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Toronto (CTV Network) — Anyone who lives in a part of the country with trees that shed their leaves in the fall – like beeches, birches, oaks, ashes and the ever popular maple tree – knows to expect a brilliant display of yellow, orange and red foliage each October.
Some provincial governments, like those of Quebec, Ontario and Nova Scotia, even maintain official fall foliage maps that help pinpoint the best spots to see the changing leaves while they’re looking their best and brightest.
However, scientists say shifting or intensifying weather conditions brought about by climate change could increasingly alter when this colour show begins each year, how long it lasts and how brilliant it is.
“Climate change has different impacts in different areas of Canada and globally,” said Ingo Ensminger, a professor of plant physiology at the University of Toronto in a phone interview with CTVNews.ca. “There’s this idea that the climate is just warming, it’s getting hotter and drier, but what we have instead is actually a mosaic of events that will change. It’s not just black or white.”
WHAT NORMALLY HAPPENS
There’s a lot of chemistry behind the phenomenon that gives fall its name, and the chemical processes that cause the leaves on deciduous trees to turn yellow, orange and red before they fall off are heavily influenced by a whole host of environmental factors.
At baseline, in perfect conditions, the process behind this colour display – known among scientists as autumn senescence – is triggered by the dropping temperatures and shorter daylight hours of autumn.
“There’s a process that is highly regulated, that is synchronized with environmental cues and that leads to a somewhat predictable repetition of a certain process,” Ensminger said.
“Towards the autumn or the end of the growing season, trees and plants in general actually start to cease growth and photosynthesis. At this point they stop taking up CO2 from the atmosphere which is the substrate for photosynthesis, biomass production and growth. ”
A tree’s leaves appear to change from green to yellow and orange in the fall as it breaks down and reabsorbs the chlorophyll responsible for photosynthesis during the growing season. In reality, the yellow pigments we see in the fall – called carotenoids – are present in the leaves throughout the year, and simply become visible once the eye-catching green of chlorophyll fades away.
“So it’s not that they all of a sudden show up here,” Ensminger said. “They’ve just become visible because chlorophyll is the first pigment that starts to degrade.”
Along with chlorophyll, the tree will reabsorb any other nutrients it can break down within its leaves, and then it will shed them.
On some trees, like sugar maples, the leaves turn red in the fall. Unlike with yellow and orange leaves, this actually is the result of trees producing a new pigment specifically for fall, which scientists believe helps protect the chlorophyll-depleted leaves from sun damage long enough for the trees to finish reabsorbing nutrients from them.
All of this happens predictably in response to the days growing shorter and the temperatures dropping each fall.
However, other factors can affect when this process begins, what colours emerge and how long the fall foliage lasts.
For example, bright, sunny days are more likely to yield bright red leaves, explains Loïc D’Orangeville, associate professor of forestry and environmental management at the University of New Brunswick.
“Usually when you have nice sunny days in the fall, you tend to have brighter colour, that’s the general understanding,” D’Orangeville said. “And it’s logical. If reds are produced to act as a sunscreen, then if you have lots of sun you probably have these brighter colours.”
An unseasonably warm spring or fall can speed up or delay the fall colour change by several weeks, too.
“There’s actually been a cool study out in New England that looked at that, and they found that if you have a warm (September) it pushes back the fall colours,” D’Orangeville said. “And if you have a warm spring, it makes the fall colouring (begin) earlier for some species.”
That study, published by Harvard University scientists in 2013,(opens in a new tab) looked at long-term data for eight tree species in a New England hardwood forest. It found that sensitivity to temperatures at specific times of the year depended on the tree species, but that for most species, a warm September delayed leaf colouring, and in some cases, a warm May advanced colouring.
In the case of either an especially warm spring or fall, the researchers found that while the timing of the colour change was thrown off, the amount of colour displayed by the trees increased.
Summer is a different story, though. Drought conditions in the summer can cause trees to drop their leaves prematurely, before they have a chance to go through autumn senescence. In these cases, the leaves simply wilt and fall off, both D’Orangeville and Ensminger agree.
“If the tree really undergoes extreme heat and drought stress during the summer, the leaves will simply wilt instead of going through this very well regulated and orchestrated process of leaf senescence,” Ensminger said. “And you will see that the leaves will wilt and then eventually fall down.”
THE CLIMATE CHANGE FACTOR
All of these conditions – basically everything except the lengthening and shortening of daylight hours – can be influenced by climate change.
The effects of climate change aren’t uniform and vary from one part of the country to another. Some regions – such as parts of the Prairie provinces and British Columbia(opens in a new tab) – are experiencing increasingly drier conditions and more frequent droughts as a result of climate change.
Other regions – such as the Great Lakes and southern Quebec – are actually seeing more precipitation,(opens in a new tab) which Ensminger said could be a boon for deciduous trees.
“A lot of tree species that are at the moment somewhat limited by, say, not enough annual precipitation that will actually benefit from that and they will grow much better,” he said.
However, the warming trend in Canada also creates more opportunities for forest pests native to warmer climates to the south – like insects and fungi – to migrate north. Pests like the emerald ash borer and the spongy moth defoliate trees like the ash and oak, maple, birch, alter and hawthorn.(opens in a new tab)
“The white ash is beautiful…the leaves turn purple in the fall and the green ash that we have turns bright yellow,” L’Orangeville said. “And so these are two cool species that are getting wiped out completely right now by emerald ash borers.”
L’Orangeville said even native pest species can cause extreme damage when their usual host trees are weakened by stressful conditions such as drought.
“Big infestations will be triggered by climate anomalies,” he said, “like the big mountain pine beetle infestation that destroyed one of every ten pines in B.C. was triggered by drought that weakened the trees.”
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