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.
Please let me know if you have any questions about this article or on trees’ colors in the fall. I find trees so interesting and love studying them.
Thanks so much for this article. Saw some gorgeous aspens this year, including lots of reds, and wanted to know why. Love your stunning photos too. If you haven’t been to Kebler Pass, CO, highly recommended.
Thank you, Lynn. Yes, Kebler Pass is one of my favorites. Lots of red aspens in N. New Mexico this year also. Thank you for sharing your observations. Happy Autumn 🙂
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.
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!
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!
Thank you, Mike! I hope you can see the colors of fall this year and enjoy this wonderful season.
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?
Hi Lila, I hope you can get out to see the fall colors. In the spirit of protecting these beautiful places and wildlife, I do not share any location information.
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.
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.
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???
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.
Thank you for this excellent post. I have shared it with my Wenatchee Washington readers
Thank you, Susan. I hope you are seeing some great fall colors. This year in the southern Rocky Mountains and San Juan Mountains there was a good show of red aspens.