Fall colours: Why summer's weird weather has affected the leaves

Canada has had some weird weather this year and that has had an impact on the fall colours now on display in some areas. As well, historic patterns of when trees are at their most colourful, and how colourful, may be shifting due to climate change.

Before autumn, the healthy leaves on deciduous trees get their green colour from the chlorophyll molecules in their microscopic factories, or chloroplasts, that convert water and carbon dioxide into sugars, other carbohydrates and oxygen.

The chlorophyll hides the yellow and orange pigments present in the leaves because it is so intense. In autumn, trees stop replacing the chlorophyll in their leaves, which then breaks down, allowing the other pigments to become more and more visible.

The proportion of pigments varies by species. Maples have lots of anthocyanin, for example, which gives a red colour. Anthocyanin is abundant in leaves in early spring but as the leaves grow it breaks down.

"Anthocyanins are absent from the leaves in summer and are only produced by the leaves as chlorophyll decomposes," Carleton University biologist Root Gorelick writes in an email to CBC News.

Scientists debate why some autumn leaves on some tree species turn red. "The best guess is that some of the pigments, such as carotenoids [which produce the yellow and orange pigments], help absorb sunlight, while carotenoids and anthocyanins help protect the chlorophyll from UV damage, which leaves are especially prone to in cold weather," Gorelick explains.

Scientists call this overall process senescence. In fall, a tree needs to recover some of the huge store of resources it put into growing leaves, Janice Cooke, a biologist at the University of Alberta explains.

That's what's happening between the time when the leaves begin to change and when they fall off the tree. "A long senescence is good for the tree," Cooke says, because it is recapturing more resources from its leaves.

"Large amounts of nutritive reserves must be recovered for winter storage so they can boost the array of activity which begins with new growth each spring," Robert Guy and Jodie Krakowski of UBC's Department of Forest Sciences write in the journal, Davidsonia.

Shorter days, colder temperatures

Trees get their cues about when to start their shutdown process from the interaction of day length and overnight temperatures.

"Light influences the timing, magnitude and degree of leaf colour change," Guy and Krakowski write.

The temperature threshold to initiate change, however, differs among tree species, according to Cooke.

And leaves at the top and outer edges of a canopy usually have far brighter colour than the leaves that get less intense sunlight, according to Guy and Krakowski.

Early spring, drought affect colours

The early spring this year in some parts of Canada can have an impact on the process and the drought that took hold later in parts of Ontario and Quebec should be leading to earlier fall colour, Cooke said, adding that it "may be not so brilliant."

She says that's because during a hot, dry spell "some of that senescence program would have been initiated earlier." The conditions were so hard on the trees that they began an early recapture of resources from their leaves.

Once senescence is initiated, the best weather for fall colours is "nice, crisp nights and warm, sunny days." Cooke explains that the combination is a cue to the tree to get on with the transition but that it can do it slowly.

A sudden plunge in overnight temperatures can result in trees all changing colour at once, since their different temperature thresholds for shutting down will all be crossed.

Canada's fall colours

Alberta had plenty of rainfall this year through the active growing season and now is experiencing a prolonged fall with good daytime temperatures which turn cooler at night. "We have exactly the right conditions to give us fall colour," Cooke said from Edmonton.

"We are experiencing the best fall colour since we arrived here in 2005."

She has heard from family in the B.C. Interior who say that the fall colours there are also looking good.

For those who are interested, a number of provincial websites offer up-to-date reports on fall foliage based on local conditions.

Climate change and fall foliage

Although day length, light, temperature, ground moisture and insects all affect fall colours, scientists say that climate change might also be having an impact. They cite a trend of earlier springs and warmer autumns.

"We have evidence that the growing season of the trees is being influenced each year by the climates that they experience," Cooke says.

But she says scientists don't have a good handle on whether there is "a disconnect happening between trees' sensing of conditions and the climate." Trees are genetically programmed to respond to both day length, a pattern that is not changing, and temperature, for which the pattern appears to be changing.

The natural rhythms for insects, and the birds that eat them, are also influenced by those patterns and so Cooke says there needs to be more research into the interaction of environmental cues to different species.

Her lab at the University of Alberta is looking into that but at this point scientists need to gather more data.

Elisabeth Beaubien, Alberta co-ordinator of PlantWatch, tells CBC News that the group is looking at how plants are responding to climate change. Through PlantWatch, volunteers — or citizen scientists — record and report flowering times for about 40 plant species, part of an attempt to "identify ecological changes that may be affecting our environment."

The program's 600 volunteers only track spring changes, but, Beaubien says, with more resources PlantWatch would be able to track changes in fall as well.

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