Every now and then, some trees apparently just need a jolt. When struck by lightning, the large-crowned Dipteryx oleifera sustains minimal damage, whereas the trees and parasitic vines in its immediate vicinity often wither away or die altogether. That clearing out of competing vegetation results in a nearly fifteenfold boost in lifetime seed production for D. oleifera, researchers estimated.
An Instrumented Forest
“This is the only place on Earth where we have consistent lightning tracking data with the precision needed to know [whether a strike] hit a patch of forest.”
Panama is often known for its eponymous canal. But Barro Colorado Island, in central Panama, is also home to what researchers who work in the area call “one of the best-studied patches of tropical forest on earth.” That’s because cameras and devices to measure electric fields are constantly surveying the forest from atop a series of towers, each about 40 meters high. Those instruments can reveal, among other information, the precise locations of lightning strikes. “This is the only place on Earth where we have consistent lightning tracking data with the precision needed to know [whether a strike] hit a patch of forest,” said Evan Gora, an ecologist at the Cary Institute of Ecosystem Studies and the Smithsonian Tropical Research Institute.
Such infrastructure is key to locating trees that have been struck by lightning, said Gabriel Arellano, a forest ecologist at the University of Michigan in Ann Arbor who was not involved in the research. “It’s very difficult to monitor lightning strikes and find the specific trees that were affected.”
That’s because a lightning strike to a tropical tree rarely leads to a fire, said Gora. More commonly, tropical trees hit by lightning look largely undamaged but die off slowly over a period of months.
Follow the Flashes
To better understand how large tropical trees are affected by lightning strikes, Gora and his colleagues examined 94 lightning strikes to 93 unique trees on Barro Colorado Island between 2014 and 2019. In 2021, the team traveled to the island to collect both ground- and drone-based imagery of each directly struck tree and its environs.
Gora and his colleagues recorded six metrics about the condition of each directly struck tree and its cadre of parasitic woody vines known as lianas—including crown loss, trunk damage, and percent of the crown infested with lianas. Lianas colonize the crowns of many tropical trees, using them for structure and competing with them for light. Think of someone sitting next to you and picking off half of every bite of food you take, Gora said. “That’s effectively what these lianas are doing.”
The team also surveyed the trees surrounding each directly struck tree. The electrical current of a lightning strike can travel through the air and pass through nearby trees as well, explained Gora. Where a struck tree’s branches are closest to its neighbors, “the ends of its branches and its neighbors’ will die,” Gora said. “You’ll see dozens of those locations.”
Thriving After Lightning
On average, the researchers found that about a quarter of trees directly struck by lightning died. But when the team divided up their sample by tree species, D. oleifera (more commonly known as the almendro or tonka bean tree) stood out for its uncanny ability to survive lightning strikes. The nine D. oleifera trees in the team’s sample consistently survived lightning strikes, whereas their lianas and immediate neighbors did not fare so well. “There was a pretty substantial amount of damage in the area, but not to the directly struck tree,” said Gora of the species. “This one never died.”
(Ten other species in the researchers’ cohort of trees also exhibited no mortality after being struck by lightning, but those samples were all too small—one or two individuals—to draw any robust conclusions from.)
Gora and his collaborators estimated that large D. oleifera trees are struck by lightning an average of five times during their roughly 300-year lifespan. This species’ ability to survive those events while lianas and neighboring trees often died back should result in overall reduced competition for nutrients and sunlight, the team reasoned. Using models of tree growth and reproductive capacity, the researchers estimated that D. oleifera reaped substantial benefits from being struck by lightning—particularly in regard to fecundity, or the number of seeds produced over a tree’s lifetime. “The ability to survive lightning increases their fecundity by fourteenfold,” Gora said.
D. oleifera may be essentially evolving to be better lightning rods.
The researchers furthermore showed that D. oleifera tended to be both taller and wider at its crown than many other tropical tree species on Barro Colorado Island. Previous work by Gora and his colleagues has shown that taller trees are particularly at risk for getting struck by lightning. It’s therefore conceivable that D. oleifera are essentially evolving to be better lightning rods, said Gora. “Maybe lightning is shaping not just the dynamics of our forests but also the evolution.”
These results were published in New Phytologist.
Gora and his collaborators hypothesized that the physiology of D. oleifera must be conferring some protection against the massive amount of current imparted by a lightning strike. Previous work by Gora and other researchers has suggested that D. oleifera is more conductive than average; higher levels of conductivity mean less resistance and therefore less internal heating. “We think that how conductive a tree is is really important to whether it dies,” said Gora.
Continuing to ferret out other lightning-hardy tree species will be important for understanding how forests evolve over time. And that’s where more data will be useful, said Arellano. “I wouldn’t be surprised if we find many other species.”
—Katherine Kornei (@KatherineKornei), Science Writer
