Quaking Aspen: Star of the Autumnal Sierra
As temperatures drop and hours of daylight decrease, autumn steals quietly across the Sierra Nevada. There is no pomp, no great fanfare to accompany the change of seasons, like there is in the east. Here in the west, the change is more subtle, but perhaps all the sweeter in its humility.
Undoubtedly, the star of the western autumnal show is the Quaking Aspen (Populus tremuloides). All across the mountainous west, stands of delicate aspen trees burst into glorious autumn color: liquid gold and pure sunshiny yellows are most common, but under the right conditions, leaves become brilliant shades of flame orange, salmon pink and even vivid crimson. Each heart-shaped leaf is attached to the branch by a long, flattened petiole (leaf stem) which allows the leaves to dance and shimmer at the slightest breeze, hence the common name "quaking" and Latin specific epithet "tremuloides."
Aspens prefer moist soil and abundant sunlight, and tolerate temperatures from -70 degrees Fahrenheit to 110 degrees Fahrenheit. The trees have smooth, white bark that does not peel, but is marked with dark scars at the locations of previous years' branches. (Be careful not to confuse the smooth white bark of aspens with the peely white bark of birch trees. Birches are in a completely different plant family from aspens. While aspens are members of Salicaceae, with willows, poplars and cottonwoods, birches belong to the family Betulaceae, with alders and hazels.)
A grove of Quaking Aspens in full autumn kit is a magnificent sight to behold. But each grove hides a secret that is not readily apparent to the casual on-looker. One grove appears to be made up of hundreds of trees, but is in fact one single organism. Genetically the trees are clones, shoots derived from one root system. The trees reproduce asexually, the underground system of roots sending up new shoots until acres and acres are covered in a grove of identical aspen trees. The trees are capable of reproducing by the normal method of flowering and setting seed, but do so only under specific conditions.
Aspen groves far exceed California's famed Bristlecone Pines (Pinus longaeva) in age, and may be in the running for the title of "world's oldest living thing" along with the Creosote Bush of the desert southwest (which are also clones). Scientists believe the oldest aspen clone is 80,000 years old. This clone is named "Pando" (Latin for "I spread") and is located in the Fishlake National Forest of central Utah. With over 40,000 individual trunks spread across an area of 106 acres and an estimated weight of over 6,000 tons, Pando may be considered Earth's largest living organism. (Though actually, a honey fungus in the mountains of Oregon is considered by many to be the largest. Sorry, blue whale.)
Interestingly, within a clone of aspens each tree exhibits identical branching structure and all of the trees will simultaneously change color in the fall, from green to the same varying shades. One clone can be distinguished from another by the color of its fall foliage.
But you might be wondering, why, exactly, do leaves change color in the first place?
First, we need a quick refresher on photosynthesis, the process upon which all life on earth is built.
During photosynthesis, plants take in water and carbon dioxide from their environment and with the aid of sunlight captured by chlorophyll molecules, convert it into sugar (usable energy for the plant) and oxygen. The equation looks like this:
light
As we see from the equation, the plant molecules chlorophyll, found in chloroplasts, are a vital part of the process of photosynthesis. Chlorophyll itself is unstable and readily breaks down in the presence of sunlight, so plants are required to produce it at a constant rate throughout the growing season. The production of chlorophyll requires warm temperatures and adequate sunlight.
As long as temperatures and light conditions are favorable, chlorophyll is produced and photosynthesis carries on. But many species of deciduous (winter-dormant) trees only photosynthesize during the favorable months of the growing season, going dormant to conserve energy at other times. It is as these preparations for dormancy are made that we begin to notice a change in leaf color.
Lower temperatures and shorter days bring about the miraculous change in leaf color of many species of deciduous trees. Red pigments (anthocyanins) and yellow pigments (carotenoids) are present year-round in leaves, but are masked during the growing season by green chlorophyll. With fewer hours of daylight and the cooler temperatures of autumn, trees are triggered to grow membranes between branches and petioles (leaf stems), slowing the supply of nutrients to the leaves. Chlorophyll production drops and photosynthesis slows down. Without green chlorophyll to mask the other colors, the familiar reds and golds of autumn are allowed to shine for a brief time before the leaves drop from the trees and the trees become dormant for the winter.
Underneath the thin bark of aspens is a green photosynthetic layer that allows the trees to photosynthesize to some extent during the dormant period. Aspen trees grow year-round, providing a valuable source of browse for deer and elk during tough winters.
Predicting peak fall color is tricky, since it is influenced by environmental conditions and not likely to occur at exactly the same time each year. According to what I've read, a growing season with ample moisture followed by a dry, sunny fall with cool days and frost-free nights produces the best fall color in aspens. From what I've seen so far, the trees are looking good this year in the Sierra!
Undoubtedly, the star of the western autumnal show is the Quaking Aspen (Populus tremuloides). All across the mountainous west, stands of delicate aspen trees burst into glorious autumn color: liquid gold and pure sunshiny yellows are most common, but under the right conditions, leaves become brilliant shades of flame orange, salmon pink and even vivid crimson. Each heart-shaped leaf is attached to the branch by a long, flattened petiole (leaf stem) which allows the leaves to dance and shimmer at the slightest breeze, hence the common name "quaking" and Latin specific epithet "tremuloides."
Aspens prefer moist soil and abundant sunlight, and tolerate temperatures from -70 degrees Fahrenheit to 110 degrees Fahrenheit. The trees have smooth, white bark that does not peel, but is marked with dark scars at the locations of previous years' branches. (Be careful not to confuse the smooth white bark of aspens with the peely white bark of birch trees. Birches are in a completely different plant family from aspens. While aspens are members of Salicaceae, with willows, poplars and cottonwoods, birches belong to the family Betulaceae, with alders and hazels.)
A grove of Quaking Aspens in full autumn kit is a magnificent sight to behold. But each grove hides a secret that is not readily apparent to the casual on-looker. One grove appears to be made up of hundreds of trees, but is in fact one single organism. Genetically the trees are clones, shoots derived from one root system. The trees reproduce asexually, the underground system of roots sending up new shoots until acres and acres are covered in a grove of identical aspen trees. The trees are capable of reproducing by the normal method of flowering and setting seed, but do so only under specific conditions.
Aspen groves far exceed California's famed Bristlecone Pines (Pinus longaeva) in age, and may be in the running for the title of "world's oldest living thing" along with the Creosote Bush of the desert southwest (which are also clones). Scientists believe the oldest aspen clone is 80,000 years old. This clone is named "Pando" (Latin for "I spread") and is located in the Fishlake National Forest of central Utah. With over 40,000 individual trunks spread across an area of 106 acres and an estimated weight of over 6,000 tons, Pando may be considered Earth's largest living organism. (Though actually, a honey fungus in the mountains of Oregon is considered by many to be the largest. Sorry, blue whale.)
Interestingly, within a clone of aspens each tree exhibits identical branching structure and all of the trees will simultaneously change color in the fall, from green to the same varying shades. One clone can be distinguished from another by the color of its fall foliage.
But you might be wondering, why, exactly, do leaves change color in the first place?
First, we need a quick refresher on photosynthesis, the process upon which all life on earth is built.
During photosynthesis, plants take in water and carbon dioxide from their environment and with the aid of sunlight captured by chlorophyll molecules, convert it into sugar (usable energy for the plant) and oxygen. The equation looks like this:
light
carbon dioxide (6CO2) + water (6H2O) --------> glucose (C6H12O6) + oxygen (6O2)
chlorophyllAs we see from the equation, the plant molecules chlorophyll, found in chloroplasts, are a vital part of the process of photosynthesis. Chlorophyll itself is unstable and readily breaks down in the presence of sunlight, so plants are required to produce it at a constant rate throughout the growing season. The production of chlorophyll requires warm temperatures and adequate sunlight.
As long as temperatures and light conditions are favorable, chlorophyll is produced and photosynthesis carries on. But many species of deciduous (winter-dormant) trees only photosynthesize during the favorable months of the growing season, going dormant to conserve energy at other times. It is as these preparations for dormancy are made that we begin to notice a change in leaf color.
Lower temperatures and shorter days bring about the miraculous change in leaf color of many species of deciduous trees. Red pigments (anthocyanins) and yellow pigments (carotenoids) are present year-round in leaves, but are masked during the growing season by green chlorophyll. With fewer hours of daylight and the cooler temperatures of autumn, trees are triggered to grow membranes between branches and petioles (leaf stems), slowing the supply of nutrients to the leaves. Chlorophyll production drops and photosynthesis slows down. Without green chlorophyll to mask the other colors, the familiar reds and golds of autumn are allowed to shine for a brief time before the leaves drop from the trees and the trees become dormant for the winter.
Underneath the thin bark of aspens is a green photosynthetic layer that allows the trees to photosynthesize to some extent during the dormant period. Aspen trees grow year-round, providing a valuable source of browse for deer and elk during tough winters.
Predicting peak fall color is tricky, since it is influenced by environmental conditions and not likely to occur at exactly the same time each year. According to what I've read, a growing season with ample moisture followed by a dry, sunny fall with cool days and frost-free nights produces the best fall color in aspens. From what I've seen so far, the trees are looking good this year in the Sierra!
Click on the video above to see aspens shimmer in the breeze!
(View the video in full screen and make sure it is set to HD for best results.)
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