By Eric Anderson, Center for Urban Habitats
(Above: Nyssa sylvatica [black gum])
Gazing out at these November landscapes, we find ourselves enveloped in the warm yellows, oranges, and reds so comfortingly juxtaposed against the brisk seasonal air nipping our skin and vaporizing in our lungs. Few experiences delight with the same universality as this transformation of the eastern deciduous tree canopy, its polychromatic splendor evoking something innately inspiriting, an experience one can enjoy quite viscerally with nary a thought given. But for those who do decide also to engage with this process at a more cerebral level, the marvels multiply exponentially. Dare to ask why–and how–of these processes and one’s mind opens to a vitalizing array of new observations and discoveries.
Within every leaf, autumn’s cooling temperatures and shorter daylight hours elicit a series of chemical changes that have evolved over millenia to ease deciduous plants through the winter months. As many will remember from high school biology, that old familiar pigment chlorophyll predominates throughout leaf cells for much of their lifespan from early Spring through late Fall. What exactly is the purpose of that chlorophyll? Perhaps fewer of us recall its utterly essential function, to absorb most electromagnetic wavelengths from the sun and thereby harness the solar energy needed to manufacture (plantufacture?) the complex sugar, glucose. This captured solar energy powers the photosynthetic recombination of carbon dioxide molecules from the air and water molecules from the roots into the C6H12O6 glucose molecules, thus converting that energy from the sun into a more versatile and mobile food energy package. These glucose molecules can be stored and moved around the plant to locations where cells and tissues need to be created, repaired, or cleaned out, much as batteries enable us to move about the world with our bevy of devices rather than always being tethered near a wall. It is the molecular properties of the chlorophyll that enable it to absorb the solar energy for conversion into these essential glucose molecules; chlorophyll has a fairly complex structure similar to the hemoglobin in our blood, except that the central atom is magnesium rather than iron, thus causing the chlorophyll molecule to absorb most of light’s wavelengths except of course for those around 500 nanometers, which are instead reflected away and encoded by our visual systems as the green we see when we look at leaves.1 2
Chlorophyll, though, is an energy-intensive molecule to produce3, so as the Earth gets to that point in its annual trip around the sun where our northern hemisphere is tilting away from the sun and Virginians start experiencing shorter days of autumn, the energy demanded by manufacturing chlorophyll exceeds the solar energy that can be absorbed and converted into sugar, so deciduous plants start cutting their losses by curtailing chlorophyll production. As that green curtain then begins to dissipate, from behind it emerge two other pigments that also have been present throughout the summer but in lower concentrations, carotenes and xanthophylls. Xanthophylls reflect the yellows of the electromagnetic spectrum, while beta-carotene, a carotenoid, reflects red and yellow wavelengths, which most human brains will perceive as an orangey color. Existing mostly in the “shadow” of chlorophyll, the primary role of these pigments seems to be mainly to further modulate what light energy gets absorbed and reflected by the plant, thus improving the efficiency of photosynthesis and, perhaps, protecting against the damaging wavelengths of light by reflecting them back off the plant.
As a plant cuts off the supply of chlorophyll’s ingredient atoms to the leaf, the remaining chlorophyll that already exists in a leaf slowly degrades, and the colors reflected by the xanthophylls and carotenes start to emerge from behind that dissipating green curtain, and our retinas begin to register the warm, exuberant yellows and the gentle, soothing oranges.
So why don’t the leaves stay yellow and orange throughout the winter, and return to green in the spring? It turns out that leaves also cease production of beta-carotene around the same time as chlorophyll, but because the heartier molecular structure of the flavonoids and carotenes degrades more slowly than chlorophyll, the oranges and yellows last longer than the greens, even though those pigments are simultaneously dissipating, as well.
The alert reader will notice that we have overlooked one important autumn color, those fierce, luminous reds we also enjoy, especially in sassafras, black tupelo, sweet gum, sugar maples, and red maples. Even as plants cease production of the chlorophyll, xanthophyll, and carotenoids each autumn, many plant species will at that time start to generate a different pigment, anthocyanin, for a brief period. This seems at once curious, given that most other plant functions are slowing down when this one ramps up, as well as noteworthy, since an act so brazen would seem to necessitate a clear and obvious purpose. A singular explanation, though, has eluded scientists, perhaps because the reasons are manifold, but experimental results have supported a few different hypotheses. For one thing, at the chemical level, anthocyanins produced in the fall seem to protect chloroplasts as they disassemble their chlorophyll molecules, which seems to help increase the efficiency with which the increasingly lethargic tree can salvage and reuse the dissolving chlorophyll’s valuable nitrogen atoms.4 Other studies suggest that the red coloration may [also] be a mechanism to protect plants from insects who are attracted to the yellows from the xanthophylls, and/or to reduce the harmful impacts of ultraviolet radiation.
On your next walk through the woods or even your neighborhood, consider taking a more careful look at the patterns of leaves around you. How does one leaf vary from the one next to it? Do you notice any suggestive clumps or patterns distinguishing one side of the plant from another? How about differences between the leaves at the top of the tree and those at the bottom? What color profiles seem to characterize different species of plants? What might that tell you about the chemistry inside? Can you hypothesize possible reasons that plant species may have evolved that particular chemistry? We invite you to share your observations and your hypotheses below, so that together we might all enjoy even deeper satisfaction and stimulation from the magic of autumn’s deciduous rainbow.
1 The Chemistry of Autumn Leaf Colours, Andy Brunning, www.compoundchem.com
2 Explainer: The Chemistry of Autumn, Fernando Gomollon-bel, www.chemistryworld.com
3 The Science Behind Fall Foliage, Dylan Stuntz, www.americanforests.org
4 The Science of Leaf Color Change, Scientists at Harvard Forest, https://harvardforest.fas.harvard.edu/leaves/science
Photographs by Eric Anderson
