Stronger hurricanes could decimate forests and accelerate climate change, warns study

El Yunque National Forest after Hurricane Maria, on October 4, 2017.
CNN  — 

When Hurricane Maria struck Puerto Rico in September 2017, it left devastation in its wake.

It took nearly 11 months to restore power across the island and five months to fully restore the island’s main water service. Almost 3,000 people were killed, according to official estimates.

But beyond the human tragedy, a new study has found that Maria changed the makeup of the forests that cover more than half the island, with certain species declining after the storm and others increasing.

And the research suggests this could be a portent of changes that could come to forests across much of the Atlantic Tropics, as climate change drives more powerful storms.

One potential effect is that these forests, rather than being net carbon stores, could actually become net carbon emitters, adding more carbon to the atmosphere. In other words, as the forests increasingly become victims of climate change, they would also contribute to it.

“Carbon dynamics after a storm are hard to understand but after a storm a forest loses a lot of carbon and then regains it as it regrows, so we’re looking at the long-term average,” said Maria Uriarte, of Columbia University’s Earth Institute, who led the research.

“If bad storms become the norm, if a storm of Maria’s severity becomes the average storm in the Caribbean, we expect the forest will store less carbon because trees don’t have time to get big. We expect the net balance to be negative – that’s not published yet but that’s what our models are showing.”

Damage and recovery

Uriarte has been studying Puerto Rico’s trees for the last 15 years. She returned to the island three months after her namesake hurricane and saw the damage to the forests in person.

Read: How climate change is revealing, and threatening, thawing relics

“What was very striking is that, with tree species, there were winners and losers,” she told CNN.

“I’ve been here a few times since the hurricane, and the recovery is very quick. There is a lot of damage but there is regrowth coming in – but the composition [of the forests] is definitely going to change.”

Maria hit Puerto Rico as a category 4 storm, with winds of more than 155 miles (250 kilometers) per hour, and up to 500mm (20 inches) of rain falling over 24 hours. Last year, researchers from the University of California, Berkley, used remote sensing techniques to estimate that Maria may have killed or severely damaged 23 to 31 million trees across Puerto Rico.

Read: What really happened in Puerto Rico after Hurricane Maria

For this latest study, published this week in the journal Nature Communications, Uriarte and her team were able to examine the forests up close, to get a more detailed view of the damage done.

They studied a 40-acre section of the Luquillo Experimental Forest, which has been monitored since the 1990s. That meant they had long-term data that let them compare damage from Maria with that of Hurricane Hugo, which struck the island in 1989, and Hurricane Georges from 1998.

Winners and losers

They found that Maria killed twice as many trees as Hugo and Georges – both category 3 storms, and much less intense than Maria – and broke more than three times as many trunks. Many of those with broken trunks will die in the next few years, according to Uriarte.

“One thing that was different between the storms is the percentage of trees that were broken,” said Uriarte. “Really large trees that were just snapped in half – and this is unique.”

Read: Fish are disappearing because of climate change

For the researchers, this was a striking finding, as in the past the larger trees – such as the mahogany-like “tabonucos,” used for making furniture and boats, and “ausubos,” which are so dense their wood doesn’t float in water – have been more resilient to storms.

“These are some of the trees that have denser wood, and trees with very dense wood historically have very low break rates and mortality rates,” said Uriarte. “This storm was different – trees 100 years old or more were broken. A proportion of these trees will be able rebuild the biomass and recover, and a number of them won’t.”

The El Yunque Rain Forest on September 19, 2018 in Rio Grande, Puerto Rico, a year after Hurricane Maria.

Exactly why these bigger, denser trees fared so badly is still being investigated. But the result is that the number of big hardwoods has dwindled, while other species – particularly fast-growing palms, which bend in strong winds, rather than breaking – have replaced them.

Changing forests

Uriarte says this change in the composition of the forests could be something we see across the Atlantic Tropics – with the trees overall becoming shorter and less dense, and the forests less biodiverse.

“Palms have less dense biomass than hardwood trees, and they store less carbon,” said Uriarte.

Yadvinder Malhi, a professor of Ecosystem Science at the University of Oxford, who wasn’t involved in the research, said it was plausible that there would be a shift towards more faster-growing trees that grow quickly after a hurricane.

“In the long term the total amount of carbon stored in the forest depends on the recovery time between disturbances. It drops to near zero during the disturbance and slowly builds up and drops again,” he said. “So if those disturbances become more frequent and more intense, then the average over a century timescale will be lower.”

Scientists say that while climate change doesn’t cause hurricanes, it is making oceans warmer, which is making hurricanes more intense. Oceans have warmed an average of 1 to 3 degrees Fahrenheit over the past century and as they get warmer, scientists expect hurricanes to bring heavier rain and possibly stronger winds. Heavy rainfall tends to destabilize soil, causing more trees to uproot.

Read: How climate change made hurricanes worse

The study notes that “the expected changes in hurricane winds and rainfall may have profound consequences for the long-term resilience of tropical forests in the North Atlantic basin.”

As well as affecting carbon emissions, the changing forest makeup could affect the wildlife that lives there. “For instance the endangered Puerto Rican parrot, which likes old, tall growth, can’t nest because much of the forest is disturbed,” said Uriarte. Species that favor an open forest environment will do better, she adds.