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A new study published today by Wake Forest University and an international team of scientists reveals that tree communities across the Amazon and Andes are not adapting quickly enough to climate change, with major implications for the future of tropical biodiversity and ecosystem services like climate regulation and pollination.

The research, spanning more than 40 years of forest monitoring, shows that tree species are not shifting populations upslope and communities are not reorganizing fast enough to match rapid warming trends. While some mid-elevation forests near the cloud base show signs of adjustment, vast areas of the Amazon and high Andes remain effectively stuck, researchers said, raising alarms about forest resilience.

"These forests are simply not keeping up with climate change," said lead author William Farfan-Rios of Wake Forest University. "The result is a growing climatic debt that threatens the integrity and functioning of the most diverse forests on Earth."

Their findings, "," appear in Proceedings of the National Academy of Sciences.

The scientists had expected to see a greater prevalence of warm-adapted tree species over cool-adapted ones through time, a process called thermophilization. But, after looking at how trees are appearing, growing and dying at each elevation, the scientists saw little to no evidence of thermophilization along the elevation gradient from a few hundred feet in the Amazon to over 12,000 feet in the Andes.

The authors warn that the current lag creates a "climatic debt," or a mismatch between actual warming and the slow pace of species turnover. If unaddressed, this debt will accumulate, pushing ecosystems past tipping points.

Key findings include:

• Massive dataset: Over 66,000 trees, 2,500 species, and 66 long-term forest plots were monitored along the Amazon-to-Andes elevational transects in Peru and Bolivia.
• Slower-than-warming shifts: Average "thermophilization rates"—the increase of warm-adapted species relative to cool-adapted ones—were an order of magnitude slower than the regional rate of warming.
• Cloud-base hotspots: Mid-elevation forests (1,200-2,000 m) showed the strongest signals of compositional change, driven largely by increased mortality of cool-adapted trees.
• Amazon inertia: Lowland Amazon forests showed no consistent directional change, suggesting resilience in the short term but high long-term vulnerability to heat stress and drought.
• Drivers of change: Tree mortality was the dominant factor, not recruitment of new warm-adapted species.

Why it matters

Tropical forests in the Andes and Amazon represent the highest concentration of biodiversity on the planet and play a critical role in regulating Earth's climate. If tree communities fail to adjust to warming, scientists warn of:

• Reduced carbon storage and weakening of the Andes and Amazon as a global carbon sink.
• Loss of critical habitats for plant and animal species.
• Increased risk of ecosystem collapse, especially in mid-elevation cloud forests where mortality is high.

"You have to be there for long periods of time to understand how these forests change," said co-author Miles Silman, Andrew Sabin Presidential Chair of Conservation Biology at Wake Forest. "If we lose these climate observatories, these natural labs, we blind ourselves to our future. What we found is that forests are changing, but they're not changing in the ways that make them resilient to climate change."

Silman noted that tree communities can adapt over thousands of years, but individual trees die fast, and new ones recruit and grow slowly. "They also need the full complement of animal dispersers and pollinators to help expand their range—and loss of habitat is shrinking their ranks. If you look at the magnitude of changes happening in the Andes-Amazon, the forest communities likely are not going to keep up. That's why research like this is important."

The study adds to a growing body of evidence that species in the tropics are less able to track climate change than those in temperate zones. Unlike temperate forests, tropical species often have narrow heat tolerances and limited places to migrate, particularly in the lowlands where no hotter-adapted species exist to move in.

"Amazonian and Andean tree communities are not tracking current climate warming" was authored by a team of more than 20 scientists from institutions across the Americas and Europe, including Wake Forest University, the University of Miami, Oxford University, Washington University in St. Louis, the University of Leeds and the Missouri Botanical Garden.

The research draws on decades of collaboration through networks such as the Andes Biodiversity and Ecosystem Research Group (ABERG), the Amazon Forest Inventory Network (RAINFOR) and ForestPlots.net, representing one of the most comprehensive long-term monitoring efforts in the tropics.