Precipitation? Nope, not totally.
Temperature? Can’t claim all the credit.
Sunlight? Right on. This is our primary source.
In a nutshell, as the calendar turns to fall, the length of sunlight decreases, and the duration of darkness at night increases. This initiates a biochemical process that reveals autumn’s colors.
So, how and why do the colors change?
Before we get too far down this path, we need to back up to spring and summer. Spring and summer are the growing season for deciduous trees. During the growing season, chlorophyll, a green pigment in the leaves, is continuously produced, broken down, and restored, thereby creating a consistently green color. Chlorophyll is used in photosynthesis to trap the light energy to convert carbon dioxide and water to carbohydrates such as sugar and starch. The carbohydrates are the fuel for the tree and leaves’ growth. The fuel is stored for the change in the seasons.
In the fall, our days become shorter and our nights become longer, causing the growing process to stop. This means no new chlorophyll is produced, but it continues to break down. Eventually, within the short span of the season, the chlorophyll and its green color disappear completely, thus revealing the leaves' new colors.
Also during this time, leaves’ veins begin to close, forming a layer of cork-like cells. This separation layer will allow the stored carbohydrates and water to continue to nourish the tree and not the leaves. This separation also triggers a hormone, which is what eventually causes the tree to lose its leaves after the process is complete.
Where do the colors come from?
The palette for autumn is made up of chlorophyll, which we’ve already discussed, as well as carotenoids, xanthophylls, anthocyanins, and tannins.
- Carotenoid: produces the range of orange colors
- Xanthophyll: produces the range of yellow colors
- Anthocyanin: produces the range of reds and violet colors
- Tannin: produces the brown color
Carotenoids and xanthophylls are present in the leaves alongside chlorophyll during the growing season but are masked by the green pigment. As chlorophyll breaks down and disappears, carotenoids and xanthophylls become visible. Anthocyanins are created from the sugars that are trapped within the leaves by the closing of the veins, initiating a chemical reaction to produce the bold red colors. Tannins are present in almost all types of deciduous tree leaves. After all of the colors dissipate, the leaves turn brown because of the remaining tannin.
So, what trees turn which colors?
- Oaks turn red, rust, or brown.
- Hickories turn golden or bronze.
- Aspens and poplars turn yellow or golden yellow.
- Dogwoods turn purple or reddish purple.
- Red maples turn red.
- Black maples turn yellow.
- Sugar maples turn orange or golden orange.
How does the weather affect the changing of the colors?
Temperature, light, and water supply all have an impact on the vibrancy of the fall colors as well as how long they stick around. The best combination for our most brilliant fall leaves is rain in the spring, a warm summer, and sunny fall days and cool (but not frosty) nights. Extremes such as a summer drought or an early freeze damage the foliage – either delaying the color change, causing an early end to the change, or prompting an early falling of the leaves.
Isn’t science wonderful?
Bring fall to your classroom with a hands-on, exploratory field trip. A quick Internet search will provide you a plethora of lesson ideas. And on a more personal note, might we suggest grabbing your favorite seasonal coffee, tea, or snack and getting outside? Go see fall in all its glory.