The vibrant colors of autumn make this time of year a favorite. You may be wondering why leaves change color. To understand this, it’s important to first know how plants take in nutrients. Plants absorb water through their roots and carbon dioxide from the air. They use sunlight to convert water and carbon dioxide into glucose, which serves as their food source for energy and growth. This process is known as photosynthesis. A chemical in the plant called chlorophyll aids in photosynthesis.
Chlorophyll gives leaves their green color. As summer turns to autumn, the days gradually grow shorter, resulting in fewer hours of sunlight each day. Eventually, there isn’t enough sunlight for photosynthesis to continue. The trees enter a period of rest and rely on the food they stored during the summer, much like a bear living off its food stores in the winter.
What Causes the Variety of Colors in Leaves?
Leaves turn a variety of colors in autumn. You may wonder why they don’t all turn the same color, such as orange, yellow, or red. As chlorophyll disappears, the green color fades, revealing the orange and yellow colors that were already present in the leaves, but hidden beneath the green. As the green fades, these vibrant hues become visible.
The bright red and purple colors that characterize autumn are the result of glucose trapped in the leaves. This glucose is trapped after photosynthesis stops. Cooler nighttime temperatures cause the trapped glucose to turn red.
There’s also an important function behind the red and purple colors. They help the tree retain its leaves for a longer period. By staying on the tree longer, the leaves allow the tree to absorb more nutrients before they fall. Additionally, these colors help protect the tree by lowering the freezing point, providing some defense against frost.
The brown color in the leaves is caused by waste products, specifically tannins. These tannins, trapped in the leaves, cause them to turn brown.
How Weather Affects the Colors
The intensity of the colors of autumn is affected by weather patterns, which explains the variations in color from year to year. Red and purple hues are enhanced by cool temperatures. If autumn is warm both day and night, the colors will be less vibrant because the cooler nighttime temperatures, which help trap glucose in the leaves, are absent.
Moisture also plays a role in leaf color. A series of rainy, overcast days intensifies the color of the leaves. On the other hand, a dry summer can lead to less intense colors. In years with drought, the foliage will not be as vibrant as in years with sufficient rainfall.
The best colors are produced by a weather pattern of warm, sunny days and cool nights. The warmth of the day and abundant sunlight allow the tree to produce more glucose. When the temperature drops at night (but stays above freezing), the glucose becomes trapped inside the leaves. As the temperature continues to cool, the veins in the leaves constrict, trapping even more glucose. This leads to vibrant colors. Therefore, an autumn season with mostly warm, sunny days and cool nights will produce the best fall foliage.
