What's with those colours? Sean Thomas on the science behind this year's fall foliage

Christopher.Sorensen — Fri, 21 Oct 2022 13:01:16 +0000

Christopher.Sorensen Fri, 10/21/2022 - 09:01

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(photo by David Lee)

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Forestry

John H. Daniels Faculty of Architecture

Toronto residents are enjoying a particularly dazzling autumn display this year as the city’s tree canopy changes colour amid the cooling temperatures.

That includes the fiery red leaves of the sugar maple that stands on the southeast edge of Spadina Circle near the �ؿ�ζSM’s John H. Daniels Faculty of Architecture, Landscape, and Design, where Sean Thomas is a professor of forestry and a longtime associate editor at the journal Tree Physiology.

Thomas says a perfect storm of climatic conditions this season have produced especially vibrant leaf tones.

He recently spoke to Danny Sinopoli about the science behind this year’s eye-catching autumnal foliage.

Why do leaves change colour in the fall? What’s the science behind it?

There is a common misconception that autumnal leaf-colour change is due entirely to degradation of chlorophyll that “unmasks” other pigments that are already there. This is true basically with yellow colouration, which is mainly due to carotenoid pigments. However, red colours in fall foliage are due to newly produced anthocyanin pigments, which raises an intriguing functional biology question: What is the adaptive value in leaves producing new pigments just before the leaves are to be shed?

There are a couple of hypotheses, but the explanation that I think has received the most support is that anthocyanin pigments are playing a role as a “sunscreen” that better enables trees to recover nutrients from senescing leaves. It turns out that the breakdown products of chlorophyll are highly reactive, particularly under high UV exposure and low temperatures. Without the protection offered by anthocyanins, free radicals are generated from the breakdown products of chlorophyll that disrupt the process of nutrient recovery. Some anthocyanins are also antioxidants and scavenge the free radicals, so there are likely two aspects to their protective function during leaf senescence.

This nutrient recovery hypothesis predicts that anthocyanin production should be greatest when temperatures are low – but still above freezing, since frost events kill leaves – and light levels are high. This pattern is widely supported. Demonstrating that anthocyanin production actually increases nutrient recovery itself is more difficult to demonstrate, but there is some evidence for this as well.

So, are the colours actually more vivid this year?

Compared to last year, yes. Consistent with the theory, last fall was a relatively warm one in the GTA, without near-frost events until well into November – and also relatively cloudy conditions. This year had the right combination [for brighter colour] of cold weather events, lack of drought or a hard frost and relatively sunny conditions.

A sugar maple on Spadina Circle puts on a fall show (photo by Zheren Zheng)

Why do people react so strongly to red tones – like those of the sugar maple on Spadina Circle?

I think the accepted psychological theory is that red provokes strong emotional reactions because it is a danger cue. But what happens when the entire landscape is red, or the colour is rendered on such a large scale?

To speculate wildly outside of my area of expertise, a red forest landscape may initially provoke a kind of alarm reaction, to which viewers then acclimate, and this acclimation is pleasantly stimulating. Perhaps this is a bit like spicy food: “Hot” flavours are due to pain receptors, and the relaxation of the pain response releases endorphins.

Autumn in the Rouge: changing colours is just the start, U of T experts say

ullahnor — Fri, 21 Oct 2016 16:01:26 +0000

Autumn in the Rouge: changing colours is just the start, U of T experts say

ullahnor Fri, 10/21/2016 - 12:01

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Leaves are now bright hues of red, orange and yellow in Rouge Park (photo by Ken Jones)

Don Campbell

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Don Campbell

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Rouge National Urban Park

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Exploring the Rouge during autumn? There’s more than just changing colours

It’s that time of the year again. There’s a chill in the air, the days are getting shorter, and the city is painted with a cascade of changing autumn colours.

Perhaps in no other place in Toronto are these changes more evident than the Rouge National Urban Park. With its accessible trails and abundant biodiversity, it’s a popular spot for nature walkers and bird spotters alike, especially in the fall.

The �ؿ�ζSM Scarborough is the primary research and education partner with Parks Canada, with experts who use the Rouge to conduct research and take class trips to explore the many species that live in the park. Some of those researchers shared insights on what’s taking place in the park throughout autumn.

The park is unique because it’s situated in a transition zone between the Carolinian forest and northern forests, and also happens to be located in Canada’s largest city, notes Associate Professor Marc Cadotte, an expert on urban forest conservation and biology at U of T Scarborough.

“The Rouge is unique because of its high diversity and unique combination of species,” he says.

That diversity can be experienced with a quick walk around the park, he adds. If you’re near the Glen Rouge campground, you will see plenty of maple, oak and hickory forests but as you move to more hilly terrain, you will start to see spruce, pine and cedar. This also means the animals and birds will be different from one location to the next.

Associate Professor Marc Cadotte in Rouge National Urban Park (photo by Ken Jones)

In the park, you’ll notice different colours in the leaves because, as Cadotte points out, there’s considerable variation in leaf colour even within the same species of trees.

“The colours are produced by the chemistry of the leaves," he says. "They’re typically green in the spring and summer because of chlorophyll, which drives photosynthesis.

“As the days begin to shorten, trees prepare for the winter by drawing in the important chemicals of the leaves. What’s left behind can be quite colourful.”

The reason they eventually fall and die is that even though leaves are a food source for a plant, once winter rolls around they become a burden. Cadotte says if you’re out for a stroll, you may also notice that some leaves of some plants remain green and don’t fall. That’s because they’re either invasive, like the garlic mustard plant, or they’re evergreen trees like pine that have evolved in harsher climates.

Morning is a great time to see insects, notes Cadotte. The most obvious will be the monarch butterfly, which is preparing for its long migration south to Mexico. There will also be plenty of woolly bear caterpillars, which are preparing for their long hibernation for the winter.

“During autumn mornings, you’re more likely to see insects resting on bark or the side of trees. You can walk right up to them, and they won’t take off because they need to warm up before they can move,” he says.

Mornings are also a great time to see mammals being active, says Rudy Boonstra, an ecology professor who studies animal behaviour at U of T Scarborough.

“The best time is either early morning or right before dusk when human activity is low,” he says.

There’s a whole range of animals to see throughout the Rouge, including cottontail rabbits, raccoons, squirrels, chipmunks, woodchucks, foxes, coyotes, white-tailed deer, striped skunks, and even mink along the shores of Highland Creek and the Rouge River.

Some hibernators, like chipmunks, spend all fall storing food in an underground cache. They will wake up every few weeks in winter to replenish body stores before going back in hibernation. Others, like woodchucks, store food internally in the form of fat, he adds. They also wake up every few weeks during the winter but replenish their needs from their fat stores. Both species will lower their metabolism to a few degrees above zero-degrees Celsius in order to conserve energy.

People stroll through Rouge National Urban Park (photo by Ken Jones)

Other mammals prefer the “sit and take it” approach, says Boonstra. Animals like white-tailed deer, red fox, red squirrels, grey squirrels and raccoons do not hibernate. Some remain active and can be seen, while others, like the raccoon and skunk, become inactive in a burrow or tree hollow and may reduce their body temperature slightly

He adds that male deer in rut, also called bucks, can be dangerous from October to early November since it’s the mating season.

“If males have a look where they aren’t afraid or are eyeing you up, it’s a good idea to stay away,” he says. “It’s very rare for them to charge, but it does happen… just check out YouTube.”

Boonstra adds that the City of Toronto’s Biodiversity in the City series is a great resource for those wondering what mammals and birds they can see along Highland Creek and the Rouge River.

One of the more obvious signs of autumn is migrating birds, adds Associate Professor Jason Weir, who does research on the biodiversity of new world birds at U of T Scarborough.

“There are big wetlands where the Rouge empties into Lake Ontario so you will see all sorts of birds,” he says.

At this time of the year birds, many of which live in pairs or small groups during the summer, aggregate in flocks in order to migrate south. In the forests along the Rouge there are songbirds such as warblers, sparrows and blue jays that can be seen, he says.

On recent class trips, Weir says, he’s come across wood ducks, gadwalls and northern shovelers as well as great egrets and caspian terns in the wetlands.

Walking through the Rouge, you may also come across fish like migrating chinook salmon and rainbow trout that have been stocked and are now naturalized in the area, says Nicholas Mandrak, an associate professor at U of T Scarborough who is an expert on freshwater fish.

“There’s a good chance of seeing these species, particularly after a rainfall, which triggers them to swim upstream,” Mandrak says.

Fall Colours

What's with those colours? Sean Thomas on the science behind this year's fall foliage

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Autumn in the Rouge: changing colours is just the start, U of T experts say