New England has the most beautiful fall foliage in the world! This may be my extremely biased opinion, but it is backed up by the millions of leaf peepers who visit every single year to witness the magnificent display put on by our native trees.
For a long time, scientists believed that red fall colors were a result of the breakdown of chlorophyll in the leaves, revealing the underlying colors that were being suppressed by the green chlorophyll. However, we now know that anthocyanins—the compound that produces a red pigment—are produced shortly before the chlorophyll is broken down. Because anthocyanin production is costly, there must be an evolutionary reason that trees go through the effort of turning their leaves red!
This question is a very hot topic in the scientific community, with many proposed hypotheses and no consensus on an answer. The two main hypotheses provide a physiological and ecological reason for red leaves. In the physiological hypothesis, anthocyanins protect leaves from the sun, which can disrupt nutrient (particularly nitrogen) resorption into the roots. However, one study found that nitrogen resorption was actually more efficient in trees with yellow autumn leaves (Pena-Novas & Archetti 2021. Journal of Evolutionary Biology 34(9):1423-1431). So, there must be an ecological explanation as well.
In their 2007 paper, Lev-Yadun & Gould outline multiple hypotheses that could provide an ecological reason for trees to produce red leaves. The bright leaves could signal to birds and other animals that fruit is ripe and ready, thereby getting the animals to help with seed dispersal.
Red leaves also disrupt insect camouflage—predators with good color vision, such as birds, can easily pick out an insect that is not camouflaged. Green insects would no longer be disguised among red leaves, encouraging them to find other green food plants at a time when deciduous trees are resorbing nutrients from leaves to store for winter.
The red color could also be an indication to herbivores that the leaves are unpalatable. Anthocyanins are created along similar chemical pathways as other defense chemicals such as tannins, so the presence of anthocyanins shows that other defensive chemicals are likely to be present.
The red color could also indicate to herbivores that leaves have a low amount of nutritional value because they are about to drop. Herbivores may take the color as a signal that it is not worth feeding on the leaves, especially if they are dropping soon. The authors argue that this color signaling is an example of aposematism, a strategy where organisms use bright colors to advertise unpalatability (think poison dart frogs or monarch butterflies)! In this case, the coloring is truthful about the state of the leaves and minimal benefits to herbivores.
So, this could also be an example of Zahavi’s handicap principle, which states that because producing certain traits can be costly, those traits are an honest depiction about the state of the organism. For example, peacocks have beautiful feathers that reduce the fitness of the individual, so they must have other superior traits that can outweigh the cost of these feathers. In our case, the anthocyanins in the leaves are costly to produce, but show that they may not be worth feeding on. Lev-Yadun & Gould argue that likely many or all of these hypotheses overlap, including the physiological hypothesis. Evolutionarily, traits that provide multiple different benefits are even more likely to be selected for, so it makes sense that there would be many reasons for anthocyanin production (Lev-Yadun & Gould 2007. The Botanical Review 73(4):279-289).
There are so many possible explanations for red fall color. I find it so thrilling that in nature and science, there is a reason—or even multiple reasons—for everything! To stay up to date on this year’s fall foliage throughout Maine and find ways to enjoy the beautiful colors and fall atmosphere, check out the Maine Department of Agriculture Conservation and Forestry’s Maine Foliage site. And don’t forget to buy your favorite red, orange, and yellow trees at a 25% discount at our Native Plant Sale before the end of the season!
