How 30 Million Dead and Dying Trees Are Reshaping Puerto Rico's Forests

A scence from the Luquillo rainforest shortly after Hurricane Maria hit.
Photo: USFS

When Hurricane Maria struck Puerto Rico in September 2017, it knocked out power island-wide, battered infrastructure, and stripped forests of their leaves. Over the months that ensued, upwards of a thousand people would die as a result of the storm. So would about 30 million trees.

That last stunning figure comes courtesy of research presented last week at the American Geophysical Union annual meeting. Columbia University forest ecologist Maria Uriarte spoke at a press conference about her group’s ongoing efforts to document Hurricane Maria’s impact on Puerto Rican forests, by visiting 30 long-term forest-monitoring plots across the island in the eight months following the storm and surveying damage among trees with a stem diameter of 10 centimeters (4 inches) or larger.

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Pairing this field survey with satellite data, Uriarte estimates that 20 million to 40 million trees were killed or severely damaged island-wide. That represents 15 percent to 25 percent of all large trees on the island, she told Earther. The dead and dying trees contain an estimated 5.75 million tons of carbon that could be released to the atmosphere as they decay, equivalent to 2.5 percent of the carbon taken up annually in forests across the United States.

Aerial LiDAR surveys flown by NASA in April 2017 offer another perspective on the damage. As NASA Earth scientist Doug Morton explained at the press conference, the 3D model these surveys produced reveal that Puerto Rico’s forests became about a third shorter on average as the storm tore through. About 60 percent of all canopy trees incurred some damage.

“The storm came through and literally gave the forest a haircut,” Morton said.

That means more light is now reaching the forest floor, which together with the pulse of nutrients delivered by all the downed vegetation, means things are regrowing quickly. But there’s no guarantee the forests that return will look like the ones that existed prior to the hurricane.

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As Uriarte pointed out, palm trees fared quite well during the storm, while big hardwoods suffered significant damage. As trees that broke die a slow death over the coming years, palms could become more dominant, with cascading effects on wildlife that rely on these species for food and shelter.

Researchers will continue to collect data for years to try to understand what these changes mean for Puerto Rico’s El Yunque rainforest in the northeast, the tropical dry forests in the southwest, and elsewhere. Puerto Rico is no stranger to hurricanes, but as Uriarte emphasized, Maria’s impacts were very different from previous major storms like Hurricane Hugo in 1989 and Hurricane Georges in 1998.

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Tree damage and mortality were higher during Maria, for one, but the way in which the forests were damaged also differed. While in past major storms, variation in damage across the landscape was predominantly driven by wind, in 2017, it was rainfall that mattered more.

In the northeast at least, this could be due to the back-to-back deluge of Hurricanes Irma and Maria which caused the soils to become saturated, Uriarte said. “It seems the risk factors have shifted from what we traditionally thought mattered,” she said during the press conference.

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A big concern, Uriarte went on, is what the climate change-juiced hurricanes of the future—which could feature slightly higher wind speeds and substantially greater rainfall—will mean for these forests and the ecosystem services they provide, including keeping substantial amounts of carbon locked away. Uriarte told Earther she’s “absolutely” concerned about these forests switching from carbon sinks to sources as the effects of worsening disasters pile up.

Morton emphasized that Hurricane Maria wasn’t the end of the story, and that Puerto Rico’s forests are on a road to recovery. But, he said, 2017 was “a major change that affects the trajectory of the forest over time.”

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Maddie Stone

Maddie Stone is a freelancer based in Philadelphia.