When Hurricane Maria struck Puerto Rico in September 2017, it transformed the island’s forests into tangled messes of split tree trunks, downed branches, and fallen leaves. El Yunque rainforest in northeast Puerto Rico, a 28,000-acre national forest renowned for its rugged beauty and high biodiversity, was particularly hard hit. As winds of up to 155 miles per hour whipped across the Luquillo Mountains, where El Yunque is located, Hurricane Maria stripped forest canopies bare, turning a lush, green landscape into a muddy expanse of leafless trees.
Four years later, the El Yunque rainforest still shows clear signs of damage from Maria, the strongest storm to strike the island since 1928. But the ecosystem is slowly recovering. And scientists with the U.S. Forest Service, which manages the rainforest, as well as NASA and a National Science Foundation-funded Long Term Ecological Research (LTER) site in El Yunque, are intensively studying that recovery. To these researchers, Hurricane Maria’s impacts are a bellwether for a future in which stronger, wetter storms are more frequent.
Subtle ecological clues, from which tree species fared better to which forest areas are recovering faster, are helping scientists predict how El Yunque, and other rainforests across the coastal tropics, will change as hurricane seasons intensify. Researchers are also beginning to ask whether climate change could push hurricane-adapted rainforests into a future where they are no longer rainforests at all.
Hurricane Maria “was like having 30 years’ worth of damage to the forests in a single event,” says Doug Morton, an Earth scientist at NASA’s Goddard Spaceflight Center in Greenbelt, Maryland. “It really was very different from the normal disturbances that shape the nature of tropical forests.”
A 23-foot haircut
To understand how Hurricane Maria affected El Yunque, Morton is taking a 1,000-foot view—literally. In March 2017, he participated in a series of island-wide aerial surveys to map the 3D structure and composition of Puerto Rico’s ecosystems using LiDAR, an instrument that can study the earth from above, and other instruments. The initial goal of the research was to track long-term forest recovery on agricultural land that had been abandoned. But after Hurricane Maria struck, the team pivoted its focus to measuring the historic storm’s impacts.
In April 2018, seven months after Hurricane Maria, Morton returned to the island to repeat the previous year’s aerial surveys. The damage to El Yunque was stark: What was previously a closed-canopy tropical rainforest had become a patchwork of forest fragments and open areas where light reached the ground—more like a savannah woodland ecosystem, Morton says. Most striking of all was how much shorter the forest was. On average, Morton now estimates that forest canopies lost 23 feet of height due to the storm. Hurricane Maria, he says, gave El Yunque “a haircut.”
Other scientists confirmed the extent of the damage from the ground. Returning to a long-term study site within El Yunque after the storm, Columbia University forest ecologist Maria Uriarte and her colleagues discovered that Hurricane Maria killed twice as many trees as 1989’s Hurricane Hugo, a Category 3 storm that has long served as an ecological benchmark for major disturbances in this rainforest.
“What we saw relative to the 1989 hurricane that happened in Puerto Rico is that Maria was far more devastating to the forest,” Uriarte says.
Not every rainforest species was equally ravaged. While many of the large tabonucos and other hardwood tree species at lower elevations were snapped in half by the storm, the shorter-stature palms that dominate the highest reaches of El Yunque’s cloud forests “fared very well,” Uriarte says, noting that their flexible trunks tend to bend rather than break under high winds. Beginning about six months after the storm, a flush of “pioneer species”—grasses, shrubs, and seedlings of a tropical tree called Cecropia—began popping up to take advantage of the ample sunlight on the forest floor.
Those pioneers will get edged out as the ecosystem continues to recover and the forest canopy closes, says Jess Zimmerman, an ecologist at the University of Puerto Rico and the lead principal investigator for the LTER program at El Yunque. But recent data shows that El Yunque’s recovery is proceeding more fitfully than scientists expected.
A patchy recovery
Morton returned to Puerto Rico in March 2020 to repeat his aerial surveys of the rainforest. Although that trip was cut short when NASA recalled all of its field campaigns due to the coronavirus pandemic, the team collected enough data to reconstruct what has happened to El Yunque since 2018.
The findings, which were published in the journal Ecosystems last week, surprised the researchers. About two-thirds of forest areas that lost canopy height during Hurricane Maria grew rapidly taller between 2018 and 2020, which is to be expected for a forest recovering from a major hurricane. But nearly a third of forest areas that got shorter during the storm have not grown back in the years since.
“That was an unexpected finding,” Morton says.
It’s possible, Morton says, that certain patches of forest were damaged to the point where the trees can’t regrow quickly. Instead of putting energy into getting taller, these trees could be investing more resources in rebuilding their root systems. But why some areas are experiencing a delayed recovery remains a puzzle: Morton says the slow-growing patches of forest don’t appear to be associated with specific tree species or topographic features, factors that often shape forest recovery in El Yunque.
Uriarte, who co-authored the analysis, hopes that the yearly census of her long-term rainforest study site, which began earlier this month, will shed more light on the mystery. Identifying the “combination of factors” responsible for variations in forest recovery, she says, will help scientists understand “if the trajectory of the forest is similar to what we observed after previous hurricanes or if it's going in a different direction.”
More dramatic changes to come
During the past 30 years, El Yunque has experienced three major (Category 3 or higher) hurricanes, including Hurricane Maria. Historically, Zimmerman says, storms of this intensity only occurred about every 50 years. “Whether by happenstance or not, we’ve now arrived at what we imagine things will be like in the future,” where stronger storms are more frequent, he says.
Changes to El Yunque over the past three decades offer clues as to where the rainforest is headed. For instance, Zimmerman says the number of palms in El Yunque has “increased significantly” over that period. If the forest continues to get hit with a major hurricane about every decade, he expects palms will keep taking over, resulting in a shorter-statured forest that stores less carbon.
Uriarte agrees. “A forest that becomes more dominated by palms will contain less carbon because palms contain far less carbon than [hardwood] trees,” she says.
Such a forest might be more resistant to stronger hurricanes, since palms don’t tend to break under high winds. But other anticipated impacts of climate change, such as an increase in droughts in Puerto Rico and elsewhere in the Caribbean, could hit palm-dominated rainforests harder, since palms are “extremely vulnerable to drought,” Uriarte says.
Grizelle Gonzalez, a scientist at the Forest Service’s International Institute for Tropical Forestry, says there is “some hope” that El Yunque will be able to adapt to multiple climate-induced shifts at a time, considering that it has experienced both severe droughts and hurricanes throughout its history.
But Gonzalez says it’s also possible “that at some point we will get to a threshold or tipping point” where the ecosystem will cease to be a rainforest anymore. Some models suggest that Puerto Rico’s rainforests could transition into drier, more shrub-dominated ecosystems as warming and drying trends intensify.