Trees with a view of Italy’s Mount Etna behind.
In 2001 on Italy’s Mount Etna, a thin line of pine trees started turning greener and greener, suggesting they were photosynthesizing more than their neighbors were. Credit: Alessandro Squassoni/Pixabay

In 2001 on Italy’s Mount Etna, a thin line of pine trees started turning greener and greener, suggesting they were photosynthesizing more than their neighbors were. The change was subtle enough that Sicilian hikers wouldn’t have noticed a difference, but infrared satellites recorded the enhanced vegetation over two growing seasons.

The trees occupied a narrow band on the mountain, about 30 meters wide and 2 kilometers long. When Etna erupted along the same strip in November 2002, the trees were destroyed. Afterward, experts wondered whether these supergreen trees predicted the future flank eruption.

A team of researchers investigated the area around the eruption and found that volcanic steam may have saturated the soil, providing the roots with extra moisture and causing the pine trees to turn greener. The authors published their results in Ecohydrology and suggested that their findings could help pinpoint the outflow spot for future flank eruptions as well as reconstruct eruptions from the past.

The Thin Green Line

As Europe’s most active volcano, Etna is extensively studied and monitored. In 1973, Italian researchers poring over Skylab data noticed a line of enhanced vegetation. Months later, Etna erupted beneath those trees—just like what happened in 2001. The observations, published in 1975, prompted the current research.

Nicolas Houlié, a geophysicist and a coauthor of the recent study, scoured satellite maps for more evidence of enhanced vegetation as part of his postdoctoral research. He found a line of greener trees on Etna from 2001 and 2002 as well as a similar line from 2001 on Mount Nyiragongo in the Democratic Republic of the Congo, which erupted a few months later.

“It took us 3 days to find 20 trees we could agree on” because each expert weighed in on the tree size, soils, and location that affected the cores.

To learn more about Etna’s arboreal enigmas, Houlié and his colleagues collected tree cores with support from the Swiss National Science Foundation. They focused on surviving trees within 150 meters of the flank eruption. “It’s very, very multidisciplinary work,” Houlié said of the research team, which included experts in forestry, soil science, volcanology, biochemistry, and ecology. “It took us 3 days to find 20 trees we could agree on,” he said, as each expert weighed in on the tree size, soils, and location that affected the cores.

With samples in hand, the team moved to the lab. A biochemical analysis showed that carbon isotopes in the tree rings were similar for both the control period (1992–1996) and the eruptive period (1999–2003). The signal didn’t match the expected values for deep-origin carbon, making carbon dioxide an unlikely influence. However, the oxygen-18 isotope dipped during the 2003 growing season, indicating the influence of volcanic water vapor in the soil. The authors speculate that the destroyed trees may have briefly benefited from volcanic steam saturating the soil.

Blasts of the Past

In theory, a scientist could check infrared satellite images for a line of greener-than-usual trees to see where rumbling volcanoes might erupt all around the world. But there’s still uncertainty over what spurs the increase in photosynthesis. Volcanic vapor could cause the greening, as the study suggested, but lava could have altered the soil chemistry instead. “There’s a very fine balance for lava flow to be beneficial for the trees,” Houlié said.

However, some experts say it is unlikely that enhanced vegetation will be useful for predicting future eruptions because a line of green trees does not occur on every volcano before every eruption. But “you should use everything that is available,” said Erik Sturkell, a geophysicist at the University of Gothenburg who was not involved in the research. Greening trees are at least “a very good way to date previous eruptions in places where we do not have a historic record.”

Isotopes could help reconstruct historic eruptions in places like Indonesia and Alaska, where volcanoes are common but written records are scarce.

According to coauthor Paolo Cherubini, a dendroscientist at the Swiss Federal Institute for Forest, Snow and Landscape Research, this study demonstrates that preeruptive volcanic activity influences tree chemistry, and any new volcanic indicator is useful. “To me, the most important thing is that oxygen-18 may be used as an indicator of past volcanic eruptions,” Cherubini said. Isotopes could help reconstruct historic eruptions in places like Indonesia and Alaska, he said, where volcanoes are common but written records are scarce.

That’s an intriguing prospect, Sturkell said. “It would be interesting to see the spatial distribution of this anomaly, if it dies out as you get further away.” Researchers will need to test other theories to illuminate the volcanic processes causing trees to get greener. “One anomaly is good. Two is better, and especially if it’s correlated to an eruption,” Sturkell said.

—J. Besl (@J_Besl), Science Writer

Citation: Besl, J. (2021), Supergreen trees can signal sites of eruptions, Eos, 102, https://doi.org/10.1029/2021EO210590. Published on 8 November 2021.
Text © 2021. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.