PUERTO RICO’S EL YUNQUE NATIONAL FOREST—A year after Hurricane Maria raked Puerto Rico with winds of 250 kilometers per hour and a meter of rain, the island is still struggling to recover. Estimated deaths have risen to shocking levels—nearly 3000—and although power has been almost completely restored, blackouts occur regularly. The wind and flooding also devastated ecosystems as diverse as mangrove swamps and rainforest. As they mend, scientists are watching closely.
A major focus is this rainforest in the Luquillo Mountains of northeastern Puerto Rico that scientists have been monitoring since the 1940s. The only tropical forest in the U.S. Forest Service’s (USFS’s) National Forest System, it has endured several major hurricanes. But Maria and Irma—a hurricane that struck the island a glancing blow just 2 weeks earlier—were the strongest in a century, turning lush forest into ranks of skeletal trees and piles of sticks. Maria also destroyed research infrastructure and blocked access to some experiments for weeks.
As scientists get back to work, the devastated forest presents a rare opportunity to explore how tropical forests—and their ability to store carbon—recover from the extreme weather that is becoming more common as the world warms. The wealth of ongoing research in the forest “has positioned us really well to tackle this question of how tropical forests are going to respond,” says Grizelle Gonzalez, an ecologist at the International Institute of Tropical Forestry, run by USFS in San Juan. Tana Wood, a biogeochemist and ecologist at the institute, says she was trapped in her house for 2 days after Maria, and her research site was hit hard. “But scientifically, it’s incredibly exciting,” she says. “It’s a unique chance to look ahead.”
Already the researchers have seen hints that as temperatures warm, forests may be slower to recover from damage. Other findings, from a long-term effort here to simulate hurricane damage, suggest more frequent major disturbances could also make forest ecosystems less resilient, in part by causing the downed trees and branches to release their carbon into the air rather than the soil. That could fuel further climate change and extreme weather.
Stepping into El Yunque used to offer relief from the ferocious sun. But now, Wood shades her eyes from the glare as she surveys new understory vegetation. Hurricane Maria destroyed the canopy that once blanketed the forest in cool darkness and shielded the experiment that Wood began in 2016, a year before the storms. Her project, the Tropical Responses to Altered Climate Experiment (TRACE), tracks how increasing temperatures affect wet tropical forests. Because the world doesn’t warm much during the time scale of a research project, TRACE features an infrared heating system that warms understory plants and soils by 4°C, fast-forwarding climate change.
Any small change in the uptake or release of carbon dioxide can have disproportionately large effects on the concentration in the atmosphere.
For Wood, El Yunque serves as a proxy for tropical forests worldwide, which account for more than two-thirds of the planet’s terrestrial plant biomass and store about a third of the world’s soil carbon. Wood’s original goal for TRACE was to understand how warming might affect that storage capacity. In tropical forests, she says, “Any small change in the uptake or release of carbon dioxide can have disproportionately large effects on the concentration in the atmosphere.” Now, she and her team have a new opportunity: to evaluate whether the artificial warming is affecting recovery from Irma and Maria’s damage. What the group finds could be an omen for Earth’s warmer future.
Wood is monitoring multiple actors in the forest, including the fine roots that supply trees with water and nutrients. Although roots are protected from the wind, a hurricane can shred them, she notes. “In these huge gusts of wind, the trees are moving and twisting,” pulling away from their roots. Roots also may suffer, she says, when a tree loses its leaves, where photosynthesis produces sugars and starch. Because the roots don’t photosynthesize, they depend on leaves to nourish them.
To see how well the fine roots are recovering from hurricane damage, the TRACE team lowers cameras through clear plastic pipes driven a meter into the soil. The researchers also measure the infiltration of new roots into fresh soil packed into boreholes in the forest floor. “Our preliminary results are that we saw less root production in the warmed plots,” Wood says.
She speculates that a scarcity of stored carbohydrate in thicker, intact roots could be a factor. When a hurricane strips foliage and interrupts photosynthesis, trees can draw on that stored food to regenerate fine roots. But Wood and her colleagues have seen hints that before Maria, photosynthesis was slower in the warmed plots, possibly leaving the trees with a smaller reserve to fuel recovery. The idea, she says, is that “warming can stress vegetation, but it may not be observable until you have another stress on top of that”—namely, a hurricane.
Either way, lagging root regrowth could mean the trees in warmed plots will recover more slowly and, as a result, will capture less carbon dioxide. And because root growth and die-off pumps carbon into the soil, lasting damage could mean a loss of soil carbon.
Other researchers have studied how warming affects rainforest plants in the laboratory. What is unusual about TRACE, says tropical ecologist Jennifer Powers of the University of Minnesota in St. Paul, is that it tracks the effects of warming amid the “full complexity of the coupled soil-plant ecosystem.”
Above ground in El Yunque, trees that survived the hurricane resemble bottle brushes, with new leaves sprouting from their trunks. The lush understory now reaches shoulder height in places and is “thicker and more dense than I have ever seen,” Wood says. Ongoing measurements will show whether the amount of foliage and the thickness of individual leaves have risen since the storm, perhaps for protection from drying or scorching under the storm-damaged canopy. More foliage could boost carbon uptake—and perhaps counteract any reduction in carbon storage resulting from damaged roots.
Wood’s team, which is funded to continue its work for another 3 years, is also tracking another possible legacy of the storms: changes in species composition. By counting and identifying as many as 600 new seedlings in a single 4-meter plot, the researchers are monitoring the fate of slow-growing, shade-tolerant plants, which could struggle to survive, and watching for nonnative species, which could decrease overall diversity. A shift in species could ultimately change the canopy structure, allowing more heat and light to reach the understory and exacerbating climate warming.
At an experimental site farther west, meanwhile, Gonzalez is studying the fate of the carbon and other nutrients locked up in the hurricane debris. Here, backyard-size plots are carpeted with browned leaves, sticks, and decaying palm fronds—a mess left not just by Maria, but also by the Canopy Trimming Experiment (CTE), underway since 2004. The project was sparked by a striking observation after hurricanes Hugo and Georges in 1989 and 1998, respectively: All the organic debris that the storms dumped onto the forest floor had decomposed within a year or two. Scientists suspected that the more intense sun and rain reaching the soil after the storms exposed it helped speed decomposition.
The CTE team is dissecting the process. In some plots, the scientists deposited debris but left the canopy intact; in others, they used chain saws and shears to trim the canopy but hauled away all the branches and foliage. Still other plots got both treatments, mimicking the effects of a major storm. After a decade, plots that only received debris deposits showed a significant increase in soil organic matter. Fungi and minuscule animals took up nutrients from the debris and returned them to the soil for plants to use. “That’s how the system kind of jump-starts and recovers,” Gonzalez says.
But on plots that experienced the hurricanelike treatment of both debris and canopy opening, intense sunshine and pelting rains disrupted the community of fungi and animals. Microbes stepped in, sending much of the carbon back to the atmosphere.
The results suggest “it’s kind of a race between the microbes and the physical processes that move that material down to the soil,” says Whendee Silver, an ecologist and biogeochemist at the University of California, Berkeley, whose CTE work indicates that the same carbon-stealing microbes might attack organic matter deeper in the soil. If the pace of disturbances accelerates, researchers note, the canopy would have less time to recover, and organic matter in the soil could dwindle along with the health of the forest.
“One of our really big concerns is that, if hurricane frequency and intensity increase, it will slowly deplete these ecosystems of what they need to grow and survive,” Silver says. That could be bad news—not just for this lush corner of Puerto Rico, but for the planet as a whole.
Source: Science Mag