Fall is a time of letting go and preparing.
Preparing for winter, plants sever the connection to their photosynthetic factories and move their sap below. Leaves drift off branches like confetti. They litter the ground, curling and turning brown or flattening into the mush of decay. Fruits, berries, and nuts swell then tumble to the ground where they crack and burst open, spilling their seeds. Mushrooms, the fruiting bodies of the fungal mycelium, push up from damp soil or rotting logs, dispersing their spores. The air is a pungent perfume of fermenting vegetation and humid soil that brews an earthy petrichor, that musty, barky aroma of fresh rain on the dry earth. It is the smell of escaping plant oils and geosmin, released by dying Streptomyces (soil-dwelling bacteria) who inveigle their friends, the springtails, to feed on them and distribute their spores.
I walk the forest, enraptured by all this transformation, so gloriously enacted like a well conducted symphony. So much is happening above me in the trees and below me in the ground. I walk slowly and use all my senses to take in what I can, knowing that—because of my limited senses—I am missing much of the magic. But what I still take in is truly glorious.
Many of the tree and shrub leaves in the riparian forest here in Ontario have turned yellow in autumn. The black locust, ash, elm, birch, poplar, and Manitoba maple turn a bright yellow as temperature falls and photoperiod diminishes (length of daylight shortens). The leaves on many oak and beech trees turn mostly brown or copper-coloured and may persist throughout the winter to the following spring in a process called marcescence. Some red oaks do eventually turn a deep red colour. The dogwood shrubs turn a reddish to purplish colour and the sumacs turn a vibrant orangey red. But the queen of autumn here in Ontario is the sugar maple, Acer saccharum (along with her little sister, the red maple, Acer rubrum). This iconic tree of Canada (featured on our national flag) displays colours that range from brilliant orange to deep crimson and everything in between.
Why Are Most Leaves Green in Summer?
What makes most leaves green in spring and summer is the pigment chlorophyll, a chelate with a central metal ion bonded to a large organic molecule. Chlorophyll’s central magnesium atom is surrounded by a nitrogen-containing aromatic porphyrin ring with long carbon-hydrogen phytol chain.
This set up allows chlorophyll to absorb red and blue light (high energy) and reflect green light (lower energy), the complement to red wavelength of white light. White light is the collection of all light with visible light being part of all light’s electromagnetic spectrum. Because light travels in waves, it has wavelengths (the distance between the peaks of waves); shorter wavelengths have higher energy and a higher wave, so purple light has more energy than red light.
Absorption of sunlight by chlorophyll in the chloroplast of the leaf excites electrons that kick off the photosynthetic reaction to produce sugar. Photosynthesis is basically an energy factory that uses sunlight to make carbohydrates such as starch and sugar from carbon dioxide and water and gives off oxygen as a by-product. Sugars, in turn, are used by the plant to live and grow.
What Happens in the Fall?
Signaled by a change in day length and night temperature, the tree prepares for dormancy. Tree hormones and enzymes chemically trigger the leaf sealing and shedding process: this includes the production of abscisic acid that triggers the creation of an abscission layer; special cells develop where the stem of the leaf (petiole) attaches to the tree branch; this layer gradually severs the tissues that support the leaf, leading to abscission. The tree seals the cut, so that when the leaf is finally blown off by the wind or falls from its own weight, it leaves behind a leaf scar.
The marcescent leaves of oak and beech trees result because of lack of the abscission layer enzyme. Because marsescence occurs mostly in juvenile trees, this slowing process of leaf loss may be beneficial for growth and nutrient sequestration, given that smaller trees often receive less sunlight.
As the tree prepares to lose its leaves, chlorophyll breaks down, revealing previously masked pigments. These may include yellow flavonoids and xanthophylls, yellow to orange carotenoids (e.g. beta-carotene, lutein, and lycopene). In many plants, anthocyanins, also flavenoids, are only being produced in the fall; so, while all other pigments are waning in the fall, these are increasing in the fall.
Why are Anthocyanins Produced in the Fall When Other Pigments are Waning?
Triggered by an increased concentration of sugars in the leaves, anthocyanins (deep red, magenta to purple pigments) are synthesized in the fall. CompoundChem tells us that their intensity and colour is affected by the acidity of the tree sap. Anthocyanins may protect the leaves from light damage and extend the amount of time before they shed, allowing more nutrients to be sequestered. Kew.org suggests that anthocyanins may also either protect the leaf from cold temperatures or deter pests, such as aphids. “Presence of these coloured compounds might allow the tree longer to reabsorb other valuable nutrients as temperatures drop in autumn.”
How Weather Affects Fall Colours
ESF tells us that temperature, light, and water supply influence the degree and duration of fall color. “Low temperatures above freezing will favor anthocyanin formation producing bright reds in maples. However, early frost will weaken the brilliant red color. Rainy and/or overcast days tend to increase the intensity of fall colors. The best time to enjoy the autumn color would be on a clear, dry, and cool (not freezing) day.” Kew.org shares that “a warm dry ‘Indian summer’ followed by colder nights is the best combination, keeping the leaves working later and then shortening the leaf drop process. That way, more compounds remain in the leaf and there is less time for them to slowly break down to brown before the leaves drop.”
Field, Taylor S. et al. 2001. “Why Leaves Turn Red in Autumn. The Role of Anthocyanins in Senescing Leaves of Red-Osier Dogwood.” Plant Physiol. 127(2): 566-574.
Frazer, Jennifer. 2015. “Root Fungi Can Turn Pine Trees Into Carnivores—or at Least Accomplices.” Scientific American, May 12, 2015. Online: https://blogs.scientificamerican.com/artful-amoeba/root-fungi-can-turn-pine-trees-into- carnivores-8212-or-at-least-accomplices/
Hardy, Rich. 2020. “The 500-million-year-old reason behind the unique scent of rain” New Atlas.
Lee, David W. et al. 2003. “Pigment dynamics and autumn leaf senescence in a New England deciduous forest, eastern USA. Ecological Research 18: 677-694.
Munteanu, N. 2019. “The Ecology of Story: World as Character.” Pixl Press, Vancouver, BC. 198pp. (Section 2.7 Evolutionary Strategies)
Munteanu, N. 2020. “A Diary in the Age of Water.” Inanna Publications, Toronto. 300pp.
Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press (Vancouver) was selected by Margaret Atwood in the New York Times ‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” was released by Inanna Publications (Toronto) in June 2020.