As September begins, there are little hints of fall. The air temperature cools, and a leaf here and there begins to turn yellow, red, orange, or purple. By October, the mountain shimmers with the gold of the aspen. Although we don’t have a great variety of deciduous trees with leaves that transition in the fall, smatterings of different colors can be seen across the landscape.
So why do leaves change color?
During the growing season, leaves serve as food factories for the plant. Within each leaf, plants undergo a process called photosynthesis. The ingredients needed for photosynthesis are chlorophyll, water, and carbon dioxide, and sunlight. The end result is the production of glucose and oxygen. The plant uses glucose as food, a building block for growth. Chlorophyll gives the plant its green color.
As summer comes to an end, days get shorter and shorter, and nighttime temperatures get closer and closer to freezing. In response to these changes, the leaves stop making food. Chlorophyll begins to break down, and the green colors disappear. The yellow and orange colors become visible. As sugars accumulate in the leaf, the red pigments are visible. Red and purple are added to the landscape.
There are two types of pigments in leaves that provide autumn colors.
- Carotenoids produce yellow, orange, and brown colors. It is the carotenes that give color to bananas, carrots, daffodils, and buttercups. These colors are masked by chlorophyll, but they are in the plant cell membranes all the time.
- Anthocyanins are red and purple and are responsible for the color of cranberries, cherries, strawberries, and blueberries. They are produced in the cell sap when sugars accumulate in the leaves. If the sap is acidic, the color is red; if the sap is less acidic, the color is purple.
Both chlorophyll and carotenoids are present in the leaf cells throughout the growing season. But, as the daylight shortens and temperature goes down, anthocyanins develop in response to decreased light and sugars that accumulate in the cells. As the chlorophyll begins to break down, the bright orange, yellows, reds, and purples are unmasked. In oaks, the brown color is created by wastes accumulating in the leaves. The brightest autumn colors are produced when dry sunny days are followed by cool, dry nights.
The color a leaf changes is genetic: aspens turn yellow, maples turn red. So why do some aspen leaves become reddish and most are yellow? Studies years ago at Colorado State University found some leaves have red pigments and others only yellow. It appears that some trees form the anthocyanins and thus have red colored leaves. Most aspen trees do not produce anthocyanins, so they have yellow leaves. Aspens sprout from an extensive root system creating groups of trees, called clones. These clones are genetically identical. Viewing the landscape, groups or clones of aspens will have a similar color. The question is, does a clone always turn the same color from year to year or are there other factors?
Why do leaves fall?
In response to the breakdown of chlorophyll, layers of cells at the base of the leaf (abscission layer) begin form, cutting off fluids and food to the leaf. The veins of the leaf become clogged and trap sugars, which promote the production of anthocyanins. Once the layer has completely formed, the leaf can be detached by freezing, wind, water, or even gentle breezes. The leaves slowly decay on the ground where they provide nutrients for the soil and food for various soil organisms.
How does losing leaves help the tree?
The leaves of deciduous trees are the most sensitive to freezing, so the tree drops the leaves instead of trying to carry them through the winter. Stems, twigs, and buds can survive the cold and reawaken when spring comes. The leaves of conifers (pines, fir, spruce, etc.) can survive the winter because they are covered with a wax coating, and the cells resist freezing. Conifers also lose leaves based on the age of the needle-like leaves, not due of temperature or weather.
Where can I see a forest in autumn color?
We don’t have as much variety of deciduous trees as in the Eastern woodlands, but we have some wonderful New Mexico landscapes to view the aspens. Aspens in our area have been subjected to fire—both the Cerro Grande and Las Conchas. However, because they are a species that responds to fire, they sprout readily. The road to the ski hill or a walk up the American Springs road will delight the viewer as the aspens shimmer in the sunlight. Another place to view a fall landscape is Overlook Park in White Rock, look across the valley, and see the groves of aspen on the Sangre de Cristos.
For a view of maples, go to Fourth of July Canyon/Campground in the Cibola Forest on the eastern side of the Manzano Mountains and southeast of Albuquerque. This area has a large group of indigenous Bigtooth Maple trees with beautiful orange, red, pink, and yellow foliage in October of each year. The colorful maples give the canyon its name.
Photos courtesy of Terry Foxx.