LOUISVILLE, KENTUCKY—The biggest trees, standing tall through storms and harsh winters, may look invincible. But a series of recent studies analyzing the effects of lightning, drought, and invasive pests on forests indicates that for trees, size is not strength, and forest giants are disproportionately vulnerable.
"There's always been an underlying assumption that large trees are somewhat buffered from environmental stress," says Andrew Barton, a forest ecologist at the University of Maine in Farmington. This new work "suggests that this might not be true." And with all three stresses likely to grow, big trees could become a particularly weak point in beleaguered forests, says forest ecologist David Lindenmayer of Australian National University in Canberra, whose work has shown that the loss of large trees puts entire ecosystems at risk of collapsing. Large trees are also major storehouses of carbon—one estimate suggests they hold 50% of a forest's carbon—and their deaths release it into the atmosphere, which could exacerbate climate change.
It makes sense that lightning targets the biggest trees, but the extent of the toll has emerged only now, from a project on Barro Colorado Island, in the middle of the Panama Canal. In temperate regions, lightning blackens tree trunks or burns a tree down when it strikes, so it's easy to see its effects. But in tropical forests like the Panama one, which has been studied for decades, lightning leaves no obvious marks, possibly because the trees carry more water, although a struck tree may still die weeks or months later. But Steve Yanoviak, an ecologist from the University of Louisville here, and colleagues recently outfitted the science station on the island with cameras and sensors, so they can triangulate lightning strikes and look for downed vegetation and other subtle signs a tree was struck. Yanoviak's postdoc Evan Gora then keeps tabs on the struck tree and its neighbors to document any declines. To date, the project has pinpointed 70 lightning strikes, Gora reported last month at the annual meeting of the Ecological Society of America here. Each strike kills an average of five trees and damages 16 more as the bolt's electricity hops from one tree to another.
Gora and Yanoviak divided all of the Barro Colorado Island trees into three size classes by their diameters, which correspond to their total size. They found a disproportionate toll on the biggest, tallest trees. "Lightning accounts for 40.5% of the large tree mortality," Gora told the meeting. That "is much higher than we expected." In contrast, lightning killed just 5.4% of the medium-size trees and 2.9% of the smallest. If climate change makes thunderstorms more common or intense, as many researchers expect, the big tree toll could rise.
Drought could worsen it. Some studies had suggested that because the roots of small trees are too shallow to reach groundwater, they might be more vulnerable to drought than forest giants. But Atticus Stovall, a forest ecologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, found the opposite when he studied 8 years of aerial photographs and laser measurements of tree height and canopy density in two regions of California's Sierra Nevada mountains, totaling 40,000 hectares. Using custom computer programs, Stovall traced the fate of 1.8 million individual trees.
In 2014 and 2015, the region was in the grip of the worst drought in 1200 years together with unusually hot weather. Before the drought, trees of all sizes died at about the same rate, Stovall found by examining changes in the forest canopy. But after 2014, when the severe drought began, 40% of the trees taller than 30 meters died, compared with 28% for medium-size trees and 16% for small trees, Stovall reported at the meeting.
"Height is more important than any other factor," including rainfall and temperature, for whether a tree survives drought, he says. He calculated that for every 10 meters of increase in a tree's height, its mortality rate increases 2.4% per year.
Stovall and other scientists believe taller trees are more vulnerable in part because they have to pull water longer distances from the roots to their crowns. If conditions cause water in leaves to escape too fast, bubbles can form in a trunk's water-conduction channels, disrupting flow. "Once a tree has too many bubbles, it's usually done for," he says. Drought also weakens trees' defenses against bark beetles, which in turn carry pathogens that can kill the tree, in what Stovall calls a "death spiral."
About 14% of the world's conifers grow in drought-prone climates like the Sierra Nevadas, and global warming is expected to make droughts more common in those areas. "We think this could be a major concern for forests," Stovall concludes.
Invasive pests, which are also on the rise because of human activity, are another scourge of big trees, Songlin Fei, a forest ecologist at Purdue University in West Lafayette, Indiana, has found. After watching a beloved ash tree in his yard die from being infested with the emerald ash borer, a beetle accidentally introduced from Asia, Fei began to wonder what the toll of these pests was on the carbon stored by trees. He gathered data from a U.S. Forest Service (USFS) program that tracks 120,000 forest plots across the United States, recording tree damage or death. In addition, he and colleagues tapped a federal database that keeps tabs on forest pests. About 15 nonnative organisms—insects, fungi, and other microbes—have become major threats to U.S. forests. Based on the damage these 15 pests did in the plots monitored by USFS, invasives annually cause the release of carbon equivalent to that produced by 5 million cars, the researchers reported in the 12 August issue of the Proceedings of the National Academy of Sciences.
At Science's request, Fei used that study's data to calculate the death rate for different sizes of hemlock, ash, and birch trees, with and without infestation. The largest trees suffered the most damage, which Fei called a "surprise." The biggest ash trees were five times more likely to die when infested by the emerald ash borer, whereas mortality in smaller trees increased only threefold, for example. "In general, larger diameter trees are more likely to die with pest invasion," Fei says. He suspects that, like elderly people, older trees are generally not as robust and are less able to recover from stresses such as invasive insects and pathogens.
"We must protect large old trees much better than we have done previously," Lindenmayer says. Creating a buffer of vegetation around them can help. And old-growth forests, which harbor the biggest trees, should be protected from fire or logging, he says. Besides being the most vulnerable, after all, the biggest trees are the slowest to regrow. "Many large old trees are really irreplaceable," Lindenmayer says.