As the seasons change and the days get colder, the trees around us give a wonderful show of colors. As marvelous as these reds, oranges, and yellows are, the process of this change is a key indicator to the tree’s health and longevity.

What makes them change color?

This change all starts within the leaves itself. Leaves make a chemical called pigment which is what gives them color. The chemical is called chlorophyll which is what gives them the bright green color we see in the warm months. Leaves are triggered by the summer sunlight to create more and more chlorophyll. Having this pigment is a key point in the process of photosynthesis as it allows plants to make energy from the sunlight that hits them. During the summer months, chlorophyll covers the other colors within the leaves. The other colors in leaves come from a group of chemicals called Carotenoids. These chemicals are responsible for the bright yellow to orange to red colors you see.There are more chemicals within leaves that can determine their color. Xanthophylls can be responsible for more yellow looking leaves, and Anthocyanins are responsible for the red or even purple colors. During fall when the days get shorter and there is less sunlight this is a signal to the leaves to stop producing chlorophyll. After this happens, the green in the leaves slowly disappears revealing the warmer tones that have been hiding under the surface.

Weather is also a key factor for the intensity of the color. Warmer days and cooler nights are ideal for leaves to change color and also makes them the most vibrant. If there’s too much rain or early frost it could cause the leaves to turn brown and fall early, cutting the color display short. Drought can also cause this same reaction.

Why do trees lose their leaves?

As beautiful as this show of fall colors is, it’s not just for looks. It is an adaptation that trees have to survive cold winters. When the temperatures drop and there is less sunlight, the trees will form a cork-like structure called an abscission layer between the leaf and stem connected to the branch to cut off its nutrients. Trees do this to shed their leaves and go dormant. By doing this trees are able to save energy on conserving water within rather than expend it on trying to maintain their leaves. Additionally, snow and ice can be extremely heavy. If it were to cling to large masses of leaves a tree's branches could become damaged and snap off. As it warms up after the winter, trees will “wake up” to begin the cycle once again. This seasonal cycle allows for trees to survive year round.

What's the difference between trees that lose their leaves and trees that don’t?

Trees that lose their leaves in the winter are called deciduous trees. This simply means these types of trees lose their leaves in mass during a single season. Here in Illinois, we have many deciduous trees that vary in color. The red leaves often come from red oak, white oak, northern pin oak, and staghorn sumac. The bright yellow and orange leaves are river birch, tulip tree, american pawpaw, quaking aspen, and bur oak.

Trees that keep their leaves all year round are called evergreen trees. An evergreen tree is a plant that will keep its green foliage year round and will not shed its leaves or needles. The needles on evergreen trees are thick and have a waxy like coating to protect them from the harsh weather and reduce water loss. This makes it so evergreen trees can photosynthesize throughout the winter whenever the chance arrives. As snow and ice may pile up, the cone shape of an evergreen makes it easier for the snow to slide off instead of snap branches. Their branches also tend to be more flexible which also aids in damage control during the winter time. Illinois only has 8 native evergreen tree species which are jack pine, shortleaf pine, white pine, red pine, northern white-cedar, eastern redcedar, baldcypress, and tamarack. 

“Why Do Leaves Change Color?” National Environmental Satellite, Data, and Information Service, 26 Aug. 2025, www.nesdis.noaa.gov/about/k-12-education/understanding-our-planet/why-do-leaves-change-color. Pace, Madeline. “The Science behind Fall Foliage: UNC-Chapel Hill.” The University of North Carolina at Chapel Hill, 18 July 2023, www.unc.edu/discover/the-science-behind-fall-foliage/.Jeung, Emily. “Why Leaves Change Colors and Fall off Trees.” Pacific Science Center, 25 June 2026, pacificsciencecenter.org/blog/why-do-leaves-change-colors/. “How Do Trees Change in the Fall? (U.S. National Park Service).” National Parks Service, U.S. Department of the Interior, 27 Jan. 2025, www.nps.gov/articles/how-do-trees-change-in-the-fall.htm. Kortebein, Paul. “Why Do Some Trees Keep Their Leaves through Winter?” Why Do Some Trees Keep Their Leaves Through Winter? | Three Rivers Park District, 24 Dec. 2018, www.threeriversparks.org/blog/why-do-some-trees-keep-their-leaves-through-winter.Grundmann, Bill. “Why Do Some Trees Hold onto Dead Leaves in Winter?: Organic Plant Care LLC: Organic Lawn & Plant Health Service in Hunterdon, Morris, Somerset & Union Counties, NJ and Bucks County, PA.” Organic Plant Care LLC, 20 Sept. 2019, organicplantcarellc.com/why-trees-hold-dead-leaves-in-winter/.Huang, Sophia. “Why Do Some Trees Keep Their Leaves through Winter?” A Plus Tree, 15 Nov. 2024, aplustree.com/trees-that-lose-their-leaves-during-winter/.“The Colour of Leaves.” Encyclopedia of the Environment, 22 Sept. 2024, www.encyclopedie-environnement.org/en/zoom/the-colour-of-leaves.“Illinois Fall Colors.” Illinois Department of Natural Resources, dnr.illinois.gov/education/atoz/ilfallcolors.html. Accessed 2 Nov. 2026.Clark, Rachael. “How Coniferous Trees Survive Winter.” Runyans Country Tree Farm, 10 Feb. 2025, runyanstreefarm.com/about-the-farm/blog/how-coniferous-trees-survive-winter/.