The Glorious Colors of Autumn

We are lucky in Western Pennsylvania in that, while autumn is the harbinger of cold weather to come, it also brings us a show of changing leaves that ranges from quietly beautiful to spectacular. I’m sure everyone has commented at one time or another on the intensity of the leaf color in any given year. This year (2013)leaves the leaves are not nearly as bright and the bright reds and oranges are very scarce. To understand why the range of colors is brighter some years than others we must first understand why leaves change colors.

Chlorophyll is the green pigment that absorbs sunlight and uses it to transform carbon dioxide and water into the simple sugars that give the plant fuel for growth. The oxygen that we breathe is a byproduct of this process. Chlorophyll isn’t a very stable compound. It breaks down and is replaced all through the growing season but as long as the leaf is able to produce chlorophyll the leaves remain green.

As the days grow shorter and the nights colder an abscission layer begins to form at the end of the leaf stem or petiole. This corky layer begins closing off the pathway by which water and nutrients reach the leaf. Now, when the chlorophyll is broken down the leaf does not have what it needs to make more. The green color begins to fade from the leaves the carotenoid pigments that are in the leaf all year become visible.  The carotenoid pigments carotene, lycopene and xanthophyll, are similar chemically and are more stable than chlorophyll. They absorb a different part of the light spectrum thus enhancing the range of light that a leaf can absorb. Untimely, these pigments transform the solar energy they collect to the chlorophyll, which is the only pigment that can photosynthesize.

All leaves contain carotenoids. These pigments are responsible for a wide range of colors in autumn leaves. Carotene and lycopene produce reds, oranges and yellows. Xanthophyll produces only yellow. Chemical interactions in the leaves can affect the colors of these pigments.

While the carotenoids are present in the leaf all summer there is a second set of pigments, the anthocyanins, are produced as sugars build up in the cells of the leaves. Bright, sunny days contribute to the buildup of sugars, so these pigments only appear if the weather is right. Anthocyanins are thought to facilitate the process by which the accumulated sugars flow back into the stems before the leaf drops.  This pigment group produces the bright reds and the darker maroon leaf shades. If the leaf has low acidity the leaves will appear darker. Conversely the anthocyanins are bright red in the presence of a high acid level.

Just as chlorophyll masks the carotenoids, anthocyanin covers the carotenoids. Leaves with large supplies of anthocyanin will appear red. If a leaf contains carotenoids and anthocyanins in approximately equal amounts the leaves will take on an orange color. Leaves with only carotenoids will usually be yellow. Other chemicals in the leaves can affect color depending on the amount of the pigments present. Tannins in oak leaves can cause a brown color under certain circumstances.

While the leaf chemistry plays an important part in the autumn color display, it is the weather that triggers the process and determines the intensity of the display. The green of the leaves will not begin to fade until cold weather causes the leaf to begin the process of separating from the tree by cutting off the supply of nutrients. Sunny days during this process will cause a surge of sugar in the leaves and encourage the production of anthocyanin. It also speeds up the destruction of the chlorophyll. Dry weather is another factor in the production of anthocyanin,  as it concentrates the sugar in the leaves. The brightest oranges and reds are produced by dry weather, sunny days and cool nights. When this combination does not occur the carotenoids predominate and the leaves are mostly yellow or turn brown without putting on a show of color.

A good growing season results in healthy leaves and good sugar production, so the basis of fall color begins in early spring. A warm wet spring gets the trees off to a good start. The best summer conditions consist of moderate rain and days that do not produce excessively high temperatures. When these conditions are followed by a fall that consists of warm, sunny days and cool nights the optimum production of the colorful pigments occur.

Leaf color can be adversely affected by late summer drought. Lack of rain can delay the color display by several weeks. Extremely cold weather and severe frosts will cause the leaves to turn brown and drop prematurely. Warm weather in conjunction with excessive rain fall will result in duller colors.

Leaf colors are species specific so what trees are in a given area is also important for a colorful autumn display. So which trees produce what colors? The maples are probably the peacocks of the autumn color display. They produce the widest range of colors and some of the most intense shades. The fall color of red maples is a bright scarlet. Sugar maples produce red/orange autumn color. The leaves of black maples are bright yellow.

Dogwood is another good producer of the red pigments but leaf chemistry results in a more purple shade of red. All of the birches have a yellow fall leaf color. While the birches are a bright yellow the poplars produce a more golden color. Hickories have a darker yellow fall leaf color tending toward bronze. The oaks produce more subdued colors. Their reds are darker than the maples and often the leaves are russet or brown. Oaks, though, do extend the fall foliage season as the leaves tend to stay on the trees longer.

The leaf colors this year were quite disappointing. We now know that this was due to a wet, warm period in early fall. Sun has also been a scarce commodity this fall.  Now that the nights are getting cooler some color is emerging but many of the leaves have turned brown and dropped already. Better luck next year.