Why unseasonably warm weather could endanger fall's brilliant colours

An unseasonably warm fall could keep leaves from reaching their normal brilliance this year, according to a retired biology professor.

"I suspect that we might not just get those bright bold colours if it continues, but it's still early," said former Acadia University professor David Kristie.  

He said cool nights and sunny days in the fall help bring out the red pigments in leaves. Without cool nights, those reds could lose some of their depth, and the orange leaf colours that come from the mixing of the red and yellow pigments could also suffer.

"If this warm weather continues for too much longer … the leaves are going to ultimately fall off before we can get those beautiful colours," said Kristie. 

He said the only colour that won't be affected by the warm weather is yellow.

The science of colour change

Deciduous trees change colour in the fall because as the number of daylight hours diminishes, chlorophyll, the chemical that gives leaves their green colour, is not replaced. Once the chlorophyll starts to fade away, the yellow and orange pigments start to show through. 

There's debate about why some tree species have leaves that turn red in the fall. Kristie said the leading hypothesis suggests that the red pigment protects the remaining photosynthesis going on in a plant as autumn sets in — and photosynthesis is inhibited by cool nights. So the red pigment only deepens if cool weather rolls in.   

Cold snaps could help

The long-range weather forecast suggests it will remain unseasonably warm for the coming three months, according to CBC meteorologist Kalin Mitchell. 

But he said there's still hope.  

"Even if the prediction is for an above-normal fall as far as temperatures go, that doesn't preclude that there could be some cold snaps within those three months," Mitchell said.

"Don't lose hope yet. We may still run into some colder nights that will really allow those brighter colours to pop." 

Mitchell also said there were no prolonged drought conditions in much of Nova Scotia this summer, so the trees aren't overly stressed. Sometimes a lack of water can cause trees to shed their leaves early. 

Celtic Colours not concerned

The fall colours are one of the reasons people travel to Cape Breton to take in the Celtic Colours International Festival, which runs Oct. 6-14 this year. 

While organizers aren't thrilled with possibility of muted colours, they don't think it will hurt attendance at the festival. 

"The colours are certainly part of the whole thing, but I don't think that a variation in the brilliance of the colours is going to have much impact on ticket sales specifically," said Dave Mahalik, the communications officer for Celtic Colours. 

Several of the festival's shows have already sold out.

The festival plans to give a bit back to Mother Nature this year, as it has partnered with the provincial government and Strathlorne Forest Nursery to plant a red maple tree for every concert ticket it sells. Last year, Celtic Colours sold about 22,000 tickets. 

"We can ensure that we do have the foliage for years to come and, of course, it's good for the environment.… It's a nice legacy to have," said Mahalik.