To many, Utah’s fall leaves are a dazzling display of nature’s beauty. To Eleinis Ávila-Lovera, the autumnal switch to reds, yellows, oranges and purples tells a chromatic story of survival.
“Whenever I see plants outside, I’m always thinking what’s going on at a cellular level and why they’re doing what they’re doing. It always goes back to what’s best for the plant through evolutionary time,” said Ávila-Lovera, assistant professor in the School of Biological Sciences. “We might think that they just want to put on a show for us, but the changing fall colors are part of a strategy that allows these plants to survive the climate they experience in their natural habitat.”
Ávila-Lovera is a plant ecophysiologist who researches how plants respond to and tolerate drought. She figures out the mechanisms and traits underlying plant strategies that allow vegetation to cope with extreme water limitations. For example, some desert plants drop their leaves during the hottest and driest months to preserve moisture. In the mountains, the psychedelic foliage works in much the same way—trees that are deciduous drop their leaves to preserve nutrients and prevent freezing over harsh cold winter months.
All about pigment
Fall officially begins after the autumnal equinox, when the Northern Hemisphere begins to slowly tilt away from the sun. The shortening daylight triggers the beginning of the changing colors.
“Deciduous trees want to drop their leaves in a controlled way to preserve some of their nutrients and sugars. This process is called leaf senescence,” Ávila-Lovera said. “They can transfer leaf sugar into their stems, which lowers the freezing temperature of the water in their tissue. They can also store elements like nitrogen, phosphorus, sulfur, magnesium and amino acids to use for the next growing season.”
The reason most leaves are green is because of a pigment called chlorophyll. Chlorophyll molecules absorb all wavelengths in the visible light spectrum except for green. The green wavelength reflects into our eyes, so that’s all we can see. Chlorophyll is essential for photosynthesis, the process by which plants make their food using carbon dioxide and sunlight. Chlorophyll is an unstable molecule, so leaves must synthesize it constantly.
Other pigments exist in leaf tissue—carotenoids, such as carotenes and xanthophylls, are the orange and yellow pigments that boost the plant’s light-harvesting capabilities and protect its tissue from UV damage. Anthocyanins provide the plant’s red pigments. Some leaves contain anthocyanins year-round, but many deciduous plants start producing them due to a build-up of sugars. As daylight grows shorter, the base of the leaf grows a wall of cells that slowly clogs the veins that carry fluids, sugars and nutrients into and out of the leaf. These clogged veins trap sugars in the leaves and precede the production of anthocyanins, which act as extra protection against UV damage. Once the wall of cells completely seals up, the leaf can fall off.
With less sun energy in the winter, the plants stop replenishing chlorophyll. As the molecules begin to break down, the other pigments are revealed. Yellows are the xanthophylls, oranges are carotenes and anthocyanins, reds and purples are anthocyanins.
“Without chlorophyll, the other pigments become unmasked. The colors that we see depend on the proportion of the pigments that the leaves have. Aspens for example only show bright yellow leaves because they have only carotenoids. But, other trees, like maples or oaks, produce anthocyanins and show that red and purple pigment. I think those are most beautiful—the changing color is just amazing,” said Ávila-Lovera.“These colors are so interesting. Sometimes, you may see multiple colors within the same leaf, and you think, ‘Oh, there must be a different distribution of pigments making up this leaf.”
Every deciduous plant species has a genetic code that determines what kind of pigments it will produce. However, external factors can influence when the process starts, and the vibrancy of changing leaves. If there’s a wet winter that keeps the soil at an optimum moisture level, the plant can produce lots of pigments in healthy leaves. By fall, the robust pigments will be more vibrant. Alternatively, if there’s a freak weather event that causes a cold snap and leaves freeze off, the plant will be left with fewer resources to keep it healthy over the winter.
“Species living in these conditions, they have evolved these specific strategies to allow them to continue living and surviving in this type of ecosystem,” Ávila-Lovera said. “With the rate that climate change is happening, we are not sure if plants are going to be able to keep up with these changes and acclimate andevolve. So, that’s one of the things that we can start thinking about—how do these climate change influence plan survival through the lens of drought, or extreme weather events, or how that would affect the process ofcolor changing during the fall?”
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