For thousands of years and across many cultures, fall has been a time for celebration. It’s the season of harvest when food is plentiful, and the weather is still mild. And in many parts of the world, fall also brings some of the most striking visuals of all, as the leaves explode into color on every deciduous tree.
It’s hard not to look at those leaves and wonder exactly what causes this beautiful yearly ritual. Sure, we all know the surface-level answer: leaves change color because they’re dying. Yet, the dying leaves on other plants just fall off without an extravagant show of beauty. What makes fall leaves different?
The Role of Chlorophyll
As you may have learned in grade school, chlorophyll is what makes plants green. This photosynthetic pigment is also responsible for providing plants with food: it enables them to convert sunlight into energy.
However, green isn’t the only color in leaves; they also have yellow and orange pigments. But green chlorophyll is bright enough to overpower those other pigments—most of the time.
In the fall, though, changes in temperature and sunlight signal the trees to halt the food-making process. This causes the chlorophyll to break down, so the yellow and orange pigments in the leaves can finally show through, giving you a bold display of autumnal colors. Other chemical changes in leaves in the fall also cause other pigments to appear, such as red and purple colors.
Of course, this only happens in plants that lose their leaves in the fall. When non-deciduous plants lose a leaf here and there, the chlorophyll production doesn’t stop, so you don’t get an impressive array of colors—the green simply fades to brown.
Why Do Trees Lose Their Leaves in the Fall?
Knowing what makes the leaves change colors may also raise another question: why do these trees lose their leaves at all?
The answer is simply that for lots of plants, keeping leaves on in the winter is just too taxing. Winter offers extreme cold temperatures, minimal sunlight, and dry air. Yet, leaves thrive with lots of sun, moisture, and warmth. In many climates, deciduous trees can’t maintain their leaves all year long. Losing their leaves is a survival tactic.
As the chlorophyll breaks down and the leaves change colors, the tree absorbs the old chlorophyll molecules. Then, when spring comes, it will already have the molecules needed to make chlorophyll for new, green leaves.
What Makes Some Leaves Brighter than Others?
You may have also noticed that some areas, and some years, have exceptionally bright fall leaves. There’s a scientific explanation for that, too.
Water supply, temperature, and weather patterns all affect how bright the leaves turn out each fall. It’s also helpful if there are less wind and rain early in the season, so the leaves can develop their full brightness before being dropped from the trees.
In areas with mild autumns, you’ll often see brighter leaves than in areas with sudden, harsh frosts. But the exact vibrancy of fall leaves can also change from year to year in a single area, depending on rainfall, storm frequency, and other factors. So, if you see a particularly bright display of autumn leaves, make sure to enjoy it now—it might not be as impressive next year.