How to Read Red Aspen Leaves: Anthocyanins and Sun Block for Leaves

     Why do some aspen leaves turn orange or red, rather than the more usual yellow or golden? Maizie, fearless and happy pup, and I were on our 16th annual autumn pilgrimage to Hell's Hole Trail on Saturday, pondering this very point.

      Since we've hiked the same trail on or about September 30th for 16 autumns, we've seen the same aspen tree turn yellow one year and bright orange or red the next few years, and back to yellow for a few years, then back to orange.

     
       I wrote in a 2014 PEOTS postscript (after my sign off) about it being a very orange year (lots of carotenoids). 2015 was overall much yellower; the reds and oranges were quite rare.  How much does moisture, temperature, soil conditions, or other factors affect the yellow, orange, or red of aspen? Ready, Maizie? Lead the way!

     

      First, a brief photosynthesis review: As deciduous trees prepare to lose their leaves, they begin to resorb nutrients and to slow chlorophyll production, unmasking other pigments that have been present all along. Yellow and orange leaf colors are due to xanthophyll and carotenoid pigments (see below).

Here's a handy chart showing all the leaf pigments:

• Chlorophyll for greens
• Carotenoids for orange
• Xanthophyll of the Carotenoid group for yellow
• Anthocyanins for reds and purples
• Tannins for brown as a waste product

      Reds and purples, as noted above, are due to anthocyanins and these pigments are only produced in the autumn (by some plants). It is still not clear the role these red pigments play, and why a tree would spend energy to create them at a time when they are about to drop all their leaves. Hypotheses include reducing the risk of light-induced damage to leaf cells (sort of a sun block for the leaves), protection from cold, protection from insects, and helping leaves retain water.

        Some plants almost always produce red but, in some cases like aspen, only a few trees turn red. What makes them different?

      In 1989, Kuo-Gin Chang and his research team, then at Colorado State University, analyzed the pigments of yellow and red aspen and determined that all aspen produced carotenoids, but anthocyanins were only found in red aspens. The ability to produce anthocyanins appears to be a genetic trait that some aspen trees have, but most don’t.

      However, just because a tree can produce anthocyanins doesn’t mean that it always will. The researchers followed the trees they studied for five years. The trees that started off yellow stayed yellow, but some of the trees that started off red (i.e., they produced anthocyanins) were yellow in subsequent years. This demonstrates that both genes and weather cause the red. Years that produce the best reds have warm sunny autumn days followed by cool, but not freezing, nights.

      Not only do the red pigments of the anthocyanins protect leaves from the sun they also give some species extra time to absorb their essential leaf nutrients. As chlorophyll starts to exit the leaves, anthocyanins are being created to get the leaves additional time to unload the excess nutrients. Anthocyanins are a result of excess sugars within the cells and in combination with bright light, produce red pigment. Most anthocyanins are present only in autumn. One may observe a tree turning red at first and then changing to all yellow as days lengthen and rains come. The trees then need more food supply so they go into high gear and the leaves turn yellow.

       So, clearly, Maizie and I need more data within one season to see if the reds and oranges of certain aspen turn yellow later in the autumn.

Leaving it there for now; how's your aspen?
Steph