Norway spruce trees did not predict a solar eclipse, researchers say

A controversial study published in April last year reported a remarkable observation: a stand of Norway spruce trees (Picea abies) seemed to ‘synchronise’ their electrical signals before a partial solar eclipse.

A fresh explanation now suggests something rather less extraordinary was happening.

After reviewing the evidence, ecologists Ariel Novoplansky and Hezi Yizhaq of Ben-Gurion University of the Negev in Israel have put forward an alternative account.

They argue that the electrical activity recorded in the trees was driven by a fall in temperature, a thunderstorm moving through the area and several local lightning strikes. Previous research has shown that each of these factors can provoke comparable signalling reactions in plants.

"To me, [the previous study] represents the encroachment of pseudoscience into the heart of biological research," says Novoplansky.

"Instead of considering simpler, well-documented environmental factors, like a heavy rainstorm and a cluster of nearby lightning strikes, the authors leaned into the more seductive idea that the trees were anticipating the impending solar eclipse."

Norway spruce trees and the solar eclipse claim

In October 2022, researchers recorded what they called "individual and collective bioelectrical responses to a solar eclipse" in a forest in the Dolomite mountains of north-eastern Italy. Older trees produced stronger signals both ahead of and during the eclipse.

The original researchers suggested that this activity could indicate that older trees had transferred experience from earlier events to the wider forest. Drawing on previous studies, they proposed that the spruce trees detected an approaching environmental shift and organised a response - in this instance, to a solar eclipse.

Novoplansky and Yizhaq disagree in their new paper. In their view, the thunderstorm was a much more plausible environmental trigger, and they outline several reasons they believe the first research team arrived at an incorrect interpretation.

To begin with, solar eclipses differ in their route, intensity and length, meaning older trees could not have drawn on ‘remembered’ information to forecast the next event.

In addition, any gravitational changes that could have offered an early indication would have been extremely small - broadly similar to those associated with a new Moon.

Why the thunderstorm is the likelier explanation

There was also little reason for the trees to mount a coordinated response to the solar eclipse. As it was only partial, the fall in light was similar to that experienced on a cloudy day, and would not have substantially disrupted photosynthesis or other processes.

"The eclipse only reduced light by about 10.5 percent for two short hours, during which the level of sunlight was approximately twice what the trees could practically use," says Novoplansky.

"Frequent fluctuations in cloud cover at the study location change light quality and quantity by much bigger amplitudes."

Novoplansky and Yizhaq also note that the initial study examined just three trees and five stumps, making it far from a comprehensive survey of the forest. They contend that the readings were more likely to reflect separate tree-level responses to lightning than collective cooperation across the forest.

Plants have previously been found to ‘anticipate’ environmental shifts - for example, preparing for drought when early warning signs emerge in the soil. Therefore, the notion that a forest might anticipate a solar eclipse is not entirely without precedent.

Tree electromes remain an emerging field

Even so, as Novoplansky and Yizhaq explain, the hypothesis falls apart on several points.

Research into tree electromes - the charged molecules moving through tree cells - is ongoing. Although this particular forest has prompted debate, the field is still expected to yield further intriguing findings.

"The electrical activity of trees is a real phenomenon, but it's still a nascent field of inquiry," says Novoplansky. "The idea that variations in electrical signals, observable even in dead logs, might encode memory, anticipation, or collective responsiveness requires a few extraordinary leaps, none of which were supported in the study."

"The forest is wondrous enough without inventing irrational yet superficially fantastic claims of anticipatory responsiveness or communication based only on correlation."

The research was published in Trends in Plant Science.