How a Tree Grows
by Brent Cook


Light , Leaves , and Autumn Colors





Light is something we interact with every day. It surrounds us all the time, allowing us to see and interpret the world. We are very visual creatures, meaning that we learn best by seeing. Think about how much easier it is to understand a movie by watching it rather than by someone telling you about it. We get more information through vision than any other sense; our brains spend a lot of energy just telling us what we're looking at. More important than vision, light is the energy source for the majority of life on earth. We obtain all of our energy from plants which captured their energy from light. Leaves are the most abundant light-capturing components of the biosphere.

Leaves provide us with many valuable services, some of which are often overlooked. First, they are the food production system for trees. Although it seems like common sense, we must remember that without leaves, trees would not exist. Anything that affects leaves will likely affect tree growth and survival. Second, leaves produce oxygen (O 2 ), the essential gas for animal life. Third, leaves take in carbon dioxide (CO 2 ) from the atmosphere. This is important for carbon sequestration which helps to buffer global warming (refer to A Tree From the Air? for more information on this). Fourth, leaves are the primary food source for aquatic life in forested streams. Brook trout in high mountain streams of the Appalachians got their energy from insects which ate leaves that fell into the water. Fifth, leaves can reduce noise pollution. A busy highway lined with full-canopied trees will be much quieter than one without trees. Sixth, leaves give us shade. A tree that is correctly placed near a house can dramatically lower energy costs during the summer. Shade from the leaves replaces the need to run an air conditioner. Last, but definitely not least, leaves beautify the landscape. Many people value aesthetics more than any other benefit provided by leaves. Perhaps the best time to enjoy the visual beauty of leaves is during autumn. Hundreds of thousands of people travel through the Appalachian Mountains in autumn just to behold the spectacular color displays produced by the leaves of deciduous trees. These are only a few of the benefits of leaves-there are countless others.

As mentioned before, autumn is a time when people enjoy viewing yellow, orange, red, and purple leaves that were once completely green. This phenomenon sparks the interest of many people and causes them to wonder why it happens. Popular questions include, "How come leaves only change colors during autumn?" and "How come one side of a tree's leaves will undergo color change before the other?" Since so many ask these questions, let's discuss what actually happens to allow us to see autumn colors.

Before we can understand why we see different colors in leaves, we must first understand why we can see different colors to begin with. Remember that color comes from white light. White light is nothing more than an assemblage of photons traveling at certain wavelengths. A photon is best described as a tiny packet of energy that was emitted by the sun. When these packets of energy travel at wavelengths of 0.7 to 0.4 micrometers, and are mixed together in equal amounts, we get white light. So how big is a micrometer? You can think of it like this. If you were to slice a meter stick into one billion equal pieces and then put one thousand of those tiny slivers back together, you would have a micrometer. So a wavelength of 0.7 micrometers is 700 billionths of a meter. Seven-hundred billionths of a meter can also be expressed as 700 nanometers.

Obviously, photons travel at very short wavelengths to produce white light, but there is one more component to light-speed. Just how fast does light travel? To us it seems instantaneous. As soon as a light switch is flipped, light immediately illuminates the room. Light does have a certain speed, however. In a vacuum, which is basically a chamber free of matter such as air, light travels at 186,282 miles per second. So in a perfect vacuum light could travel around the earth at the equator 7.5 times in one second. All photons of the electromagnetic spectrum travel this fast whether they are radio waves or gamma rays. White light has just the right speed and energy for leaves to harness some of its energy.


If a plane could travel around the earth 7.5 times in one second then it would be traveling at about the speed of light


When wavelengths of white light are separated, we see color. We've all witnessed the perfect example of this-a rainbow. When we see a rainbow we are seeing white light that was bent, or refracted, by water droplets in the sky. As light passed through the droplets, the photons got refracted and separated according to the wavelength at which they were traveling.

Rainbows show that color comes from light.


So how can objects be only one color if white light is the combination of all colors? This question deals with reflection and absorption. If something is only one color it is reflecting photons of a certain wavelength, which is the color you see. It is therefore absorbing all other colors of white light. For instance, a red shirt would reflect wavelengths of red and absorb wavelengths of blue and green. What about black and white objects? If something is completely white then it is reflecting all photons of white light. If something is completely black, then it is absorbing all photons of white light. Recall that photons are small pieces of energy which explains why you would want to wear a white shirt on a hot day instead of a black one. A black shirt will be hotter since it absorbs more energy.




Leaves exhibit color for the same general reason as any other object. They reflect and absorb certain wavelengths of visible light. There are three main pigments that allow leaves to do this. The pigments are chlorophyll, carotenoids , and anthocyanins . These pigments are primarily just different arrangements of three types of atoms-carbon, hydrogen, and oxygen-which have bonded in different ways to form molecules. Other atoms such as nitrogen and magnesium may also be present. The atoms of these molecules are arranged in a manner that allows them to reflect and absorb light differently. If you've ever been in the forest and scraped away the top layer of leaves on the forest floor you may have noticed that the partially decomposed leaves appear black.

As leaves decompose they become darker

This is because the decomposition process has broken large molecules down into smaller ones of varying shapes and sizes. The presence of so many different molecular structures, in relatively equal amounts, causes all light to be absorbed. The same phenomenon is occurring when leaves are still alive and colorful, except that now, there will be a dominant molecule. Therefore the leaf will reflect the corresponding color of the molecule, or pigment, found the most throughout the leaf.

Chlorophyll, as you probably know, is the pigment that makes leaves green. It is one of the most important molecules to the majority of life on Earth because it traps energy needed to make food. Without it we would not exist. Since it is green, chlorophyll traps red and blue light and reflects green light. This means a lot more than just what color we will see. It implies that most of the energy within our bodies came from photons traveling at red and blue wavelengths. The energy from these photons was trapped by chlorophyll and then used to make food for plants which became food for us directly or indirectly. Now it's easy to see how much we depend on light and leaves.

Chlorophyll is the most dominant pigment in this sugar maple leaf


We said before that we will see the color of the most abundant pigment in a leaf, so if a leaf is green, it obviously has more chlorophyll than any other pigment.

Carotenoids are the pigments that make leaves yellow, orange, and sometimes lighter shades of red. Hence, they absorb blue and violet wavelengths. Carotenoids are often called accessory pigments because they aid in energy absorption. These pigments are not near as numerous as chlorophyll during spring and summer. That's why leaves still appear green, even though they contain yellow pigments. The abundance of chlorophyll simply masks the yellow reflectance.

Carotene is the most abundant pigment in this sugar maple leaf


Anthocyanins are the pigments that make leaves red and purple. They therefore absorb blue and green light. These pigments are not always present in leaves, however. They are formed in the central vacuoles of leaf cells during autumn due to changes in how sugars are broken down. Anthocyanins are also only produced in certain trees. You may have noticed that some trees always turn red while others always turn yellow. It is not known whether or not anthocyanin production provides any direct benefit to trees.

Anthocyanin is the most abundant pigment in this flowering dogwood leaf


Autumn Colors


We used to think that leaves changing color in autumn was triggered by cooler temperatures. This was a good guess, but it's actually caused by the days getting shorter. More specifically, it's the amount of daylight. This is referred to as a photoperiod, which is the amount of sunlight in one day. A chemical called phytochrome "measures" the photoperiod. When day length reaches a certain point, phytochrome "tells" the tree to start preparing for winter. You could think of this as someone standing outside with a stop watch keeping track of how long each day was. Wavelengths of red light activate phytochrome and make this process possible-just another example of how light controls many life processes.

Deciduous trees perform one obvious task to prepare for winter. They drop their leaves. This is known as abscission. Keep in mind that red light activated phytochrome which, in turn, activated abscission. It all ties back to light. One of the first stages of abscission involves growing a layer of corky cells where the petiole of the leaf is attached to the tree. These cells will clog the transport tunnels to the leaf so no water or nutrients can get in, and no sugars can get out.

As the abscission process develops, chlorophyll begins to breakdown and disappear. It will breakdown to where its molecular structure no longer reflects any portion of white light. Chlorophyll is constantly being broken down by sunlight; it can only absorb so much energy before its bonds break apart. During the spring and summer leaves are continually building new chlorophyll molecules, but rebuilding becomes impossible in autumn due to the abscission layer of corky cells. No water or nutrients can get into the leaf to make new chlorophyll.

Since all leaves contain carotenoids , we will see shades of yellow and orange once the chlorophyll has broken down. Leaves are always reflecting yellow light, we just can't see it until the reflected green light from the chlorophyll is gone.

Tulip poplar leaves before and after the chlorophyll broke down


(Left) A green leaf was dissolved in rubbing alcohol. (Right) The dissolved pigments were pulled up through the filter paper. The yellow bands are the carotenoids which proved they were in the leaf even though it was completely green.


Abscission also causes anthocyanins to form in some trees. Just before a tree produces the corky cells, many nutrients and sugars in the leaf will move from the leaf to the tree. This is known as translocation. The sugars remaining in the leaf will break down in a different fashion since the nutrients, especially phosphorus, went into the tree. Anthocyanin pigments result and the leaf will reflect red or purple light.

Flowering dogwood leaves before and after color change.
Once the chlorophyll broke down anthocyanins were produced

(Left) A red leaf was dissolved in rubbing alcohol. (Right) The small yellow band on the filter paper is showing the carotenoids that were in the leaf. Since there was much more anothocyanin , the leaf still appeared red


Some leaves may turn yellow, orange, and red at the same time. This may be because nutrients and water got blocked from entering some parts of the leaf but not others. Another reason may be that certain parts of the leaf were receiving more sunlight. If you've ever seen a tree that has green leaves on one side and red, orange, or yellow leaves on the other, it was probably a result of different amounts of sunlight. In the northern hemisphere, leaves that are on the southwest side of a tree will receive much more sunlight than leaves on the opposite side. Leaves near the top of a tree will also receive more sunlight than leaves at the bottom of the canopy. Consequently, phytochrome will trigger abscission sooner in leaves getting more sunlight.

This striped maple leaf has turned yellow on one side due to differences in sunlight.
The side that is yellow was hanging out in the sunlight while the other side was in the shade


These sourwood leaves were pulled from the same tree on the same day. The leaf on the far left was at the bottom of the tree in deep shade while the leaf on the far right was at the top of the tree in full sunlight. You can easily see how leaves will receive varying amounts of light depending on their location


Here you can see that the Virginia creeper leaves in the shade are still green while the ones in the sunlight
have turned to a dark red



  1. What benefits do leaves provide?
  2. What 3 pigments allow the reflection and absorption of certain wavelengths of visible light?
  3. Why is chlorophyll so important for life?
  4. What causes one portion of a leaf to be yellow while the other is still green?



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