Around 14 hours before a partial solar eclipse passed over the Dolomites in Northern Italy, a group of spruce trees showed a sudden, synchronized increase in electrical activity. A widely publicized paper by Chiolerio et al. claimed that the trees were anticipating and preparing for the impending solar eclipse. In an opinion paper publishing February 6 in the Cell Press journal Trends in Plant Science, researchers debunked this claim by examining the evidence and offering a simpler explanation: a local thunderstorm coincided with the trees' increased electrical activity, during which a cluster of lightning strikes struck near the study site.
"To me, this paper represents the encroachment of pseudoscience into the heart of biological research," says first author Ariel Novoplansky, an evolutionary ecologist at Ben-Gurion University of the Negev in Israel. "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."
It is well documented that plants can perceive and respond to changes in environmental conditions and can even anticipate and prepare for future competition from currently harmless neighbors or impending stresses. However, this type of response only occurs when the anticipated event poses a significant challenge and is tightly correlated with predictive cues. In the case of the solar eclipse in question, the event was not significant enough to warrant anticipation, the researchers say, because it was short and involved only a small reduction in light—less of an interruption than some passing clouds.
"The eclipse only reduced light by about 10.5% 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."
Even if the solar eclipse was significant enough to warrant a preemptive response, the researchers showed that the trees would not have had the ability to anticipate its coming. The solar eclipse was the 53rd in the Saros 124 sequence, a series of eclipses that occur every 18 years, 11 days, and 8 hours. Chiolerio et al. showed that older, larger trees displayed greater increases in electrical activity compared to younger trees and claimed that this showed that the older trees were communicating with the younger trees based on their previous eclipse experiences. However, the debunkers note that each solar eclipse is unique in its path, magnitude, and duration, so even if older trees could "remember" previous eclipses, that information wouldn't help them anticipate future eclipses. Additionally, they note that the gravitational changes associated with solar eclipses—the signal the trees were supposedly responding to—are similar in magnitude to the gravitational differences that occur during a new moon and are therefore uninformative.
