If you’re wondering whether or not fall colours are unusually vibrant this year, they absolutely are.
You might have noticed the bright yellow leaves with orange tips beaming in the sunlight, the multiple shades of green leaves on a single tree and the scarlet red, rich purple and pink trees bursting with pigment.
There’s a scientific explanation for all this. In simple terms, the trees are a little stressed.
“Certainly with the weather conditions, there are certain factors that really amplify some of those colours this year,” said Sean Fox, manager of horticulture at The Arboretum.
Fox said some fall colours changed a bit earlier than usual this year which is most likely a reaction of the drought stress (dry soil and loss of water) that Guelph and other parts of Ontario have experienced.
“We just weren't getting the rain as often as we needed them. When that happens a tree will shut down a little bit early,” said Fox.
“That’s what we experienced a lot of earlier towards the third week of September, a lack of rain, a lot of sunlight and that sunlight was causing the leaves to be very effective at producing their food or their sugars but then those sugars are being converted to anthocyanin, the pigment that shows really bright scarlet red and dark purple.”
He said the plants have not had sufficient water to photosynthesize, which hinders their ability to produce chlorophyll, the green pigment.
“When that production halts, that’s when the other colours that were hidden by the green start to show up,” said Fox.
“Every year there’s something that seems to stand out but this year, in particular, it seems that most species are really kind of showing their best,” said Fox.
He said the best fall colours appear as a result of bright sunny days and really cool nights.
The sunny days help with the anthocyanin pigment and the cool nights trap anthocyanin and slow the release of the pigment energy back into the plant.
Typically when chlorophyll leaves a plant, the carotenoid pigment — which reveals the yellow and dull oranges — begins to show.
“Carotenoids are the same things that give carrots their orange colour,” said Fox.
“Those are there. They're kind of hidden off in the summer by the green.”
Fox said every year, the aspens, and the birch trees and the walnuts will typically turn a yellow colour as a result of carotenoid production. And while maple trees and oak trees produce carotenoids, they’re unable to convert their sugars into anthocyanin which produce the vibrant purples, pink, scarlet reds and fiery orange colours.
“Not all trees can produce anthocyanin, only specific species can,” said Fox.
Fox added that also because of the pandemic, people are likely noticing the colours more because they're spending more time outdoors.
From a climate change standpoint, He said there isn't sufficient evidence to point out that fall colours have changed this year specifically as a result of climate change.
“In the years ahead or the decades ahead, if we end up having on average hotter drier summers, we might see some of that drought stress, causing earlier colour change,” said Fox.
“That earlier colour change is not necessarily a good thing. It means the trees are stressed every single year.”
Fox said the trees are pretty resilient, so the odd year like this is pretty normal and the trees can bounce back the following year.
“It's kind of that combination of different weather factors, and some years it all comes together, at least from our perspective and we get to really enjoy it,” said Fox.
“And that’s what we seem to be seeing with a lot of things this year.”
