Summer Solstice Triggers Mass, Synchronized European Beech Tree Reproduction, Study Finds

A common “old world” tree, European beech can be found across Europe — from southern Scandinavia and Spain to Sicily and northwest Turkey.

A new study by an international team of researchers has found that the summer solstice triggers synchronized beech tree reproduction all over the continent, influencing ecosystem functions.

“We got inspired by a recent Science paper where researchers from Switzerland found that the effects of temperature on leaf senescence switch at the summer solstice. The summer solstice is the longest day of the year, and happens at the same time anywhere in the Hemisphere,” said Dr. Valentin Journé, lead author of the analysis and a postdoctoral researcher at Poland’s Adam Mickiewcz University, in a press release from University of Liverpool.

The researchers examined the relationships between the seed production of perennials like the European beech and weather patterns, as well as looked at how trees synchronize their reproduction across thousands of miles.

Earlier research by the team had revealed that an external factor like weather triggered the synchrony, but the mystery remained as to how the beech — which grows in highly diverse climates —  carried this out.

The team looked at small changes in the beech trees’ temperature responses and concluded that the summer solstice was the trigger.

“A celestial cue that occurs simultaneously across the entire hemisphere is the longest day (the summer solstice),” the authors wrote in the study. “European beech abruptly opens its temperature-sensing window on the solstice, and hence widely separated populations all start responding to weather signals in the same week. This celestial ‘starting gun’ generates ecological events with high spatial synchrony across the continent.”

The study, “Summer solstice orchestrates the subcontinental-scale synchrony of mast seeding,” was published in the journal Nature Plants.

“The sharp response of beech trees is just remarkable. Once the day starts to shorten after the summer solstice, the temperature sensing-window opens simultaneously, all across Europe,” said Jessie Foest, co-author of the study and a Ph.D. researcher in the Department of Geography at University of Liverpool, in the press release. “What’s truly jaw-dropping is that the change in day length that the trees are able to detect is really small – we are talking about a few minutes over a week. Apparently, trees are able to recognise the difference.”

Many perennials only reproduce every few years in order to build up resources and produce an abundance of seed. Ecosystems are greatly affected by the European beech’s wide-ranging, coordinated annual seed production.

“Such large-scale regional synchronisation of seed production by trees has important consequences for ecosystems. Large-seeding years result in a pulse of resources for wildlife, while reproductive failures result in famines for seed-eating animals. When this variation is synchronised at sub-continental scales the consequences include far-reaching disruptions in food webs, including rodent outbreaks, migration of ungulates and birds, and spikes in wildlife-borne human diseases,” the press release said.