Scientists Are Warming the Rainforest on Purpose

In the Luquillo forest in eastern Puerto Rico, scientists are doing something that sounds strange. They are heating the rainforest on purpose. Glowing panels hang over small circles of plants and warm them by about 4°C (about 7°F). The project is called TRACE. Its goal is to find out what a hotter world will do to a tropical forest—before that hotter world is fully here.

In the Luquillo Experimental Forest of eastern Puerto Rico—the wider area that includes El Yunque—researchers run an experiment that sounds backward. They deliberately heat patches of rainforest. Infrared panels hover over small plots and raise the temperature by roughly 4°C, the amount of warming scientists project for tropical forests by the end of this century. The project is named TRACE, short for the Tropical Responses to Altered Climate Experiment. By turning up the heat now, scientists can study the forest's future early—while there is still time to learn from it.

Each warmed plot is a circle about 4 meters wide—roughly the size of a small classroom rug. Six infrared heating panels hang in a ring above it, like warm lamps pointed down at the plants. The scientists also keep plain, unheated circles nearby to compare. The heaters are careful: they hold the warmer plots almost exactly 4°C hotter than normal, all day and night.

Each heated plot is a circle about 4 meters across, ringed by six infrared heating panels raised roughly 2.6 meters above the ground. Nearby, scientists keep unheated control plots—ordinary forest used for comparison—so they can tell which changes come from the heat and which would have happened anyway. The system is remarkably precise: it holds the warmed plots at the target 4°C above the controls to within about a tenth of a degree, hour after hour. That accuracy lets researchers trust that what they see is the forest reacting to warming itself.

One big surprise is that plants don't all answer the heat the same way. Scientists watched two understory shrubs. One, called Psychotria brachiata, learned to handle the warmth—it kept making food from sunlight even when things got hot. The other shrub did not adjust as well. So a warmer forest may favor some plants over others, slowly changing who grows where.

A striking lesson from TRACE is that not every plant responds to heat the same way. Researchers compared two understory shrubs. Psychotria brachiata showed acclimation—it adjusted, broadening the temperature range where it could still photosynthesize (turn sunlight into food) by roughly 6°C in the heated plots. A second shrub, Piper glabrescens, did not up-regulate its photosynthesis the same way and performed worse as temperatures rose. That difference matters: if some species adapt and others struggle, warming could gradually reshape which plants make up the forest understory.

El Yunque faces a sharp kind of climate danger. Far away, the Hoh rainforest in Washington changes slowly, as the mountain glaciers that feed it shrink over many years. But El Yunque gets sudden, hard hits. In 2017, Hurricanes Irma and Maria tore through Puerto Rico. The TRACE heaters had to shut off while the team rebuilt. Warming started again in 2018. So this forest must face both hurricanes and rising heat at the same time.

El Yunque's climate threat is acute—fast and forceful—unlike the slow change of the Hoh rainforest in Washington State, where the mountain glaciers that feed the forest shrink gradually over many years. Here the danger arrives suddenly. In September 2017, Hurricanes Irma and Maria swept across Puerto Rico, and TRACE itself was knocked offline; the team spent roughly a year rebuilding before warming restarted in 2018. That makes Luquillo a rare place to study two stresses at once—storms and steady heat. Encouragingly, researchers found the understory plant community was more resilient, or able to bounce back, than expected, even as warming nudged it toward faster-growing, drought-tolerant species.