Mother Nature thinks of everything, doesn’t she? She even gives us a spectacular show of color as she prepares deciduous trees to withstand the winter wind, snow and ice.
As you rake your leaves, do you ever wonder why leaves turn color and drop? Many people do because they ask me why green leaves turn yellow, red, and orange. The answer is quite simple, yet quite complex. Leaf pigments, the physics of light, weather conditions, plant species, and geography all play a part in creating autumn colors.
Yellow, a pigment in the carotenoid family, is a leaf’s natural color, and therefore it is always present. Chlorophyll gives leaves their green color, and masks the yellow. Chlorophyll is present as leaves manufacture food during photosynthesis.
When the amount of daylight dwindles and temperatures fall, photosynthesis slows down and finally ceases. As it does, less and less chlorophyll is produced until the leaves’ natural yellow color can be seen.
But what about red leaves? This is due to anthocyanins, a pigment produced only during the autumn months. These complex, water soluble compounds in leaf cells react with excess stored plant sugars and exposure to sun light, resulting in brilliant pink, red and purple leaves. A mixture of red anthocyanin pigment and yellow carotenoids often results in the bright orange color of some leaves.
Weather conditions that occur before and during the decline of chlorophyll production can affect the color that leaves may display. Carotenoids are always present so the yellow and gold colors are the least affected by weather. The red tones, created by anthocyanin, are most affected by weather.
On warm and sunny days, lots of sugar is produced in leaves. Trees exposed to brighter sunlight generate the reaction between the anthocyanins and the excess sugar, creating the bright red hue. Sharp changes in climate can paint the most spectacular display of color. Cooler temperatures cause the veins in the leaves to gradually close, preventing the sugars from moving out, which preserves the red tones. Warm sunny days followed by crisp, cool nights can be responsible for the lush tones of fall we see all around us.
Soil moisture can also affect autumn color. A particularly dry summer, like the one we just experienced, can delay the onset of color change by weeks. A warm, wet spring, favorable summer weather, and sunny fall days with cooler temperatures at night are the ideal conditions for producing the most radiant colors.
Tree genetics help determine what color leaves will turn. Color depends on the levels of iron, magnesium, phosphorus and sodium in the tree and the acidity of the chemicals in the leaves. Some tree species displaying yellow foliage are ash, birch, beech, elm, hickory, poplar, and aspen. Red leaves are most often seen on dogwood, sweetgum, sumac, and black tupelo trees. Some oaks and maples present orange leaves, while others range in color from red to yellow, depending on the specific species.
So, what causes leaves to drop? As photosynthesis ceases, the base of the leaf, known as the petiole, closes up since no food is leaving. Consequently, no water and nutrients flow in, either. Meanwhile, below the petiole, next year’s leaf bud has formed and is growing and growing, until it pushes the leaf, disconnecting the tissue that holds it to the branch, and the old leaf falls. There you have the story of autumn.