By Jennifer Meikle, Environmental Educator

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One of the most amazing parts of the fall season is when the leaves begin to change. We go from the lush green of summertime to the beautiful yellows, oranges, and reds of autumn. Leaves of all colors litter the ground and most trees’ branches stand bare until springtime. But why and how does this big change occur?

It all goes back to leaves and why trees have them in the first place! Deciduous trees usually have large, broad leaves. Conifer trees have leaves called “needles,” because they are usually long and spiky. Conifer trees stay green all year long, unlike deciduous trees that lose their leaves come fall time. While the leaves of these two types of trees look completely different, they have the same job. Leaves provide food for the entire tree! They do this through photosynthesis. There is chlorophyll inside each leaf, which is a chemical that absorbs sunlight and converts it into sugar to give the tree energy. Chlorophyll is also what gives the leaves their green color. Because conifers have adapted to stay green all year long, their needles don’t need to drop during fall or winter. The needles on conifer trees are smaller, thick, and covered in a waxy layer that helps them to retain moisture through the winter. As winter comes, the needles stop photosynthesizing, but they seal the water inside the needles and are able to overwinter on that moisture. The high resin content of conifer trees also acts as a form of antifreeze for the needles and helps prevent damage.

During most of the year, the leaves on a deciduous tree are a rich green color because of the abundance of chlorophyll, however the yellow and orange pigments that you see in fall are there hiding beneath the surface the entire time. These orange and yellow pigments are called carotenoids. This is the same thing that makes carrots and pumpkins orange. When the daylight shortens and the hours of sunlight start to decrease, it causes the chlorophyll in the leaves to break down, revealing the yellow and orange pigment underneath. If you see leaves that are red or purplish colored, that is caused by a different process. This chemical change occurs when sugars get trapped in the leaves and produce new pigments in the leaves called anthocyanins. Not all trees have this special ability! Weather also plays a large role in the brightness of the leaf colors. This is why you might notice that some fall seasons bring much brighter leaves. Because what is causing the leaves to change colors is a chemical reaction, the amount of rainfall, temperature and humidity all play a role in the brightness of the fall colors. The most vibrant leaves will be seen after a very wet spring followed by a cool and dry fall.  

After the leaves change color, they drop off the trees, leaving a colorful carpet of leaves behind. The leaves would not be able to survive the winter and would risk causing damage to the trees. (Just imagine how many trees would not be able to bear the weight of snow on every leaf and branch, and how much energy it would take for the tree to keep the leaves alive in freezing temperatures!) Because the leaves are responsible for feeding the trees, you may be wondering how the trees stay alive through the winter. As the amount of sunlight decreases, the trees stop photosynthesizing and absorb as much of the nutrients from the leaves as possible, before working on forming a protective seal between the branch and the leaves. Once the branch becomes fully sealed off from the leaf, the leaf drops, as their fluid supply has been severed. The trees survive off the stored nutrients through winter. These trees are not so different than a bear hunkering down for hibernation all winter long. When spring comes and sunlight increases again, the trees grow new leaves and return to relying on photosynthesis!

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