Deerfield Spa (Pennsylvania)
During a cardiovascular walk this hot, swampy August day I sensed autumn's approach on the early morning breeze. Having experienced the deep tenacious cold of the previous two winters, my immediate response was dread. In an attempt to dispell my dismay, I reminded myself that experiencing the change of seasons, even winter, is one of the perks of living where there are four seasons.
The autumn palette is not only unique to every year but is ephemeral as well, shifting and changing with each passing day. That stated, some years the display is more spectacular than others. What makes the palette of color represented some years more vibrant than others? The autumn tapestry of color is determined by several factors. Temperature, light, water supply and soil pH all affect a leaf's change from green to warmer colors.
Just like stars are always present in our sky but we can't see them until the light of day fades to night, many fall colors are already present in spring and summer leaves but are concealed by the chlorophyl produced greens.
As days shorten and there is less light and chlorophyl production ceases, the chlorophyl breaks down and the greens fade to reveal warmer hues of yellow and orange. This is when the chemistry of autumn colors really comes into play. The pigment anthocyanin produces the deep crimsons and purples of autumn. Warm sunny days and cool nights, where temperatures stay above freezing are ideal for creating these deep colors.
Trees use stored glucose to get them through the winter. Cool nights and the closing of vessels leading into the leaves helps keep glucose in the tree where it will be used during winter months when food cannot be produced. In the process of storing glucose, some glucose is trapped in the leaves. The combination of sugar trapped in these leaves, and sunny fall days enables the anthocyanin to produce the darker intense colors enjoyed during the fall spectacular. Since anthocyanins are water soluble, the availability of water also influences the presence of reds in autumn leaves.
Carotenoids are natural pigments always found in living leaves and guarantee fall leaves with a warm yellow hue. The presence of anthocyanins adds red to the carotenoids, producing variations of orange colors and intensity. Eventually, veins that once delivered fluid and glucose close off and leaves begin to fall as a result of their weakened connection to branches. So it helps enhance color density if winds don't blow leaves off trees.
Different tree species have specific color characteristics. Aspens, for example, turn golden yellow while dogwood leaves become reddish purple. The color of maple trees in autumn depends upon the species of maple with sugar maples yielding the well known orange-red leaves.
Perhaps Nature's autumn exhibit is a gift to placate us, a way of apologizing in advance for the hardships of the coming winter, a gift to make it easier to let summer go and yield to the inevitable passing of time from one season to the next.
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