When the Sun erupts, it can fire streams of high-energy particles toward Earth, triggering solar storms that disrupt satellites, power grids, and communication systems.
We can track these events today, but the most extreme storms happened long before modern instruments existed.
To find them, scientists turn to an unlikely archive: tree rings. New research, led by a team from Northern Arizona University, shows that trees don’t just record solar storms – they sometimes blur them.
How trees grow, store carbon, and build wood can smear these signals across years, making plant biology a crucial piece of the puzzle in reconstructing Earth’s space weather history.
Solar storms leave carbon clues
High-energy particles from solar storms strike Earth’s atmosphere and create a rare radioactive form of carbon called carbon-14.
Plants absorb carbon dioxide from the air during photosynthesis, and trees lock carbon inside wood during growth. Each tree ring represents one year, making rings a natural timeline.
Sudden rises in carbon-14 levels inside rings mark rare solar storms known as Miyake events. Evidence of such events appears in tree rings across many continents.
Some past storms were far stronger than any recorded in modern times. Learning how trees store carbon helps scientists estimate how strong future solar storms could be.
Trees record carbon slowly
Tree rings work as natural archives, but wood formation does not happen instantly after carbon enters leaves. Carbon follows a complex path inside a tree.
Photosynthesis creates sugars, and trees store part of those sugars as nonstructural carbohydrates. Storage can last months, years, or even decades before use in new wood.
“Although tree rings are one of our best tools for reading Earth’s history, they’re not perfect instruments,” said study co-author Amy Hessl, from the Department of Geology and Geography at West Virginia University.
“This paper shows how tree biology shapes the stories they tell.”
Because stored carbon mixes with newly absorbed carbon, radiocarbon spikes from solar storms can spread across more than one ring. Such mixing makes exact timing harder to detect.
Growing seasons shift solar storm signals
Tree species absorb carbon at different times of year. Leaf growth, leaf loss, and photosynthesis vary across climates and species.
Deciduous trees only photosynthesize during leaf seasons. Evergreen trees keep needles year round but still follow seasonal patterns due to temperature and light limits.
Carbon uptake also depends on local climate. Forests in warmer regions often absorb carbon over longer periods than forests in colder regions.
Variation in growing season length changes when trees sample atmospheric carbon. Solar storm signals may appear earlier or later depending on tree growth timing.
Wood formation adds another delay
Wood growth, called xylogenesis, begins only after certain temperature and moisture conditions appear. Some trees finish wood growth early in summer, while others continue into autumn.
Earlywood forms first, and latewood forms later. Carbon used for earlywood often comes from stored reserves rather than fresh photosynthesis.
These timing differences explain why solar storm signals can appear across several rings. A storm happening late in one year may show its strongest signals during growth periods in later seasons.
Carbon storage shapes records
Trees store nonstructural carbohydrates as sugars and starch. Storage occurs in roots, stems, and branches.
Storage capacity differs across species. Deciduous trees often store larger carbon reserves than evergreen trees. Some stored carbon can remain unused for many years.
Research shows stored carbon may support respiration, root growth, leaf growth, and wood formation long after initial absorption.
Mixing old and new carbon blurs sharp radiocarbon signals. During extreme events like Miyake events, this blurring becomes more noticeable.
“Understanding how trees acquire carbon from the atmosphere, store it for future use and then use it to grow new wood is critical,” said Mariah Carbone, associate research professor at Northern Arizona University.
“The biology determines how faithfully the atmospheric signal is preserved.”
Geography reshapes storm records
Tree anatomy also affects carbon signals. Ring porous species often rely more on stored carbon for early growth than diffuse porous species.
Latitude also plays a role. Higher latitude regions receive stronger carbon-14 production during solar storms due to Earth’s magnetic field.
Growth timing and storage patterns combine with geographic effects to shape tree ring records.
Such variation explains why scientists observe small timing differences when comparing tree species or regions. Understanding these patterns allows more accurate reconstructions of solar storm history.
Benefits beyond space weather
Improved knowledge of tree carbon pathways benefits more than space weather research. Radiocarbon dating relies on accurate carbon timelines.
A better understanding of carbon storage improves dating of archaeological sites and historical artifacts.
Study co-author Andrew Richardson is an ecologist at Northern Arizona University.
“It’s amazing that one way to improve our understanding of solar storms and solar physics is to better understand tree growth processes, which in turn is critical for improving how we use radiocarbon for carbon dating,” said Richardson.
Preparing Earth for extreme storms
Research described in New Phytologist forms part of a larger National Science Foundation effort.
Scientists aim to learn how extreme past solar storms were and what similar events could mean for modern technology. Satellites, astronauts, and power systems remain vulnerable to such storms.
Trees continue to act as silent witnesses of solar storm activity. Learning how wood records carbon helps scientists read that history with greater clarity and confidence.
The study is published in the journal New Phytologist.
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