While the weather might seem a little bit like fall, one of the most notable signs of autumn here in BC is the leaves changing colours.
We’ve spotted some trees in and around Vancouver showing signs of changing colour, even though the fall season is over a month away.
Isn’t it too early? Well, yes and no.
According to Dr. Santokh Singh, a professor of teaching at the UBC Department of Botany, there could be several reasons why the leaves in your neighbourhood are changing colour, some unrelated to seasonal changes. Singh has been studying this for a number of years.
We’re talking about seasonal leaf senescence, a fancy way of describing the physiological reasons for the seasonal changes in tree leaf colour.
“Normally, in most of these broad-leaf trees, those leaves will only turn to yellow, red or purple at the end of September or in the fall season.”
That’s not always the case; it boils down to plant hormones called cytokinins, which Singh calls greening hormones. In contrast, ethylene and abscisic acid are the senescence-causing hormones in plants.
Environmental factors, such as drought, light and temperature conditions, or nutrient deficiency, appear to impact the relative levels of these hormones.
“The early leaf colour change, as observed in some trees in Vancouver, could be the result of one or more of these environmental stress conditions. So that can cause the change in the production of some of these hormones,” Singh said.
“Nitrogen deficiency can also be one of the reasons.”
Singh added, “Nitrogen is a key nutrient that plants absorb. It’s important for the synthesis of the chlorophyll pigment, which is what keeps the plant leaves green.”
“Very high temperatures can also cause the browning or yellowing of the leaf,” Singh said, adding that he has observed it both in his laboratory and in the field.
We asked if climate change could impact the seasonal change in leaf colours. Singh touched on the heat dome in 2021 when temperatures reached over 40˚C in parts of BC, which had a significant impact due to heat and dehydration stress. He added that more studies must be done to have a complete picture.
With all that in mind, the leaves of a tree in your neighbourhood could change colour, and the same tree species a block away might not see its leaves change due to the tree’s surroundings, including the relative availability of water, light and nutrients. Some of these factors have been tested in the field.
Have you noticed the leaves already changing colour in your BC neighbourhood? Let us know in the comments.