sought-after season of the year. Many people travel to deciduous forests and go on hiking trips just to see the breathtaking beauty the fall foliage can create, choosing this time to celebrate special occasions or get outdoors more before the cold weather sets in.

As the Earth prepares for winter, the days get shorter and a chemical change happens in deciduous leaves that changes their color to vibrant reds, oranges, and yellows.

This process takes place across the globe, but climates that receive a healthy mix of all seasons are the most likely to have “peak fall foliage.” Areas with mild winters are less likely to have colors as vibrant as leaves need less preparation for the cold weather.

So, why do leaves change color in the fall for these peak foliage areas of the world?

Whether you are learning about this process as an adult, or if you are teaching your kids about the process, HerbSpeak is here to guide you through the process these leaves undergo to produce such beautiful colors.

Why Are Leaves Green?

For some, the reason why leaves are green may seem elementary, but it is an important step in understanding why the leaves change color in the first place.

In short, the answer is that leaves are green because they store a chemical called chlorophyll. This chemical is an important substance that allows the plant to capture sunlight and use it to synthesize its own energy and “food” through a process called photosynthesis.

Leaves are green because this chlorophyll substance is absorbing the red and blue light wavelengths produced by sunlight. Sunlight produces light wavelengths in the colors of the rainbow and some colors that are invisible to our eyes. The plant absorbs the wavelengths that can help it produce energy and rejects unused light, such as the green wavelength. Our eyes see that rejected color as it bounces off the leaf.

This works for all objects, not just plants: if you were to look at a yellow shirt, the shirt is absorbing all wavelengths of light except the yellow wavelength. That yellow wavelength is rejected from the fibers of the shirt, and that light bounces off the object and we perceive it as yellow.

Plants and objects that reflect all wavelengths of light are white, and plants and objects that absorb all wavelengths of light are black. Plants with white accents or full colorations do appear in nature, however, there are no plants that are pure black.

What Does Chlorophyll Do?

As humans, we cannot generate our own source of food internally, so we must eat to supply our bodies with the energy we need for our cells to divide and our bodies to grow. Plants can manufacture their own energy for cell division and growth.

This makes plants a part of the building blocks of all life because they provide a lot of the calories, sugars, and other nutrients necessary to sustain most mammals and insects. It helps that oxygen is a byproduct of the photosynthesis process, allowing us to breathe.

Chlorophyll is also a strong pigment that produces a green color. If the leaf has a green coloration, then you know it is working hard to produce energy for the plant to continue growing.

Leaves with beige, tan, or white colorations are unable to photosynthesize, which means they are unable to process sunlight into energy in those areas of the leaf. If the leaf has green ribbing or spots, then chlorophyll is present in that part of the leaf.

Sometimes, Leaves Aren’t Green:

Even outside of autumn when leaves are known to change color, there are plenty of plants that have purple and red leaves year-round. These leaves still contain the green-pigmented chlorophyll underneath, but there are other chemicals in the leaves that produce a red or purple pigment in amounts that supersede the chlorophyll.

In other cases, there are plants that do not produce chlorophyll at all, rendering them unable to go through the process of photosynthesis. This means that they are unable to produce their own energy and must rely on other plants to help them. Most commonly, these plants belong to a species of parasitic plants which attach themselves to a host plant and steal nutrients from the host plant’s production. Some of these parasitic plants provide benefits to the host plant in return for the exchange of nutrients, while other plants simply use the nutrients they can until they must find another host plant to attach themselves to.

As we discussed above, there are no plants that are pure black. Some plants appear black to our eyes in most lighting, but these plants are exceedingly rare in nature. These plants are really a deep purple or deep blue in pigment, not black.

In recent years, horticulturalists have bred plants to appear black or deep purple by introducing a stronger concentration of a pigment called anthocyanin in select areas of the plant. This pigment is prevalent in many berries, as well as autumn leaves, producing a strong scarlet to deep blue pigment depending on the concentration.

We learned that leaves with green colorations are filled with chlorophyll. In Greek, this name comes from Khlōros (green) and phullon (leaf). This coloration is present in almost all plants that photosynthesize.

Leaves with yellow colorations contain a more prevalent concentration of xanthophylls. This name comes from the Greek xanthos (yellow) and phullon (leaf). This coloration is also present in fruits such as banana skins, in vegetables such as squash, autumn leaves, and in flowers such as sunflowers.

Leaves with orange colorations contain a more prevalent concentration of carotenoids. The pigment’s name comes from a German word meaning “carotene-like pigment found in living things”. Carotene is German derived from Carota in Latin, which is the Latin name of carrots. This coloration is present in photosynthetic bacteria, in vegetables such as carrots, flowers such as the orange daylily, autumn leaves, and many other plants.

Leaves with red or seemingly-black colorations contain a more prevalent concentration of anthocyanins. The name for this pigment comes from the Greek words anthos (flower) and kyaneos (dark blue). This coloration is also present in fruits such as pomegranate seeds, vegetables such as purple cauliflower, autumn leaves, and flowers such as borage.

Why Do Leaves Change Color in the Fall?

It’s no secret that the colorful change of leaves is an iconic time of the year where the weather begins to get colder, days get shorter, and fall flavors are abundant. This is a time to celebrate what harvests the year has brought before the cold winter nights begin in earnest.

Just as we can sense this change in season, trees and other plants can sense it just as well as we can. There are several mechanisms that plants are equipped with that allow them to sense what time of year it is better than humans could prior to the advent of calendars. [1]

Leaves are green in the spring and summer because they are producing a lot of chlorophyll. Where there is sunlight, there is chlorophyll production.

Plants are extremely sensitive to changes in their environment, as they must adapt where they are, rather than migrate like animals and humans tend to. The sense of any change in day and night length is called photoperiodism and is controlled by light receptor chemicals in the leaves called phytochrome and cryptochrome. [2]

As plants sense these changes in sunlight, they slowly stop producing as much chlorophyll, and the pigment breaks down inside the leaf. This allows other pigments, such as anthocyanins, carotenoids, and xanthophylls to begin showing, producing vibrant colors of reds, oranges, and yellows.

When the nights get colder and the days get shorter, chlorophyll begins breaking down because the plants are taking the water and nutrients from the leaves and storing it in their trunks or stems, packing as much energy away as possible to help them regrow leaves when spring comes.

Before the introduction of synthetic preservatives and agricultural developments that allow farmers to grow year-round, humans had to do the same by taking the harvests from the year before and canning vegetables, making jellies, and drying herbs and spices to make it through the winter.  

Does Cold Weather Make Leaves Change Color?

If you live in a climate with peak foliage seasons, then you might have noticed a correlation between the weather and how long you can expect those bright colors to last. The weather is a difficult balancing act of survival for plants, as the plants must decide whether to shed their leaves early or to gradually fall and display their vibrant colors as long as possible.

If a region experiences warm, sunny days and cool nights, then you can expect the fall foliage to be phenomenal that year. Consistently bright sunlight allows the plants to continue capturing energy and turning it into sugars as it prepares to sever the leaves for winter. The crisp nights close the veins in the leaves, locking in these sugars and preventing it from moving into the trunk until the weather warms up again. These sugars lead to a higher production of anthocyanins and other color-producing pigments, which show through stronger as the chlorophyll breaks down.

Early frost, extreme drought, and strong winds or rains can trigger faster leaf fall, however, as the plant goes into survival mode to conserve energy and prepare faster for winter. Plants take these environmental cues to mean that winter is beginning early, and it is time to batten down the hatches.

Can Leaves Change Color Overnight?

In mild weather, leaves gradually change colors over a month or two, peaking in mid-autumn and fading out as the leaves fall in preparation for frost and snow.

Leaves can change color overnight if there is an early frost or a drought which speeds up the plant’s clock for leaf fall. The breakdown in chlorophyll pigments can also be affected by temperature, elevation, and latitude, meaning some areas are more prone to overnight changes in coloration than others. Likewise, some areas will have a longer period of fall foliage color than others.

Why Do Leaves Fall?

Leaves contain a layer of cells close to the stem called an abscission layer, essentially meaning “a layer to cut.” This layer remains dormant until it is triggered in fall.

What triggers leaf fall?

A hormone called ethylene is largely responsible for leaf abscission. Ethylene production is triggered by the slight changes in temperature. [3] As mentioned before, plants have the ability to sense the length of the day and night through a sense called photoperiodism.

When the day begins to grow shorter, the overall production of a phytohormone called Auxin lowers. Auxin regulates plant growth, so this means the plant stops growing as the days gets shorter, and this lowers the plant’s tolerance to Ethylene, as well as triggers its production.

Ethylene is responsible not only for degradation of chlorophyll [4] but also triggers the growth of the abscission layer.

When abscission cells are triggered, they begin to push into the leaf cells, cutting off the vascular cells which transport nutrients – while the plant is taking back what nutrients it can into the trunk or stem – and over a few weeks, these cells grow and push the leaf away from the stem. The leaves are left hanging by a thin cellular connection, ready to be removed by the slightest breeze.

Image: abscission layer in leaf stems, cells shown in dark purple.

Plants cut their leaves before winter to prevent them from photosynthesizing in the middle of winter if there is a warm day before another freeze. If a leaf freezes without protection from the winter temperatures, the leaf will die, taking some vital nutrients with it.

This might not be a big deal if one or two leaves make that mistake, but let’s look at a tree as an example: if the full tree were to keep its leaves without any winter protection, would have no energy reserves to make more leaves when spring comes. Without leaves, it cannot make energy through photosynthesis, therefore, it cannot live.

Plants cutting their leaves off is an essential part of winter survival, and the dead leaves help house wildlife and other creatures that use it for nesting, warmth, or nutrition. What leaves aren’t used by the local wildlife decay, where the roots suck up the remaining nutrients, recycling the dead matter.

Environmental factors can also play a major factor in how quickly leaves fall. If the plant experiences drought or early frost, this abscission may be amplified as the plant hastens its preparations for winter.

Do Evergreen Leaves Fall?

Evergreens, which consists of most conifers such as pines, firs, hemlocks, cedars, and spruce trees, are termed for their persistent green color despite the changing foliage. Many evergreens native to northern climates have modified leaves called needles, rather than the typical flat, broad leaves. Even in snow-covered winter landscape, these needles do not fall completely, staying on for two to four years at a time.

While there are plenty of evergreen plants, for the sake of simplicity in terminology, we are going to focus on evergreen trees that have evolved to withstand snowy winters.

You may think that evergreen leaves do not fall because they continue to go through the process of photosynthesis throughout winter.

That is a reasonable hypothesis, but studies have shown that most evergreens have a lower metabolic process over the winter, much like a hibernating bear, so they could not possibly benefit from the same rate of photosynthesis. [5]

The secret is in the energy cost of creating leaves. Needles take more energy to create than deciduous leaves. The tree must gain more energy from the leaves than it spends in creating it, and it cannot do this over a single growing season.

To compensate for this, the needles stay on the evergreen tree for two to four years so the leaves can photosynthesize throughout multiple growing seasons.

With overwintering comes a lot of environmental stress for trees, however. Evergreens must compensate for freezing temperatures as well as strong, biting winter wind.

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In most plants, if the cells inside the leaves freeze, it could be lethal, or at least detrimental to the tree’s energy stores, putting them at a disadvantage for the remainder of winter.

Deciduous trees prevent this by severing the leaves in fall before freezing weather begins. Evergreens, on the other hand, produce a cryoprotective substance that acts like a natural antifreeze. [6] This makes some needles waxy to the touch.

Likewise, the longer the leaves need to withstand harsh winter winds, the more rigid and thick the leaf’s cells need to be. A dense, tough needle will last through many more winters than a brittle needle will.

Evergreens do lose their leaves as the needles get older, but they do not lose all their leaves every season like deciduous trees. Evergreens need to keep these leaves as long as possible to continue producing energy, but eventually these needles get old and stop photosynthesizing as efficiently. These needles will fall, making way for more modified leaves to continue the hard work of photosynthesis.  

Do Evergreens Change Color?

Which brings us to the question about color: do evergreens change color? If the act of changing color is just the tree undergoing different concentrations of certain pigments, where all pigments can photosynthesize to a degree, what is the benefit of staying green?

Almost all evergreens do go through a color change, however; in some evergreens, this change is barely noticeable, and unless you really look, they seem to retain a green color until they fall and turn brown on the ground. These needles are typically closer to the trunk, so less visible to us, and any loss in the green color is so individual, and so hidden behind younger needles, that it is not often noticeable when the rest of the tree is green.

Some species of evergreens do go through a full change in color at certain times of year like deciduous trees. These colorful evergreens are a breathtaking sight to behold. In the right climate at the right time of year, full trees can take on vibrant golden or red hues. Most evergreens that change color to such a noticeable degree are shrubs or vines, however, not trees. These colorful evergreens are not limited to autumn – some will display stunning hues of red, purple, or gold in spring and summer as well.

References
  1. Plant adaption to cold climates, Christian Körner, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5130066/
  2. Photoperiodic Flowering: Time Measurement Mechanisms in Leaves, Young Hun Song, Jae Sung Shim, Hannah A. Kinmonth-Schultz, Takato Imaizumi, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4414745/
  3. Ethylene Role in Plant Growth, Development and Senescence: Interaction with Other Phytohormones, Noushina Iqbal, Nafees A. Khan, Antonio Ferrante, Alice Trivellini, Alessandra Francini, M. I. R. Khan, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5378820/
  4. Involvement of an ethylene response factor in chlorophyll degradation during citrus fruit degreening, Xue-ren Yin, Xiu-lan Xie, Xiao-jian Xia, Jing-quan Yu, Ian B. Ferguson, James J. Giovannoni, Kun-song Chen, https://onlinelibrary.wiley.com/doi/full/10.1111/tpj.13178
  5. Overwintering evergreen oaks reverse typical relationships between leaf traits in a species spectrum, Hisanori Harayama, Atsushi Ishida, Jin Yoshimura, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4968473/
  6. Cryoprotectants: the essential antifreezes to protect life in the frozen state, Barry J Fuller, https://pubmed.ncbi.nlm.nih.gov/15660165/