In last month’s blog post, we explored evolutionary reasons why leaves turn red in the fall. But this phenomenon is not uniform across the globe. In North America (and East Asia), deciduous trees have a much higher proportion of species that turn red instead of yellow. In Europe, only 24 species have red autumn leaves, compared to 89 species in North America. Given the benefits of red fall leaves, why aren’t they as prominent in Europe?
Researchers Simcha Lev-Yadun (from Israel) and Jarmo Holopainen (from Finland) suggest a hypothesis that goes way back in evolutionary time. During the Tertiary Period, around 35 million years ago, the earth went through cycles of hot and cold phases. During the cold phases, glaciers covered large parts of the northern hemisphere, forcing plants and animals to migrate south to avoid freezing. North America (and East Asia) have mountains that run north to south, allowing species to easily migrate southward. However, in Europe, the Alps run east to west, which blocked species from migrating farther and caused them to go extinct due to the ice and snow. When the northern hemisphere warmed up again, trees in North America were able to migrate back northward and retain their red autumn leaves.
So why didn’t trees in Europe once again evolve to have red autumn leaves? The authors suggest that during the Tertiary period, trees had co-evolved with animal predators which required their leaves to be red to avoid predation. However, these animal predators ran into the same mountain problem that the trees did, and therefore went extinct along with their food source. With the selective pressure for red leaves gone, trees in Europe had no reason to develop red autumn leaves, and maintained their yellow autumn leaves instead. In North America, these animal predators survived just as the trees did, which means the selective pressure to have red leaves is still present (Lev-Yadun & Holopainen 2009. New Phytologist 183(3):506-512).
There are other factors that could influence the red and yellow difference between North America and Europe. Scientists Susanne Renner (from Germany) & Constantin Zohner (from Switzerland) found that North America receives higher solar irradiation, has more temperature fluctuations, and has a shorter growing season than Europe. Production of anthocyanin (the blue, red, and purple pigments found in plants) is therefore that much more crucial in North America to protect leaves and ensure that nitrogen can be resorbed into the roots before winter (Renner & Zohner 2019. New Phytologist 224(4):1464-1471). In another paper, Renner & Zohner note that there is less nitrogen in our poor North American soils. They found that trees that had symbiotic relationships with nitrogen-fixing bacteria did not have any red autumn leaves, because they could already produce enough nitrogen. But many trees without nitrogen-fixing bacteria did have red autumn leaves—when there was limited nitrogen, it was worth the extra cost of producing anthocyanins to protect leaves long enough to resorb the nitrogen (Renner & Zohner 2022. Journal of Evolutionary Biology 35(1):183-188).
So as peak foliage season starts to wind down, remember to appreciate our New England fall colors—truly some of the most brilliant and unique in the world.
