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Autumn Leaf Colour

Hurricane Fiona took many things from PEI last year, including our fall leaf colour. As we move into the shorter, cooler days of October, I’m happy to see our hardwoods resuming their annual autumn show. This is an orderly biological process called senescence that I think deserves a place among the top natural wonders on Earth. Let’s look at how and why it happens.

Photo 1: A variety of autumn colours in PEI Red Maple leaves.

The broad leaves of our hardwood trees produce huge amounts of energy in summer but would be huge liabilities in winter. Their large surface area would not only allow water to be lost from the tree during cold, dry days, but would also catch wind and snow, breaking branches and topping trunks. Deciduous trees shed their leaves and avoid those consequences.

Leaves are green in the growing season due to chlorophyll, a pigment that allows plants to convert sunlight, carbon dioxide, and water into food. Chlorophyll is expensive for the tree to produce – it takes a lot of energy. Shorter day lengths and lower temperatures trigger trees to break down and reabsorb chlorophyll so it won’t be lost when leaves fall. Nature is a great recycler, and the building blocks of chlorophyll – magnesium, nitrogen, carbon, hydrogen, and oxygen – are all dismantled and transported out of the leaves and back to the tree for future use. (It’s not unlike how we put the summer outdoor equipment away so we can use it again next year).

As green chlorophyll disappears, yellow and orange pigments that were there all along – carotenoids – can now be seen. Carotenoids are the bodyguards for the all-important chlorophyll, protecting it from light that would damage it and absorbing wavelengths that chlorophyll can’t.

Reds and purples indicate the production of new pigments called anthocyanins. These are mostly late season sunblocks, protecting the leaves from too much sunlight. They also provide a bit of protection from freezing. Because anthocyanins are produced in late summer, they are sensitive to local climate conditions and can vary greatly from year to year.

Once the tree has recovered everything it needs from the leaf, it seals off the connection. This creates a spot where the leaf can break cleanly from the tree without tearing or damaging it. Autumn winds and rains help break the leaves away and eventually the tree is leafless and prepared for winter.

Cold, sunny days are best for production of the showy red anthocyanins. PEI’s late summer and early autumn temperatures were above average this year, and so I expect our 2023 leaf colour will be mostly yellow and orange. (Though I note the Red Maples through PEI's Miscouche swamp are looking fiery). Climate change is having several effects on the timing and colours of fall hardwoods. Warmer August and September temperatures mean we may expect (on average) less red and more yellow and orange in fall leaves.

Whether it’s red, orange, or yellow, our beautiful fall colours are the outward expression of trees preparing for winter. I have a front-row seat for this annual phenomenon from my home overlooking the hardwood hills of Central PEI. Come Thanksgiving weekend, my quiet country road becomes almost bumper-to-bumper with sightseers enjoying PEI untamed.

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