Small climate changes have drastic effects on tropical plants

A team of biologists from Brown University has recently aimed to understand the impact of climate change on plant species in tropical mountain regions. 

The researchers discovered that even slight variations in temperature and moisture could have substantial effects, threatening the survival of plants and the ecosystems they support.

Plants in tropical mountains 

Emily Hollenbeck, who conducted this research while earning her Ph.D. in ecology and evolutionary biology from Brown, conducted extensive experiments in the Monteverde mountain region of Costa Rica. 

The research revealed that small changes in climate conditions drastically affect the survival of epiphyte species, strengthening concerns about widespread extinctions due to climate change in these areas.

Hollenbeck, passionate about the impact of climate change on tropical forests, spent five years documenting occurrences of epiphyte species across three mountains in Costa Rica and Panama. 

Tropical plants struggling to survive 

On one mountain, she transplanted plant species to different sites varying in elevation, temperature, and aridity, and observed the effects on the plants. 

The study, published in Nature Communications, found that most epiphyte species struggled to survive outside their native climate ranges. This research highlights the heightened extinction risks from climate change in tropical mountain ecosystems.

The natural world is shifting 

“It’s already obvious to people who live here, even those who aren’t biologists, that the natural world has been shifting in striking ways over the last 20 or 30 years in response to climate change,” said Hollenbeck, now president of the Monteverde Conservation League, a Costa Rican nonprofit focused on conserving tropical ecosystems. 

“It felt really important to conduct a very specific, well-controlled scientific study to provide context and evidence to support what we’re seeing.”

Extinction risks are not well understood 

Study co-author Dov Sax is a Brown professor of ecology, evolution, and organismal biology who served as Hollenbeck’s dissertation advisor. 

Professor Sax noted that while it is well-known that climate change poses extinction risks, the specifics of these dynamics are less understood, especially in tropical mountain regions, which host some of the world’s most diverse ecosystems.

According to Sax, most extinction risk estimates are based on statistical models that consider current climate conditions and whether these conditions will exist nearby in the future. 

Poorly documented tropical plants

The statistical models work well in regions like the United States and Europe, where species distributions are well-documented, but they are less effective in tropical settings with poorly documented species distributions and unclear tolerance levels to new conditions.

“The field doesn’t have a good handle on just how bad the extinction risk to tropical organisms might be under different levels of climate change,” Sax said. This study directly addresses these limitations for tropical epiphytes in the mountains of Costa Rica and Panama, requiring a significant amount of work. 

Adaptable plants are still vulnerable

Hollenbeck and Brown research assistants surveyed about 70 epiphyte species across three mountain ranges, transplanting around 1,500 individual plants to various elevations and climates. The plants were monitored every three months over three years.

“We found that these species really are as sensitive to small changes in climate conditions as has been widely feared, and which some previous, much smaller-scale experiments had found in the past,” Sax said. Even the most adaptable plants fared worse than expected, emphasizing their vulnerability.

A tipping point for tropical plants 

The study also indicates vastly different outcomes based on temperature scenarios predicted by 2100. Under a low-emission scenario (1.5 degrees Celsius above pre-industrial levels), most plant species studied will survive. 

However, under a moderately high-emission scenario (3.2 degrees Celsius above pre-industrial levels), 5% to 36% of species may go extinct in the region, with 10% to 55% disappearing from the specific mountain studied.

“It is a little alarming just how close to a threshold or tipping point we are for these tropical species,” Sax said, noting that Earth’s surface temperature has already increased by one degree Celsius since the pre-industrial era. 

“On the one hand, this gives us hope if we really can dramatically slow climate change, but on the other hand it shows just how little additional warming is needed to lead to the massive extinction events that we have feared might occur.”

Cascading effects of plant extinctions

Professor Sax added that with moderate warming, a third of all epiphytes could be lost by the century’s end, leading to unpredictable and far-reaching effects on the ecosystem and the local economy.

Hollenbeck, living in Costa Rica and involved in teaching science and running research fellowships, described epiphytes as the canaries in a coal mine for tropical forests. 

“Getting this level of concrete data involved a stupefying amount of fieldwork over a long time and applies to about 70 species of plants,” she said. “But this research just scratches the surface in terms of how climate change is affecting different species.”

This extensive research underscores the urgent need to address climate change to protect vulnerable tropical ecosystems and the myriad species they support.

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