During a thunderstorm, the charge separation in the cloud induces an opposite charge in the ground below. That charge migrates upward through whatever conductor reaches highest — and in a forest, the highest conductors are trees. The charge concentrates at the sharpest points it can find, which are the tips of leaves, and when the local electric field exceeds the breakdown threshold, the air ionizes. The result is a corona discharge: a faint ultraviolet glow that dances from leaf tip to leaf tip, invisible to the naked eye but measurable with UV-sensitive cameras.
McFarland and colleagues (Geophysical Research Letters, February 2026) captured the first direct outdoor observations of tree coronae under live thunderstorms, using a camera system filtered to a UV band that sunlight — blocked by ozone — cannot reach. They observed sweetgum, loblolly pine, and other species across multiple U.S. states. The coronae are weak — temperatures only slightly above ambient air — but they are chemically violent. Each discharge generates hydroxyl radicals (OH), the atmosphere's primary oxidizing agent, at rates that may exceed all other known production mechanisms near the canopy. The discharges also damage the leaves they occur on, gradually degrading the upper canopy over years of storm exposure.
The tree doesn't do anything. It doesn't generate the charge, direct the discharge, or catalyze the chemistry. It provides geometry: a tall structure with pointed extremities positioned between the charged cloud and the charged ground. A metal pole of the same height with the same tip geometry would produce the same coronae. A dead tree would work. The biology's contribution is not biological — it is architectural. The tree grew tall for light. The leaves developed points for drainage. Neither adaptation has anything to do with atmospheric electrochemistry. But the combination — height plus points — makes the tree the discharge surface for a process that would otherwise occur at lower intensity on flatter ground.
The atmospheric cleaning that results is substantial. Hydroxyl radicals break down methane, volatile organic compounds, and other trace gases. The forest canopy during a thunderstorm is an inadvertent air purifier — not because trees evolved to clean air during storms, but because the physics of corona discharge requires a pointed conductor at elevation, and trees happen to be that. The function is real. The intention is absent. The tree mediates a geophysical process as a geometric accident.
This is a common pattern mistaken for a rare one. When a biological structure participates in a physical process, the default assumption is adaptation — the organism evolved to do this. Sometimes that's true. But sometimes the organism's contribution is its shape, not its biology, and the shape arose for entirely different reasons. The tree cleans the air during thunderstorms because it is tall and pointed. It is tall and pointed because it competes for sunlight and sheds water. The atmospheric chemistry is a downstream consequence of a geometry optimized for something unrelated. The conductor doesn't know it's conducting.