Why Do Trees Drop Their Leaves In The Fall?

There’s nothing more beautiful than driving through Colorado to see the leaves start to change in the early Fall. For a short window of time you’ll see colors on deciduous trees quickly change from green to gold and red; right before the trees shed their leaves and the temperatures start to dip. While admiring the changing leaf colors, have you ever wondered why the colors change at a specific time of year? 

Basically, it’s science and the environmental cues that make leaves change their color, but let’s take a deeper dive into the reasons why. 

The Purpose of Leaves

First, we need to take a look at the purpose of tree leaves to truly understand why leaves change color throughout the year. 

There are several purposes for leaves, but primarily they are there to provide food for the tree Through a process called photosynthesis, they turn sunlight into energy, which creates food and nutrients. The tree will store this food during the winter months, when there is less sunlight available. 

Leaves also allow the tree to “breathe”. Leaves have pores that can take in carbon dioxide and turn it back into oxygen. As you can see, leaves have several important functions to maintain its health and vitality. 

Why Leaves are Green

Beginning in the Spring, you’ll start to see tree leaves bud and then in mid-Summer trees become full and lush with a vibrant array of green colored leaves. Tree leaves are green because they are filled with chlorophyll which is needed to convert sunlight, water, and carbon dioxide into glucose. Chlorophyll also provides the leaves with their pigment.

When Chlorophyll Start to Break Down

The longest day of the year occurs towards the end of June in Colorado. After that day, the days slowly get shorter and shorter as we get closer to Fall. The shortening of daylight means that the trees have less and less access to their food source produced by the sun. 

With no food source, the chlorophyll in the leaves begins to break down along with the color green. With this change, the leaves will start to change to yellows, reds, or purple which provides the vivid colors you see during the Fall season. 

Different Colors Mean Different Things

Yellow to orange pigments are actually carotenes and xanthophyll pigments which, for example, give the orange color. These are the same pigments that give carrots their vibrant orange color. Most of the year these colors are masked by great amounts of green coloring.

Anthocyanins are pigments that add the color red to tree leaves. You can also find these same pigments in including red apples, cherries, and strawberries.

Additional Changes During the Fall

The changing colors of tree’s leaves aren’t the only thing that changes during Autumn. Leaves and their stems also deliberately become detached from the branches of the tree. The area where the branch is attached to the leaf’s stem will gradually sever itself so that the leaf will eventually be blown off by the wind or fall due to its own weight. 

Trees by Leaf Colors:

  • Oak Trees: Red or Brown
  • Hickory Trees: Gold
  • Dogwood: Purple-Red
  • Birch: Yellow
  • Poplar: Golden yellow
  • Maple Trees:
    • Sugar Maple: Orange-Red
    • Black Maple: Yellow
    • Red Maple: Scarlet

If you’re curious to learn more about how trees change during different seasons, you can read one of our articles here on preparing your trees for stormy weather.

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