ABOVE: © Istock.com, jodi jacobson

Trees bowed to 45-degree angles and flying leaves crisscrossed the sky as Hurricane Florence ravaged North Carolina’s coast and inland regions in mid-September 2018. The storm, which peaked as a Category 4 hurricane before making landfall near Wilmington as a Category 1, deluged parts of the state with nearly three feet of rain. It stripped the leaves off black walnuts, crape myrtles, and their entwining wisterias, especially on the north and northeast sides of the trees, which bore the full brunt of the 100-plus-mile-per-hour wind gusts. An estimated 1.25 million acres of timber, valued at nearly $70 million, suffered varying degrees of damage.

Whoppers like Florence are a reality that North Carolina—not to mention the rest of the Eastern seaboard and the Caribbean—may have to get used to in the near future. Historically, a given location might only see such destructive hurricanes every few decades. But with global temperatures on the rise, the risk that a fledgling storm system will grow to “major” status, defined as category 3 and above, is likely to climb. Warming oceans mean more water vapor in the air, and that vapor is what fuels the storms. “One of the signals that we expect from climate change is that the strongest hurricanes will get stronger,” says Gary Lackmann, an atmospheric scientist at North Carolina State University in Raleigh.

What does that mean for trees? The scene in the woods after Florence was one of seeming devastation. In every direction, trees, branches, and brush littered the ground. Yet just a few weeks after the storm, the stripped trees sprouted fresh leaves and flowers. It may have been autumn, but the trees already had leaf and flower buds in waiting for the upcoming spring, explains Jim Slye, assistant regional forester with the state forest service in Goldsboro. Re-leafing after storms helps keep the trees’ circulation going, and flowering allows trees to drop seeds in case they end up succumbing to storm damage. The trees won’t necessarily die, though; tree ring studies make it clear that many survived past storms.

Tropical Trees Bend, Not Break Tropical trees have adapted to the ravages of harsh storms. They are shorter than trees in hurricane-free zones, and many species are all roughly the same height, so no individual tree rises above the canopy to be exposed to high winds. Here are other tactics Caribbean species use to resist hurricanes: PDF See full infographic: WEB © Julia Moore

Sierra palm Palms and other tropical trees toss their leaves quickly in a storm. With nothing left but trunk and branches, the tree presents a smaller target to the wind. Tabonuco tree In addition to dropping their leaves, tabonuco trees intertwine their roots to create a network to share nutrients and form a strong anchor on exposed ridges. Muskwood The mahogany-like guarea, or muskwood tree. tips right over in a storm, then quickly sprouts new growth along its fallen trunk. These sprouts grow into tree clones that at first rely on the original tree for nutrients and water, before sending their own roots into the soil.

During and after a hurricane, rains can cause landslides, but winds do the bulk of the damage. Trees, particularly tall ones, bear the brunt of a hurricane’s force, says University of Cambridge plant ecologist Edmund Tanner. In fact, trees in hurricane-prone regions have evolved to weather the storms, and tend to grow to around the same height so none are exposed above the canopy.

Ecologists like Tanner, monitoring forests around the globe over the last several decades, have documented how trees stand strong and seen forests regrow following damage. Processes set in motion by scientists’ artificial hurricanes, created by ripping down branches and trees, also point to forests’ resilience. The conclusion, says Skip Van Bloem, a forest ecologist at Clemson University in South Carolina: “When hurricanes hit natural forests, it’s not a huge disaster.”

If storms become more powerful, will forests still serve their vital role of removing carbon dioxide from the atmosphere?

Whether such ecosystems can withstand the stronger storms that scientists predict will accompany climate change is an open question. Researchers in the field speculate that the makeup of the forests may change in response to more-intense storms, potentially influencing the animals and microbes that make the woods their home. “The forest will still be there. It might be slightly shifted in terms of the species composition,” says Aaron Shiels, a US Department of Agriculture (USDA) research biologist.

And those forests are good for more than a walk in the woods. Trees and soil sequester carbon by turning carbon dioxide into organic matter; after oceans, forests are the greatest sink for the world’s carbon.1 But storms transfer carbon from branches and leaves to the ground, where their decomposition can release the greenhouse gas back into the atmosphere. If storms become more powerful as Lackmann and others predict, will forests still serve their vital role of removing carbon dioxide from the atmosphere? Now, scientists are beginning to look for the answers.

When opportunity blows in

To forecast what will happen to forests in the wake of big storms, scientists can look to the fallout from past hurricanes, as Tanner has done. Every five years or so since 1974, he takes a trusted colleague or student from his lab in England to Jamaica. They travel to Cinchona Botanical Gardens in the Blue Mountains, where they camp. If it’s muddy, even a four-wheel-drive vehicle can’t get up there, but Tanner knows the locals, who can provide mules. The researchers visit their 38 plots, each 10-meter square marked at the corners with steel bars, and spend a couple of weeks collecting data. They measure the girth of every tree, score crown size, and root in the soil for the aluminum tags from downed trees.

Tanner originally set up the plots to study nutrient cycling. But he soon realized that given the tropical location, sooner or later a hurricane was likely to hit. “I don’t wish a hurricane on Jamaicans,” he’s quick to point out. “I just realized, there was going to be an opportunity.”

Sure enough, on September 12, 1988, Hurricane Gilbert made landfall in Jamaica. With winds upwards of 125 to 150 miles per hour, the massive Category 5 storm traversed the whole 235-kilometer-long island, the eye passing less than 10 km south of Tanner’s study site.

When hurricanes hit natural forests, it’s not a huge disaster. —Skip Van Bloem, Clemson University

A few years after the hurricane, Turner was surprised to find more growth among the surviving trees. For example, between 1989 and 1992, the increase in the area of ground covered by trunks of Alchornea latifolia, a flowering tree in the spurge family, was eight times the rate of change from the pre-hurricane data.2 Growth remained high for more than a decade following the storm. “We thought we were going to document the damage, not an increase [in growth],” Tanner recalls. His and others’ studies make it clear that tropical forests are well-adapted to hurricanes. Two decades after Gilbert, 54 percent of the 1,670 trees Tanner had been tracking since 1974 were still alive.

That’s not to say that Gilbert had no effect. While tropical trees often remain standing, hurricane-force winds knock off the leaves and branches in the canopy. Then, “a big race begins,” says Ariel Lugo, director of the International Institute of Tropical Forestry of the USDA Forest Service in Puerto Rico. Surviving large trees sprout new leaves to capture the light they need to grow. But they have new competition, as seeds and seedlings of so-called pioneer species, once light-starved in the shadows of the larger trees, germinate and thrive in the sunshine pouring through the ripped-open canopy. The pioneers grow quickly, rushing to produce more seeds before the canopy closes in again or they’re knocked down by another storm. Indeed, Tanner documented an increase in diversity in most of his Jamaican plots, with eight new species appearing between 1984, four years before the hurricane, and 1994.3

“People used to think that hurricanes were bad for forests, but actually they increase diversity, and they increase growth,” says Tanner. “I’m not saying that’s good or bad, but it’s a fact.”

Even decades later, Gilbert’s effects can still be seen, with trees damaged during the hurricane more likely to die than uninjured ones.4 Tanner speculates that healthy plants outcompete the damaged trees, which eventually fall into the shade of the closing canopy.

So while tropical forests are resilient to hurricanes, some effects are long-lasting. “We think the forest is continually recovering from previous damage, still recovering from previous hurricanes when hit by the next,” says Tanner. If big storms become more frequent, forests will be in an earlier stage of recovery when they’re knocked down again.

So far, Tanner’s plots have avoided another direct hit, but the next major hurricane could be around the corner. Jamaica’s Blue Mountains are typically hit hard every 25 years or so, and it’s been three decades since Gilbert.

Puerto Rico, on the other hand, has seen three major storms in that same time frame: Hugo in 1989, Georges in 1998, and the strongest and wettest of the trio, Maria, in 2017. That makes the island territory a hurricane scientist’s dream.

TOPLESS: El Yunque National Forest, home to the Luquillo Experimental Forest in San Juan, Puerto Rico, before (top) and after hurricane Maria blew off the canopy and rained debris on the ground below. © NASA

How to make a hurricane

Stepping off the plane in San Juan, you encounter a concrete jungle, says Shiels, who led a major forest experiment in Puerto Rico’s Luquillo Experimental Forest before taking his current position with the USDA. But look up from the airport, and you’ll see the green Luquillo Mountains rising above the city, dotted with a few homes and the two field stations of the experimental forest.

The research site encompasses 11,300 hectares of mountainous, humid rainforest. The thick canopy means little sun reaches the understory, so few plants grow there, making it easy for scientists to hike. “One of the really special things is going out at night, because that’s when all the coqui frogs are singing,” says Shiels. When the frogs (Eleutherodactylus coqui) are particularly vociferous, they can drown out ecologists’ conversations.

Or rather, that’s what it was like when Shiels was there during the early 2000s, and up until September 2017 when Maria ripped off much of the canopy and rained debris on the forest floor below. These days, ecologists slather on sunscreen before wading through or crawling over the mess of dropped branches and new plants coating the ground.

“There’s just this riotous growth,” says Shiels’s collaborator Jess Zimmerman, a forest ecologist at the University of Puerto Rico in Rio Piedras. “There’s a whole slew of shrubs”—for example, post-hurricane pokeweed (Phytolacca riparia)—and “this is their opportunity to grow like gangbusters because there’s so much light in the understory.”

It’s a change ecologists have witnessed before. In 1988, the Luquillo site joined the National Science Foundation’s Long Term Ecological Research Network, driving researchers in Puerto Rico to focus on rare events and long-term studies. Just one year later, Hugo slammed into the island; it was the biggest storm to hit the territory since David in 1979. In addition to the understory bloom, researchers documented a rise in animals such as invasive rats, and a drop in others such as walking stick insects. The researchers wondered: What controls these changes to flora and fauna? The sunlight shining through the newly open canopy, or the new habitats and fertilizers created by the branches and leaves dropped to the ground?

To answer this question, the Luquillo site leaders recruited Shiels to manage an ambitious experiment. The researchers collected baseline data starting in 2003, then created an artificial hurricane between October 2004 and June 2005. Shiels hired arborists who used a giant slingshot to get their ropes into the canopy. They’d climb the rope, then stay in the canopy, swinging from tree to tree. “They were so acrobatic,” he recalls. The arborists used handsaws and chain saws to slice off any branches greater than 10 centimeters in diameter. Meanwhile, on the ground, a small army of volunteers gathered up the branches and leaves, chopped and weighed them, and spread them evenly under trees in select areas. “Turns out, recent college graduates, for a free ticket to Puerto Rico and some food and lodging, will do just about anything,” jokes Zimmerman. The process took months.

MANMADE STORM: Researchers at the Luquillo Experimental Forest in Puerto Rico employed arborists to swing through the canopy cutting down branches to simulate a hurricane’s destruction. Aaron b. shields

The researchers established 12 plots, each 30 meters square. Three of the plots underwent both canopy trimming and debris deposition, while three received only tree trimming, three got only the debris, and three were left alone as controls. In the ensuing years, a variety of scientists descended on the forest. They measured microclimate features such as light, temperature, and soil moisture. They collected microbes from soil and leaf litter. They bagged leaves and branches and recorded how long it took the material to decompose. They assessed arthropods, ferns, snails, slugs—and, of course, the coqui frogs.

The researchers published several papers on their results in a special issue of the journal Forest Ecology and Management in 2014. “Some organisms respond to the debris on the forest floor,” Shiels summarizes, but “the majority responded to the light.” The open canopy promoted the growth of pioneer plants and ferns, while minimizing animal habitats. Many arthropods disappeared, perhaps decamping for darker pastures, though mites and other microbe eaters thrived. The sun exposure dried up the leaf litter, and the coqui frog population diminished; they may have hopped off in search of wetter habitats with more bugs to eat. The lack of moisture also blocked the activity of fungi and slowed their decomposition activities.5

The researchers hope the results could help foresters better prepare for future storms. Shiels suggests conservationists might bank seeds from species that struggle after a hurricane so they will be preserved even if they disappear from local forests. Forest and plantation managers might also select the best species to plant after a storm, adds Zimmerman. For example, Lugo has reported that Puerto Rican plantations full of Honduran pine (Pinus caribaea var. hondurensis)—a species that provides good quality lumber for a variety of purposes—were damaged heavily by Hugo and Georges. The pine almost disappeared from one site.6 A better post-storm planting option, Zimmerman suggests, might be tropical hardwoods, and foresters might consider planting wind-resistant trees near waterways used for drinking water, so they would stay in place and prevent erosion of the banks into the water supply.

“If we can better predict how these forests are going to respond to these changes in [strong hurricane] frequency,” says Shiels, “then we can hopefully better manage the forests.”

How the Luquillo Forest Responded to Artificial Hurricanes A hurricane dramatically changes the ecosystem, allowing certain plants and animals to flourish while sending others packing. But what is it about the storm or its aftermath that determines who survives or stays, and who dies or goes? To find out, researchers in at the Luquillo Experimental Forest in Puerto Rico created their own hurricane, exposing some plots to both a wide-open canopy (by downing branches) and forest-floor debris, other plots to only one or the other treatment, and some to neither. Then they analyzed the post-hurricane forest to reveal a cascade of effects, with changes to microclimates affecting myriad organisms and biochemical processes. Overall, the opening of the canopy had the larger influence on the forest. PDF See full infographic: WEB © Julia Moore Opening the canopy led to: More pioneer plants Drier leaf litter, but wetter soils Fewer coqui frogs Fewer types of invertebrates in litter overall, but greater numbers of mites and springtails More diverse soil microbes Scattering debris led to: 6. More diverse gastropods

How hurricanes affect temperate forests

Although North America’s East Coast faces hurricane threats far less regularly than the tropics, such storms are still the most important natural disturbance to forests in certain areas, says Audrey Barker Plotkin, site manager for the 4,000-acre Harvard Forest in Petersham, Massachusetts. A region might only experience a large storm once a century, or even less often, she says, but when big hurricanes do come, they cause widespread, costly damage. For example, early estimates put Florence’s impact, including property and vehicle damage as well as losses in economic output, at $38–50 billion.

The last big one at the Harvard Forest site was in 1938. The storm was nicknamed the “Long Island Express” for the speed at which it tore through New York and into Connecticut, Rhode Island, and Massachusetts. Unlike in the tropics, where many trees are battered but remain standing, trees in temperate climes tend to tip over or snap. The 1938 storm felled 70 percent of the timber in central New England, says Barker Plotkin, wiping out much of the eastern white pine (Pinus strobus) prominent in the region.

We think the forest is continually recovering from previous damage, still recovering from previous hurricanes when hit by the next. —Edmund Tanner, University of Cambridge

Workers in public programs, part of Franklin Roosevelt’s New Deal during the Great Depression, removed much of the fallen wood. The government’s newly created New England Timber Salvage Administration purchased logs, paying out a total of $8.3 million to landowners; much of that lumber would later go to the World War II effort, as well as to paper factories. Private companies also cut and paid for the fallen timber. After the clearing, a mix of hardwoods, such as red oak (Quercus rubra) and red maple (Acer rubrum), naturally replaced the pine forests.

Harvard Forest researchers wondered how the forests would have responded if the government had not orchestrated salvage logging. So, as the Luquillo group would do years later, they decided to create their own hurricane. In 1990, the forest staff attached a winch to 276 trees, one by one, and used a skidder—a kind of forest construction vehicle—to pull them over. These fell on others, ultimately damaging about two-thirds of the 888 trees in the two-acre plot, for a damage rate similar to that of the 1938 Long Island Express.

About half of the plot’s total trees died within three years. Barker Plotkin first started working in the forest eight years after the artificial hurricane, and even then, “it was so hard to walk through the area because there were so many downed trees,” she recalls. “All their branches were still really intact and sticking way up in the air.” New saplings had reached heights about the level of her face, making the environment dark and overwhelming, she notes. Nearby, in the control plot, she could easily stroll under tall oaks and maples.

Harvard Forest after the pulldown, October 1990 John O’Keefe

Nowadays, nearly three decades on, the branches of the dead trees in the hurricane plot have settled to the ground. The trunks are beginning to decompose, and the canopy soars over Barker Plotkin’s head. “The evidence that there was a massive disturbance is a little more subtle.” Much of the new forest grew from seedlings that were already waiting in the understory before the artificial storm. Survivors that didn’t get knocked over, such as some large oaks, also make up a proportion of the live wood on the site today. Of the original trees, 31 percent were still there in 2010.

Tree diversity measures remained fairly steady. Since the experimental hurricane, the plot gained three tree species for a total of 17. During the same amount of time, the control site gained one species and lost another, maintaining its total of 13. There were also short-term changes to the herbaceous plants and shrubs populating the understory, but after two decades, the understory had mostly returned to its original makeup.7

“The major point,” says Barker Plotkin, “is that a hurricane blowdown area looks catastrophic, and like the forest is in real trouble, but it is incredibly resilient.”

In fact, some of the trees damaged by the Long Island Express might have recovered if left in the forest, Barker Plotkin says. And even the dead ones could have returned their carbon and nutrients to the ecosystem. With the salvage effort, “it was basically two disturbances on top of each other,” she says. “Maybe leaving it alone is the best answer, in many cases.”

The coming storm

A challenging task ahead will be predicting how forests will fare if climate scientists are right that there will be bigger, stronger hurricanes in the future. To get at this question, Zimmerman and colleagues recently analyzed data from four tropical forests, including Luquillo, that had experienced zero, two, three, or four major storms between 1982 and 2015.

Last summer, the researchers reported that the more often a forest is hit with a hurricane or typhoon, the less it’s affected in terms of features such as tree mortality, growth, and species composition.8 The regularly bombarded forest is like a lawn that undergoes frequent mowing, explains study author Aaron Hogan, a PhD student in tropical forest ecology at Florida International University in Miami. The plants are sheared regularly, and the hardier ones remain standing, so the next disturbance makes less of an impact. A forest that weathers storms often might take less than a decade to return to its pre-cyclone state, Hogan estimates. In contrast, he thinks a forest less accustomed to hurricanes or typhoons could take a decade or longer to fully recover.

The forest that stands tall in the face of gales is one that might continue to suck up and store carbon, slowing future climate change. Based on computer modeling of carbon dynamics, as yet unpublished, Columbia University forest ecologist Maria Uriarte says that a forest is usually a sink, but right after a hurricane, it starts to lose carbon, becoming a source. Over time—a few years to decades, depending on storm severity—the forest returns to its sink status for a net neutral effect.

What Uriarte can’t be sure of is what would happen with more-intense storms. Indeed, the jury is very much out on the carbon storage effects of stronger hurricanes in the future. Zimmerman speculates that under those conditions, the forest would spend more time emitting carbon, thereby serving as a less powerful sink. Lugo, on the other hand, thinks the forest sucks up more carbon after a storm, because the growth of survivors and pioneers rises quickly, using carbon, and thereby outweighs the effect of decomposition-based carbon release from debris on the ground.

Clemson’s Van Bloem and colleagues evaluated trees in a different region of Puerto Rico, the Guánica Dry Forest, before and after Hurricane Maria. They researchers announced in December that many leaves took up more carbon dioxide after the storm, but they also contained less chlorophyll, which converts carbon from the atmosphere to sugar. Future computer modeling will help determine whether these factors might lead the forests to act as a carbon sink or source.

To get a better picture of how more-powerful storms will affect forests, the Luquillo scientists plan to recreate their artificial hurricane every 10 years—a rate that approximates the gap between Hugo and Georges. They did a second round of trimming trees and depositing debris in 2015, and Shiels predicts they’ll see more dominance of pioneer species and mid-successional trees, the type that come in after pioneers, as the damage builds up. He also expects there would be less carbon stored in such a forest.

Then again, forests have impressed scientists with their resilience before, so who’s to say they won’t surprise ecologists again? “There have been so many lessons learned,” says Zimmerman. “The surprises have turned into knowledge.”

Amber Dance is a freelance science journalist living in the Los Angeles area. The Scientist’s associate editor Ashley Yeager contributed to the reporting for this story.

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