The amount of remaining warmth for the summer is predictable by July, meaning trees can prepare to make seeds or nuts, as well as new buds for the following spring.Fred Thornhill/The Canadian Press
For those who live in the Northern Hemisphere, June 21 marks the day when summer is officially underway.
Yet a new study suggests that the annual summer solstice may be the moment when plants are making a subtle calculation based on temperature to determine that it’s time to start closing up shop for the year.
The finding could upend expectations about how plants will respond to shifts in their growing season as Earth’s climate warms.
“Surprisingly, what we found is that, on average, the solstice is a very optimal time for a plant to make a decision,” said Elizabeth Wolkovich, an associate professor of forest and conservation science at the University of British Columbia, and senior author on the study, published last week in the Proceedings of the National Academy of Sciences.
On the face of it, the idea seems more than a little counterintuitive.
The solstice marks the time when daylight hours are longest in the Northern Hemisphere. But it takes time for the atmosphere to respond, so the warmest days of the year generally occur about a month or more later.
So, shouldn’t plants keep making hay – literally – while the sun shines?
Earlier work suggests this is not necessarily the case. For example, a Polish-led study published last year reported that the solstice somehow acts as a starting gun that determines when European beech trees will produce nuts in a remarkably synchronized fashion across a geographic range that extends some 2,000 kilometres.
Dr. Wolkovich said she initially thought this didn’t make sense and set out to investigate the question with Victor Van der Meersch, a post-doctoral researcher.
They began by ruling out the idea that plants are somehow identifying the longest day of the year based on daylight hours – a challenge even for humans unless they have clocks and a knowledge of astronomy. Instead, they focused their study solely on temperature, which correlates well with plant growth.
“A warm season is generally a better season,” Dr. Wolkovich said.
Looking across Europe, the study used meteorological data on each day of the growing season over many years to average out differences. Not surprisingly, the warmest days occur, on average, well into the summer. But the highest variability in temperature occurs long before, in the early weeks of spring.
This creates a trade-off for a plant that is adjusting its growth based on temperature. Conditions in spring are too uncertain to commit to a growth strategy for the whole season, but by July the amount of remaining warmth for the summer is far more predictable. The study found that the increasing predictability is balanced by the decreasing days of remaining growth potential around the time of the summer solstice.
Around June 21, Dr. Wolkovich said “plants have most of the information about what’s going to happen,” even though they have not yet experienced most of the warmth they will get for that season.
This means that trees can set about preparing to make seeds or nuts, as well as new buds for the following spring.
The timing of the trade-off varied somewhat with latitude but was always relatively early in the season. And it applied both in the distant past and the future when the researchers used climate models to test the idea.
The study suggests that even if climate change lengthens the growing season in the fall at a given latitude, trees that are native to that region will not be able to take advantage of it because they will have already committed to when they shut down for the year.
Richard Primack, a biologist at Boston University who was not involved in the UBC study, called the work a valuable contribution. But he added that more investigation is needed to deduce how the effect might manifest itself among various plant species as the planet warms.
“It is almost certain that different tree species, such as beeches, pines, oaks and maples, will each have their own combinations of environmental cues and ways of responding to future climate change,” Dr. Primack said.
Editor’s note: This story has been corrected to state that Victor Van der Meersch is a post-doctoral researcher, not a PhD student. It has also been updated to correct the spelling of senior researcher is Elizabeth Wolkovich's surname.