In the Garden:
Western Mountains and High Plains
The magic colors of fall are a feast for the eyes and the soul.
The Magic of Autumn Colors
That golden, reddish and purplish foliage of autumn that blankets our high plains and mountainous regions is not the result of Jack Frost. We often take it for granted that this foliage -- its spring and summer work finished -- simply has reached old age and will soon fall to the ground.
But it's not that simple. The processes that led to colorful fall foliage started back in summer, and the leaves floating to the ground do not signify finality. The death of the leaves in autumn sets the stage for next spring's rebirth of plant life.
There are several factors that affect the brilliance of fall foliage. Color development in trees and shrubs is determined by both genetics and environmental conditions. We're fortunate to have sunny days and cool nights, which prompt bright yellow and orange hues. Additionally, drier soils with high iron content help bring on more intense colors. The abeyance of hard freezes also means a more colorful and long-lasting foliage show.
Have you ever noticed how much more showy the aspen foliage is in the mountains compared to aspen leaves in lower-elevation, metropolitan landscapes? Higher elevations have greater sunlight intensity and much cooler nights. Mountain soils generally are drier and drain water better than the clay soils common in home landscapes. Plus most gardeners provide additional irrigation to lawns and surrounding areas. Whereas home landscape soils are alkaline, high country soils are usually more acidic, which makes iron more available to the trees and shrubs.
Leaves are green because of the substance known as chlorophyll, a complex pigment needed for photosynthesis. As days shorten in late summer and early fall (and night temperatures cool down), synthesis of chlorophyll comes to a halt, and enzymes and sunlight break down the remaining chlorophyll in the foliage. A transformation occurs as this chlorophyll breaks down and other colorful pigments become more apparent.
The scarlet colors are made in the senescent leaves late in summer from sugars that are trapped in the leaf tissues and converted into a pigment called anthocyanin. This pigment is water-soluble and can be washed out. Therefore, if there is a rainy period during this time of transformation, the red coloration will be diminished.
Yellow is a leaf's basic color, present from the beginning. All summer long it has been masked by the green chlorophyll. With shortening days and the cessation of chlorophyll, the yellow pigments, including xanthophyll and carotenoids, are revealed. These pigments and the tannins that give the russet and brown colors are least affected by rainfall.
The miracle of fall colors also signifies new life. Before the leaves drop in autumn, they send many important nutrients back through the tree's vascular system. This transfer takes place before the abscission layer forms between the leaf stems and twigs. Then all circulation ceases and the leaf truly dies. Fallen leaves -- their year's work done -- will leave a legacy to the plant that produced them.
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