August begins the march into fall. There are always a few early maples that start to show some fall colors. Oftentimes these are the result of environmental changes. Something as subtle as excess moisture around the roots. Or an insult to the tree in the way of damage to the bark. Whatever the case the result is a tree with some bright red leaves.
If the reason is damage to the bark the colorful leaves may be confined to one or two branches. An altogether green tree with a couple of brightly colored limbs usually has some physical damage to the limbs in question. If the stress is caused by something more the entire tree may show its true colors and create a flag of color in the forest.
IT'S NOT ONLY THE MAPLES THAT BEGIN THIS PARADE THAT WILL END IN LATE OCTOBER WITH THE WILLOWS FINALLY GIVING UP THEIR YELLOWING LEAVES. Late August is often marked by the bright yellow of the spreading dogbane leaves. These are not reacting to some insult. They are finished with their season and are among the first plants to shift as a group into fall foliage. They are not particularly tall and may fail to attract our attention but they will, as a group, shift to bright patches of yellow. Coupled with their bright red stems they make a lovely introduction to the cavalcade of color soon to follow.
These are all good questions and we will explore the answers starting with where they come from.
For most of the summer chlorophyll is the active pigment within the cells of the leaves, but it is not the only one. There are carotenes, orange pigments whose job it is to absorb additional light and share it with the chlorophyll. Too much light can cause chlorophyll to become over-excited. In this state the chlorophyll can't continue to function, and it can even damage the leaf's ability to photosynthesize. A second chemical found in leaves is xanthophyll. These compounds are responsible for the yellow colors. Their job is to dissipate heat. Any excess energy from the chlorophyll and carotenes is bled out of the leaf and into its surroundings by the xanthophylls. The final major chemical compound found in leaves is anthocyanin. They may be red, purple, or even blue and they absorb the very high energy of ultraviolet light. These are the same wavelengths that cause sunburn in humans,
and like humans this "sunburn" in leaves can be damaging. Anthocyanins are the leaves sunscreens. They prevent damage to the leaf's tissues by screening out the UV waves.
So the leaf is a soup of different chemicals, but the chlorophyll is the dominant ingredient for most of the period of leaf out.
Chlorophyll absorbs every color of light except green, and that is reflected back to our eyes. In the fall - with shorter days and cooler temperatures - the tree makes a decision to abandon its leaves. The process is called senescence and it doesn't start with a frost as is often thought. The tree begins by constructing several layers of cells at the base of the petiole. On one side is a layer of cells designed to separate the leaf from the tree. This is known as the abscission layer. On the other side the cells are quite corky and are made to protect the tree from invasion once the leaf has dropped. The resulting bud scar is so distinctive that the trees identity can be determined simply by close examination of the scar.
Before the tree releases the leaf it pulls back certain chemicals in a process known as resorption. Primarily among the recovered chemicals are nitrogen and phosphorus. Pulling back these chemicals and allowing the photosynthetic process to cease causes the underlying colors to come to the fore. The nitrogen and phosphorus are stored in the twigs that previously held the leaves. This places them for immediate re-insertion next spring when the tree puts out its new leaves. By storing the chemicals right near where they will be needed next spring the tree gains an extra few weeks of production in the spring.
There is always much anticipation about the intensity of the fall colors. Many northeast and north central states publish weekly estimates of where the colors are at their peak. These are popular sites and are perhaps the most visited of all the postings put up by a state.
Colors are the result of the concentrations of chemicals found in the leaves. Red colors are possibly the most sought after of all the fall colors. These reds are the result of the anthocyanins and they are likely to be more intense in years where the growing season has been good. Anthocyanins are dependent on sugars. The better the growing season, the more soil moisture, the more sugars are produced. The higher the concentrations of sugars the more intense are the red colors. Conversely, in dryer years the reds are less intense.
A good dry fall following a good wet summer will increase the length of the spectacular colors. Clear skies, cool temperatures, and dry weather prolong the color season. As leaves senesce the cells within the leaves begin to break down. Rains will wash away much of the anthocyanins, causing the colors to fade faster. Not to mention that rainfall striking the leaves can dislodge them from the trees before they would normally fall. So when you notice the first maples turning color in the fall think back to the weather we have had over the summer and anticipate what sort of a color season we may experience.