There are years when the colors of the autumn leaves are stunning and there are other years when the display is a little muted. A lot of this difference has to do with the red, crimson and purple colors of fall.
While the yellow colors (carotenoids) of fall are generally consistent from year to year, the same isn’t true for the various shades of red. The pigments that cause these colors in the leaves are anthocyanins, the same pigments that give color to cranberries, apples, cherries and grapes. While carotenoids are present in the cells of the leaves all year-long, anthocyanins usually are not. Also not all trees produce anthocyanins – for example, you won’t see red leaves on an aspen. Trees that do produce this pigment, like maple trees, do so in the fall.
In order to have a bright red display of leaves, the conditions need to be just right. The best reds are produced when the days are warm and sunny and the nights are cool but not freezing. During a warm, sunny day in the fall, the remaining chlorophyll in the leaves is able to photosynthesis and produce a lot of sugar. This it the same process that goes on throughout the growing season.
But when the days become shorter, the vascular tissue of the leaves begin to shut down. The sugars that easily moved from the leaves to the tree during the summer now become trapped in the leaves. Cool nights mean that there’s little breakdown of the sugar. The result is that the leaves become saturated with sugars.
When there’s lots of sugar in the leaves and the days are sunny, the tree leaves convert the sugars into anthocyanins. These pigments are water-soluble and are stored in organelles within the leaves called vacuoles. Vacuoles are membrane enclosed structures used for storage of water and other materials. When the days of fall are sunny and warm and the nights are cool, these vacuoles become filled with anthocyanins. Later in the fall when the chlorophyll of the leaves finally breaks down, the anthocyanins can be seen and the leaves appear red, crimson and/or purple.
Since these pigments are found in the water-filled vacuoles of the leaves, the amount of moisture in the soil can also affect the fall display. If there’s a lot of water in the leaves, the anthocyanins can end up being diluted and the resulting colors will be less intense.
With all of the varying factors of light, temperature and soil moisture, no two autumn displays of color will be the same. Some years the red pigments will be intense; other years they’ll be absent. It all depends on the leaves’ ability to produce anthocyanins and the amount of these pigments that are stored in the leaves.
It’s exciting to know that each year will be different and the combinations of these environmental factors will lead to different shades of red, crimson and purple. Add to this the fairly consistent yellows and each autumn display is unique. Someone should take pictures of the fall colors from the same location on the same day for a number of years. It would be fascinating to see the differences in the display from year to year. While they’d all be different, they’d also all be beautiful in their own way.