On a recent trek to the site, Wood brushed aside thick branches and leaves the size of laptops on her way to three plots surrounded by infrared panels that heat the air and soil by 4C. The vegetation there was shorter and a bit browner compared with the three unheated control plots.
The warmed plots run on 480 volts of electricity, and while the lines are isolated from the soil, the scientists use insulated boots to avoid getting electrocuted in an accident.
Nearby, plant physiologist Rob Tunison clamped what looked like a small compact mirror around a dark green leaf to measure photosynthesis, spending 30 minutes to an hour per leaf.
They are looking at how temperatures affect basic processes such as photosynthesis — by which plants transform sunlight into energy while absorbing carbon dioxide and releasing that gas along with oxygen into the atmosphere — as well as how soils respond.
The researchers are also studying nutrients and microbes in the artificially warmed plots of land, sending off frozen samples to a lab in California for analysis.
Knowledge about tropical plants and soils could eventually be plugged into models to determine how vastly broader ecosystems respond to changes.
"We are also able to look at the potential for tropical plants and soils to acclimate to consistently warmer conditions over time," Wood said.
Tropical forests played a key role in recycling carbon dioxide, and they stored about a third of the world's carbon, she said.
They also help generate rainfall across the world by releasing water vapor, which in turn creates clouds.
"Anything that happens in these systems can have an effect on the world's climate," she said.
US, British and international climate agencies reported this month that 2018 was the fourth-warmest year on record, and global emissions of heat-trapping carbon dioxide saw their largest spike in seven years.
Overall, global carbon dioxide emissions have increased 55 per cent in the past two decades, and Earth has warmed on average about two-thirds of a degree Celsius, according to the US National Oceanic and Atmospheric Administration.
British meteorologists said in early February that the next five years could see record-breaking temperatures. Scientists expect the world this year will spew 37.1 billion tonnes of carbon dioxide, up from 36.2 billion tonnes last year, according to studies by the Global Carbon Project.
Kim Cobb, a climate scientist at Georgia Institute of Technology who is not involved in the experiments at El Yunque, says she is not aware of any other long-term warming experiments in tropical rainforests.
"What will happen at their site is highly uncertain, because the rainforest itself controls so many aspects of the regional water cycle. It's not a system that we can model extremely well today, let alone under climate change scenarios," Cobb said.
"But there is little doubt that these kinds of long-term monitoring sites are extremely valuable in advancing our understanding of the water and carbon cycle, and how they might change with climate change."
The US$3 million ($4.4m) project, partly funded by the US Department of Energy, is in its fourth year.
Wood said she hoped it could run indefinitely.
Scientists took a one-year hiatus after Hurricane Maria hit Puerto Rico on September 20, 2017, so they could separate the effect of warming from the effect of the storm, which caused more than US$100 billion in damage and toppled trees like dominoes.
A couple of miles from Wood's experiment, scientists including those from the International Institute of Tropical Forestry are looking at how hurricanes affect the tropical forest.
They began by trimming away the canopy leaves above patches of forest to mimic the effects of a storm. They hired arborists to cut tree branches and spread them across the forest floor to study how light and water move through the changed ecosystem and the impact that the debris has on soil microbes.
They also monitor cloud base heights to get a sense of how changes could affect rainfall.
Maria suddenly gave them a real-life test.
"It poses a lot of challenges but a lot of opportunities to move the science forward," said Grizelle Gonzalez, a project leader.
The experiments are expected to continue for several years, barring any interruptions from storms as the Caribbean prepares for another Atlantic hurricane season that starts June 1.
Cobb, the global warming scientist, praised the ongoing experiments.
"It is well worth the effort," she said.
"The raw beauty of these environments is really only matched by their immense scientific potential."
- AP
