- Scientists studying Peru’s Boiling River found 11% fewer tree species for every 1°C (1.8°F) increase in temperature, offering insights into how climate change might affect the Amazon Rainforest.
- The research team discovered that hotter areas not only had fewer species overall but were dominated by heat-tolerant trees that typically grow in the warmest parts of South America.
- The study site is protected by Indigenous Asháninka people as sacred land, but the forest still faces threats from nearby deforestation and fires, reflecting broader challenges across the Amazon.
- The Amazon is experiencing climate pressures, with fire-affected areas in the Brazilian Amazon increasing 18-fold in September 2024, covering a combined area nearly the size of the Netherlands.
“You’re engulfed in waves of steam and sweating buckets and there’s no cool water anywhere,” Kenneth Feeley, a professor at the University of Miami, tells Mongabay. He’s describing the Boiling River in Peru’s Amazon Rainforest. “If you look at the heat index, everything says immediate danger of heat stroke.”
The Boiling River gets its name from thermally heated water bubbling up from underground, creating distinct temperature zones in the surrounding forest. Within less than a kilometer (just over half a mile), temperatures vary by up to 11° Celsius (19.8° Fahrenheit), providing scientists with a natural laboratory to study how rising temperatures may affect tropical trees. This natural hotspot has existed for thousands of years, offering a window into how forests might look in a warmer future.
In a study published in Global Change Biology, researchers found that for each 1°C (1.8°F) increase in temperature, there were 11% fewer tree species in the area. These temperature effects on tree diversity are significant, the authors say, given that scientists predict the region could warm by 3-6°C (5.4-10.8°F) by 2100.
To study these effects, the research team, led by Riley Fortier, a graduate student at the University of Miami, established 70 circular study plots, each with a diameter of 6 meters (20 feet). In each plot, they measured and identified every woody plant with a trunk diameter of 2 centimeters (nearly an inch) or larger. They also measured air and soil temperatures, and humidity.
The team discovered two key patterns in the hotter areas of the forest. First, there was a significant drop in tree species diversity, from an average of 8.6 species per plot in cooler areas to just 5.7 species in the hottest areas. Second, the surviving trees weren’t random; they were species known to thrive in hot conditions across South America.
“The surprising part,” Fortier said, “is that the species in the hot part of this forest are species that grow in hotter forests throughout the continent. I didn’t think that we would be able to detect that kind of change just based on our small, little plots.”
The team’s analysis went beyond just counting trees. They collected soil samples from 15 locations to analyze nutrients, used satellite imagery to map surface temperatures, and employed sophisticated statistical methods to understand how temperature shapes forest communities. The results showed that temperature alone can strongly influence which tree species survive in an area, even over very short distances.
“This is a clever study conducted in a unique location that helps us understand which tree species might survive or disappear as temperatures rise,” William Farfan-Rios, a biodiversity fellow at Wake Forest University, U.S., who wasn’t involved in the research, told Mongabay.
However, some scientists urge caution in extrapolating these findings. “Careful considerations must be placed in generalizing the findings from this study into the entire Amazonian Forest,” said Jingjing Liang, a professor at Purdue University, U.S., who wasn’t involved in the research.
Water availability, not just temperature, might be more important for tree diversity in much of the Amazon Basin, where seasonal rainfall patterns and drought stress often determine which species can survive. The Boiling River site is unusual because it maintains high humidity even in hot areas, whereas most parts of the Amazon tend to become drier as they get hotter.
Alyssa Kullberg, who worked on the study as a Ph.D. candidate at the University of Miami, acknowledged these limitations. “We don’t really expect to see these hot and wet conditions that we see at the Boiling River [in other parts of the Amazon],” she said, “but knowing that just heat alone is leading us to this observation of an 11% decline in diversity per 1° increase in mean temperature, I would call it a pretty stark warning.”
The research site near the city of Pucallpa lies in one of Peru’s most heavily deforested regions. The forest persists, the researchers say, because it’s protected by the Indigenous Asháninka people, who consider it sacred.
“This is really a little haven for the forest in the region,” Fortier said. “The surrounding forest is just completely gone. This is here because of the people who protect it and view it as sacred.”
The study was conducted with permission from the local Asháninka family that owns and protects most of the land around the river. They maintain the site as a spiritual center where ayahuasca ceremonies are held. This Indigenous stewardship has created what Kullberg describes as a “bubble” of well-preserved forests in a region otherwise known for high rates of deforestation.
Yet even this protected area faces threats. “The deforestation is coming closer every day,” Feeley said. “This year they had to clear out the camp because the fires were near enough that it was smoky and people were having trouble breathing.”
The Boiling River research findings about temperature effects on forest diversity emerge as the Amazon faces multiple threats. Brazil contains about two-thirds of the Amazon rainforest, and what happens here impacts the entire Amazon. There was an 18-fold increase in fire-affected areas in September this year from the same time last year, to nearly 40,000 square kilometers (15,400 square miles) in the Brazilian Amazon, or an area nearly the size of the Netherlands. (In the Amazon, fires aren’t naturally occurring, but rather are set after deforestation to clear the land most often for cattle ranching and soy farming.)
The rainforest depends on its trees to generate much of its own rainfall through transpiration, the process by which trees release water vapor into the atmosphere. This self-sustaining cycle is under threat from deforestation, fire, and rising temperatures. When trees are cut down or die less moisture is released into the air. This makes the remaining forest drier and more susceptible to fires, which in turn kills more trees.
Scientists warn that if this cycle continues, parts of the Amazon could reach a tipping point where forest species are lost and areas transition into more degraded, savanna-like environments.
Protecting the Amazon Rainforest requires both local and worldwide efforts. In many areas, Indigenous peoples protect the forest on their lands, so many advocate for demarcation or official recognition of Indigenous land tenure rights. Since greenhouse gases from burning fossil fuels are the primary contributor to rising temperatures, lowering fossil fuel emissions is also a global priority.
“This [study] can tell us a lot about which species we can continue to rely on, for timber, for foods, for medicines, and which species we should think long and hard about,” Fortier said. “Are we going to see these species thriving in the lowland Amazon in the next 50 or 100 years?”
Banner image of study author, Dr. Alyssa Kullberg, next to rising steam of Peru’s boiling river and one of her study species, balsa (Ochroma pyramidale) in Peru. Photo courtesy of Riley Fortier.
Citation: Fortier, R. P., Kullberg, A. T., Soria Ahuanari, R. D., Coombs, L., Ruzo, A., & Feeley, K. J. (2024). Hotter temperatures reduce the diversity and alter the composition of woody plants in an Amazonian Forest. Global Change Biology, 30(11), e17555. doi:10.1111/gcb.17555
Liz Kimbrough is a staff writer for Mongabay and holds a Ph.D. in ecology and evolutionary biology from Tulane University, where she studied the microbiomes of trees. View more of her reporting here.
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