More than yellow!

Fall is my favorite time of the year.  The air becomes crisp and cool, while colorful trees light up the landscape.

Groves of aspen trees proudly decorate dark conifer forests.  Sometimes the yellow groves of aspens are blessed with trees that turn red.

Red, orange, and yellow aspen trees in the Sierra Nevada.

A mosaic of multi-colored aspen trees lights up the darkness of an incoming autumn storm in the Eastern Sierra Nevada, California. October 16, 2015.

Chlorophyll — Green Leaf

In summer, when the days are long, aspen leaves are green.  Trees produce chlorophyll during this time of abundant light and warm temperatures, which they utilize in photosynthesis.

Photosynthesis occurs when light energy absorbed by chlorophyll is used to convert carbon dioxide and water into oxygen and carbohydrates.  The trees consume the carbohydrates to grow, flower, and produce seed.

Chlorophyll absorbs red and blue light.  Light reflected from leaves containing chlorophyll has reduced amounts of red and blue light and appears green.  When chlorophyll is present, the leaves’ green color also masks out any other inherent colors the leaves may have.

Carotene — Yellow Leaf

Tree leaves contain another pigment, carotene.  Carotene absorbs blue-green and blue light, resulting in a yellow reflected light.  Carotene is more stable than chlorophyll and remains in leaves even when trees stop chlorophyll production during the shorter and colder days of fall. When chlorophyll disappears from a leaf, it no longer masks the yellow light reflected from carotene. The bright yellows of aspens in the fall indicate the end of photosynthesis until the longer spring days arrive.

“Embrace your uniqueness. Time is much too short to be living someone else’s life.”

– Kobi Yamada

The last rays of sunlight shine through an aspen grove as the sun sets behind a mountain ridge, San Juan Mountains, Colorado. October 6, 2020.

“Delicious autumn! My very soul is wedded to it, and if I were a bird I would fly about the earth seeking the successive autumns.”   

–George Eliot

Rays of sunlight pierce storm clouds,  illuminating a patch of the aspen belt, Sangre de Cristo Mountains, New Mexico.  You can see the large groves of orange aspen trees which managed to retain their leaves during this fall snowstorm on October 9, 2018.

Anthocyanin — Red Leaf

Another class of pigments in tree leaves are anthocyanins.  Anthocyanins absorb blue and green light.  Light reflected from leaves containing anthocyanins is red in color.

Anthocyanins are dissolved in the cell sap of the tree leaves.  The color of anthocyanin pigments depends on the pH of cell sap — redder when cell sap is very acidic and purple when it is less acidic.  Chemical reactions between sugars, proteins, and light in the cell sap create anthocyanins.  These reactions do not occur until the sugar concentration in the sap is high.

Anthocyanins are responsible for the red skin of ripe apples and the purples of ripe grapes.  The side of an apple that faces the sun is red due to sunlight aiding the reactions between sugars and proteins needed to create anthocyanins.

Aspen trees that can turn red do not turn red in all years.  It turns out that both genes and seasonal conditions determine whether an aspen tree will turn red.  Seasonal conditions that promote the most vibrant red aspens include fall days with warm sunny days and cool nights above freezing.

Scientists hypothesize that red anthocyanin acts as a sunscreen for tree leaves. When photosynthesis stops, the “sunscreen” protects the tree leaves from drying out and allows additional time to absorb leaf nutrients (anthocyanins are associated with excess sugars in the cell sap).

Although most anthocyanin production occurs in the fall, it can be observed in the early spring when a tree turns red first then changes to yellow when the days get longer and chlorophyll production and photosynthesis kicks in.

Red aspen trees in the Sierra Nevada

Some red aspen trees shine in this colorful grove in the Eastern Sierra Nevada, California.  October 16, 2015.

Multiple colors of aspens in the Sierra Nevada, California.  This grove contained green, yellow, orange, light red, and dark red colors in the aspen leaves. October 16, 2015.

Diagram explaining why some aspen trees turn red

After chlorophyll production stops in the fall, the aspen trees reflect yellow due to carotene in their leaves.  The chlorophyll reflects green light in the summer, masking the colors of other chemical compounds in the aspen leaves.  Aspens with more acidic cell sap, due to increased sugar content, contain higher amounts of anthocyanins.  Anthocyanins reflect red and are most visible in the fall when chlorophyll production is reduced.

Red aspen trees in a New Mexico forest.

Aspens showing off their fall color, Carson National Forest, New Mexico.  October 10, 2020.

I do not know why seeing red aspen trees means so much to me?  Perhaps it is a vivid childhood memory of running through a grove of red aspens while fishing a trout stream in the Eastern Sierra?  All I know is that each fall, I search them out in cool fall air.

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11 replies
  1. Dave Brooks
    Dave Brooks says:

    Ed, thank you for bringing this bouquet of autumn aspens to me. I was going to make a run up to the eastern sierra this year but have not done so yet and its not looking like it will happen. I have noticed the year to year variation in aspen reds before but never knew the science of it all. Beautiful images much appreciated. Best wishes to you and yours.

    • Ed MacKerrow
      Ed MacKerrow says:

      Thank you, Dave. I hope you still have a chance to see some of the fall colors this year. Warm and dry falls, with cool nights that do not drop below freezing, seems to be the recipe for the best colors. This year that recipe was achieved in most of the west. I went yesterday up to the mountains, and just like that, the aspens were mostly bare except in sheltered canyons. I find the Eastern Sierra to be wonderful though at most any time of the year. Have a wonderful fall season!

  2. Mike Seyssens
    Mike Seyssens says:

    I love the fall colors too, the forests turn on the rainbow of colors and I love it!
    Your photographs are stunning to say the least!

  3. Lila McClellan
    Lila McClellan says:

    Autumn is my favorite time of year. I haven’t gone to alot of places this year, but hope to catch some fall colors on a trip to Utah in a few weeks. would you share the location/directions of the photos in Colorado and New Mexico?

  4. Margaret Lewis
    Margaret Lewis says:

    Thank you for the stunning photographs. I enjoyed reading about the colors of the trees and why and how they occur. Thank you for permitting those of us quarantined at home to see the beauty of fall. The Eastern Sierra are such a special place.

    • Ed MacKerrow
      Ed MacKerrow says:

      You are welcome, Margaret. Thank you for reading the article and your kind words. I am glad you have experienced the Eastern Sierra. I think it is one of the most stunning landscapes in North America.

  5. Corry Clinton
    Corry Clinton says:

    Thank you Ed, for this article and the beautiful pictures. One by one, they are superb, and the blog is very informative. Never knew why some trees turn red maybe one year and not the next.

    I hope you stay healthy and wish you the best for the season and New Year. Looking forward to another workshop, maybe???


    • Ed MacKerrow
      Ed MacKerrow says:

      Corry, it is great to hear from you. I am glad you enjoyed the article on “Why Are Some Aspens Red?.” I have not been doing any workshops during the COVID-19 pandemic. I will begin doing some private tours soon, social distancing where each client drives in their own vehicles, no indoor sessions during the workshop, and masks required. I will let those who subscribe to my mailing list know in advance of when this will happen. Stay safe, and I hope you are able to get out and enjoy nature during these challenging times.


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