All leaves contain various types of chemicals; one of these chemicals, chlorophyll, is responsible for absorbing sunlight and giving leaves their green color. The leaves of all trees contain chlorophyll, a green pigment that has the unusual ability to capture light energy and (with the help of other leaf components) convert that energy into a chemical form such as sugar.
Leaves change color because their chlorophyll pigments stop being produced. Plants normally produce a series of pigments, and chlorophyll is the dominant one. However, they stop producing it during the cold months, and other pigments predominate.
Many leaves also contain other pigments, and although these pigments cannot photosynthesize as chlorophyll does, some of them are able to transfer the light energy they capture to chlorophyll. Different trees have different proportions of these pigments; the amount of chlorophyll remaining and the proportions of other pigments determine the color of the leaves.
How Pigments Combine to Produce Red Colors
The combination of anthocyanins and chlorophyll creates the brown color, while anthocyanins and carotenoids create the orange color of the leaves. Some trees produce anthocyanins in autumn, and these pigments cause yellowed leaves to turn orange and then red. This vibrant color is only due to the fact that the leaves of some plants, including the maple, actually produce new pigments called anthocyanins.
In addition to the green pigments of chlorophyll, yellow and orange pigments are always present in the leaves of plants. While chlorophyll only absorbs blue and orange light, other pigments in leaves absorb other colors.
The most common pigment is chlorophyll, which we see as the green color of summer leaves. The green color we see on the leaves of most plants in spring and summer is caused by a pigment called chlorophyll. Chlorophyll absorbs red and blue light from sunlight hitting the leaves. Chlorophyll absorbs both blue and red wavelengths of sunlight and reflects green wavelengths.
An Overview of Photosynthesis
Sunlight is absorbed by a chemical in leaves called chlorophyll. In summer, when the plant is green, its leaves contain the chlorophyll pigment, which absorbs sunlight of all colors except green. In summer, leaves are the plants that produce sugar from carbon dioxide and water by exposing chlorophyll to light. During the growing period, chlorophyll is continuously produced and decomposed, and the leaves turn green.
In summer, chlorophyll is constantly broken down and restored in the leaves of trees. But as the day length shortens and temperatures begin to drop, the amount of chlorophyll in the leaves begins to decrease, and other pigments in the leaves of many plants become more visible. In autumn, plants decompose and reabsorb chlorophyll, revealing the colors of other pigments. The color change usually occurs before the leaves fall from the tree.
If plants break down chlorophyll and remove it from the leaves before the leaves fall, the plants save energy. To maintain the amount of chlorophyll in their leaves, plants must constantly synthesize it, but this requires sunlight and warm temperatures. Since the plant needs sunlight and warmth to produce chlorophyll, chlorophyll production starts to decline as the colder months begin, so the leaves change color in autumn.
How Pigments Change During Temperature Falls
When temperatures drop, some plants stop producing chlorophyll, a green pigment found in nearly all plants. Abundant sunlight leads to abundant chlorophyll, making it the main pigment.
An article written by the Pennsylvania Department of Conservation and Natural Resources explains that as fall arrives, chlorophyll loses its stimulant and hibernates as sunlight dwindles and temperatures drop. Since the leaves can no longer produce food for the tree, they break down chlorophyll and produce orange and yellow pigments called carotene and lutein.
Carotenoid pigments are also lost from plastids during aging, but some remain in plastids after chlorophyll removal; this gives autumn leaves their yellow color. When the chlorophyll is destroyed, the green color of the leaves fades, leaving the carotenoid yellow color. The yellows and oranges of the leaves are always there, but in the spring and summer they are often masked by green chlorophyll, Vito said.
As chlorophyll production declines, yellows and oranges become more prominent. Previously green leaves, depending on the species, can acquire bright shades of yellow, orange and red, as well as brown. Thus, different leaf colors are caused by changes in pigments. When the weather changes, some plants destroy all the green pigment.
The leaves change color in autumn because the amount of pigment changes as the leaves prepare to fall from the trees. Summer sunlight causes the leaves to keep producing more chlorophyll. When chlorophyll is destroyed, the leaves of the plant become vulnerable to harsh sunlight. Leaves may turn red in autumn to protect themselves from the sun’s ultraviolet rays.
However, another interesting theory is that the red pigment acts as a kind of sunscreen for the leaves, protecting them from autumn sunlight and allowing the leaves to stay on the tree longer than if they were green or even yellow. This brightness is especially characteristic of red leaves, such as sugar maple and red maple leaves.
Dry Conditions in Summer Trigger Leaf Falloff
Dry conditions in late summer and early fall can also trigger early “closing” of trees in preparation for winter, causing early leaves to fall off trees before they reach their full potential. Of course, if low temperatures and severe frosts hit, they can kill the processes inside the leaves, resulting in poor fall color and early leaves.
For example, early drought or frost can cause leaves to fall from trees before they have had time to change color. The slightest change in September weather—too hot, too cold, too wet, too dry—can slow down color change or cause tree leaves to fall before they change color. Autumn is often a combination of bright sunshine and cooler air, and it is under these conditions that leaves can take on amber, red, and purple hues.
The number and brightness of flowers that develop in a given fall season are related to the weather conditions that occur before and during the decline in leaf chlorophyll. When the leaves start to change color is more dependent on light than temperature, so the leaves begin to change color around the same time each year.
The combination of low light, lack of nutrients and less water causes the trees to begin the process of reducing their chlorophyll content and the green color to fade. As the days shorten and temperatures drop, chlorophyll decomposes and turns from green to colorless, causing orange and yellow colors to appear. With less chlorophyll giving the leaf its bright green color, we begin to see other colors like orange, yellow and red that have always been there.
However, as photosynthesis slows down and chlorophyll breaks down, the “hidden” orange and yellow pigments become more visible and disappear much more slowly. Carotenoids and xanthophylls (orange or yellow pigments) also aid in photosynthesis and are produced throughout the season, but are masked by the “green machine” of chlorophyll production.