Why leaves may change earlier than usual

If you’ve noticed trees in parts of Canada have been changing colours earlier than usual in recent years, you’re not alone, according to a botany expert who spoke with CTVNews.ca.

Santokh Singh, a biology professor at the University of British Columbia and author of numerous studies on the phenomenon, noticed the leaf changes in B.C.’s Lower Mainland region, as well as parts of Vancouver Island.

“I have seen some trees that have been changing colour a bit earlier. And that has been since the last three or four years,” Singh said in a video interview with CTVNews.ca.

Singh specializes in senescence, or the seasonal effects on leaves. He says he has observed some trees as especially sensitive to early colour changes in recent years — in particular, those that struggle to grow.

“It’s just sometimes the older trees or trees, which are not doing well, which are not growing well in the previous season or couple of seasons, they could start to show early change in leaf colour,” he said, noting that more research was needed in the area.

What is leaf senescence?

Leaf senescence is the change in leaves, both seen and unseen, that signal the transition from a tree’s growing season to dormancy, Singh said.

In addition to the change in colours, leaves also lose proteins and undergo many changes in their genes and hormones during the process.

Trees then recycle many nutrients from the leaves for use for later growth the following spring.

Why leaf senescence occurs

As days become shorter this fall, decreased exposure to light will impair the ability of trees to make their food through photosynthesis. But numerous factors can spark leaf senescence at any time during the year, Singh said, usually related to environmental stresses such as a lack of water.

“When we don’t really see much precipitation or much rainfall, plants start to undergo what we call a drought senescence, and that can also trigger change in the colour a bit earlier,” he said.

Issues with soil quality, such as poor drainage or nutrition — particularly a lack of nitrogen — can spark the process as well. Trees by the roadside, which may have compacted soil with insufficient nutrients and troubled water conditions, are vulnerable to premature senescence.

“That could also mean that there will be a lack of oxygen in that soil surrounding the tree roots,” he said.

Similarly, extreme low or high temperatures can trigger the process, though it’s unclear whether climate change is a factor, Singh said.

“More research is needed to really see whether this phenomenon is happening more and more as we are proceeding from one year to the next.”

Should we be concerned?

With the premature change in leaf colour, a tree’s ability to survive winter could be affected. Those with leaves that change colours earlier will have a shorter growing season — green leaves, still filled with chlorophyll, are vital to producing sugar for photosynthesis, Singh explained.

Singh says he isn’t aware of any long-term studies indicating major issues that could affect humans.

Timing of leaf colour change

Because of the numerous factors involved, colour changes can occur at any time, and are difficult to predict, Singh says.

“It all depends again on how severe these environmental stresses are,” Singh said. “Let’s say that if there’s a year of a normal temperature, normal type of rainfall, then I think we might not really see this type of early leaf change in the leaf colour compared to the years when we start to see drought stresses or very high or very low temperatures.”