"October is the fallen leaf..."
–Hal Borland
"My days are in the yellow leaf..."
–Lord Byron
Catharine and I have just returned from a trip to La Paz and a visit with our friends Lynn and Skip. Leaving Laguna three weeks ago, the liquidambars and sycamores had not yet begun to turn color. A week later, on our return to Laguna, we were treated to a display of fall foliage color. We strolled downtown recently, and Catharine asked, "Why do some trees have leaves which turn color in the fall?" And I answered...
Trees are planted wherever one chooses, but grow naturally only where they can survive and prosper. In the wild, environmental factors determine success or failure for a particular species of trees.
Northern latitudes and/or higher altitudes like our recent mountain visit are characterized by cold winters and periods of winter drought (unavailability of water due to snow). Deciduous trees (cottonwood, maple, etc.) must shed their leaves to protect against adverse conditions, and are native to these climes.
The desert, which has an excess of summer heat and Laguna Beach, which does not provide sufficient winter cold, are inhospitable to many species of deciduous trees. Although many cultivars of trees provide fall color in Laguna, one must travel as far away as Maine, or as close as Big Bear to view the truly spectacular turning of the leaves during the fall.
Every broadleaf, deciduous leaf is attached to its tree by a stem called the petiole. The petiole not only holds the leaf, but contains the conducting pipelines which carry water (xylem) to the leaf and returns food (phloem) out of the leaf to the plant. This versatile structure also rotates the leaf to the proper angle to receive more sunlight and elongates to ensure that each leaf reaches enough light.
As a leaf matures, it changes in color from light green to darker green. As temperatures become chillier and water becomes less available, a biological signal is sent to the tree that winter is approaching. From late summer to early fall, a ring of cork grows across the petiole of many deciduous trees, slowly blocking the pipelines of water and food to and from the leaf. These cork cells are called incision cells. By early October, the conducting structures of the leaf are completely sealed off. Depending on the tree, leaves may hang on the stem for just a few days or remain for several weeks.
This biological process, of abscising leaves for both winter survival and to prepare for the spring's new foliage, is the reason we enjoy fall foliage color. With the absence of water, the leaf stops photosynthesizing and loses its green color from chlorophyll and instead, reveals hidden chemical colors. Xanthophyll, a bright yellow pigment becomes clearly visible. Another substance called carotene (carrots contain this chemical in quantity) is brilliant with red or orange hues. Deep reds and purples are created by anthocyanin , a chemical that is newly formed upon the departure of chlorophyll.
The fall color of a tree depends on which of the three compounds it has in the greatest abundance within its leaves. Birch, ginkgo and elm turn yellow with xanthophyll. The Sugar Maple turns golden, bright orange, or red with carotene. Liquidambar and ash turn deep red or purplish from anthocyanin production. Frequently, multiple presence of these chemicals will yield combinations of colors within the leaf.
Catharine and I quietly witnessed the ending of life for a liquidamabar leaf, as a gentle wind caught it, and the twig released the leaf cleanly between the incision cell and petiole. The leaf moved downward slowly, first end over end, then in a spiral as it flipped lazily towards the ground. It joined its brothers and sisters that already carpeted half of the sidewalk with a brilliant yellow. I hoped that the city gardeners wouldn’t sweep up the leaves too soon. See you next time.
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