Steve Yanoviak remembers walking through a forest in Panama in 1996 as a storm was brewing. He watched as lightning hit a tree just 10 meters away. That brush with death didn’t deter the University of Louisville (Ky.) ecologist, however. He has returned to Panama for the past 25 years to study ants, spiders, and, yes, lightning.
Now, Yanoviak and his colleagues have shown that lightning strikes on Barro Colorado Island in the Panama Canal disproportionately hit the tallest trees. Lightning sends electrical current spreading among tree crowns, affecting as many as 32 trees at once. The researchers also found that lightning was responsible for most deaths of the tallest tropical trees, some believed to be up to 500 years old.
This top-heavy tree mortality has likely had a significant effect on the island’s forest ecosystem, said S. Joseph Wright, an ecologist at the Smithsonian Tropical Research Institute in Panama who was not involved in the research. “These forest giants, with trunk diameters larger than 60 centimeters and heights taller than 35 meters, are responsible for most of the seeds produced by their species populations each year,” he noted. “Their deaths by lightning have disproportionate effects on population and community dynamics.”
Lightning affects ecosystems in many ways, direct and indirect. In addition to electrocuting trees, lightning strikes ignite tens of thousands of wildland fires per year globally. Lightning also changes Earth’s atmospheric chemistry and can alter the composition of rocks. “It’s a unique and concentrated disturbance,” said Evan Gora, an ecologist at the University of Louisville and a member of Yanoviak’s team.
Round-the-Clock Forest Watch
Simply finding where the disturbance took place often proves challenging, Gora noted. Because it is “impossible to predict where lightning will strike,” he said, “it’s difficult to locate [the aftereffects of] lightning.”
To improve their ability to discover stricken trees to study, Yanoviak and his colleagues took a systematic approach. Using cameras mounted on towers in the forest, the researchers filmed a 50-hectare forest plot on Barro Colorado Island in 2015 and 2016. “We filmed the forest 24/7,” said Gora.
When at least three cameras recorded simultaneous flashes of lightning, the team used triangulation to pinpoint the location of the strike. The researchers then hiked into the forest to investigate the aftermath, carrying with them camera-equipped drones and climbing equipment for venturing into the trees’ canopies, which soared 30 or so meters off the ground.
Hidden Wounds
Yanoviak and his team recorded 18 lightning strikes but found that the trees around each strike area did not typically show any outward signs of damage for days or even weeks. That’s because the harm—caused by intense heating as thousands of amperes of current coursed through the trees’ living tissue—was internal and likely consisted of cell wall damage or deformation of vascular tissue, the team hypothesized.
After months of monitoring the stands of trees where the lightning strikes occurred, the researchers noticed that some of the trees exhibited outward signs of distress, such as areas of dead branches or a loss of leaves. Furthermore, it wasn’t just one tree that was affected: Groups of trees, their crowns separated by just a meter or so, often responded together.
Jumping Through the Air
“We’re seeing ‘lightning flashover,’ a physics term that references electric current flowing through one object, jumping through an air gap, and flowing through another object,” said Gora, who presented the group’s results on 9 August at the Ecological Society of America conference in Portland, Ore. The researchers found that the tallest trees were most commonly hit first and that the current then traveled through other nearby tall trees—as many as 31—before finally dissipating into the ground.
The team estimated that of all the tall trees that died in the Barro Colorado Island plot, 56% of them died from lightning. Tree deaths caused by lightning will likely increase as a warming climate stokes a higher rate of lightning strikes, other research has shown.
The systematic removal of these tallest trees could affect the structure of tropical forests, said team member Phillip Bitzer, a lightning physicist at the University of Alabama in Huntsville. “Lightning may play a role in the ecological turnover of forests,” he said.
The loss of towering trees, often among the oldest, has a “huge effect on what’s near them,” said Gora. Their massive trunks are ecosystems providing shelter for beetles, ants, and microbes too numerous to count, he explained. “Lightning might be causing little pockets of extermination of biodiversity,” speculated Yanoviak. The loss of these tallest trees is also a blow to another aspect of the forest ecosystem, noted Wright: These oldest and largest trees sequester more than half of the forest tree community’s carbon, he said.
A video of lightning-killed trees appears below.
Tree death and damage from a single lightning strike, filmed by a drone, from Evan Gora on YouTube.
Next month, the researchers will begin expanding the lightning strike monitoring area over the entirety of the 16-square-kilometer Barro Colorado Island. The scientists are looking forward to studying whether some tree species are more likely to be killed by lightning, work that could inform the design of planned forests. If some kinds of trees are better able to withstand the effects of lightning, forest managers could selectively plant those species to ensure survival in lightning-prone areas, said Gora. The team is also planning to investigate how tree damage is correlated with the properties of a lighting strike, such as its intensity, duration, and peak current. “We’re really just getting started,” said Gora.
—Katherine Kornei (email: [email protected]; @katherinekornei), Freelance Science Journalist