Trees of the Amazon and Andes are not adapting quickly enough to climate change

"Climate debt" threatens entire equatorial forest biomes

An international team of scientists found climate change has accelerated to such a degree that Amazonian and Andean tree species are not adapting fast enough to the changing conditions brought about by a warming world.

As temperatures increase, many trees and other plant species shift their range to stay in temperatures consistent with what they have been adapted to historically. In general, species in the Northern Hemisphere are shifting northward; in the Southern Hemisphere, they shift south. Plants and other species will also shift upslope on mountains.

The research reveals that species in the Amazon and Andes rarely make the geographic shift seen elsewhere. Tree species are not moving to higher elevations and plant communities are not reorganizing at a sufficient pace to respond to warming trends.

“These forests are simply not keeping up with climate change,” said the lead author of the paper, 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.”

The scientists found no consistent directional change for trees in the lowlands of the Amazon. While this may indicate resilience for lowland biomes in the short term, long-term vulnerability is likely, mostly due to heat stress and climate change-driven drought. There were areas that showed some degree of climate adaptation, notably in mid-level forests, which lie at around 4,000 to 6,500 feet in elevation.

Collecting data over more than four decades, the scientists included over 66,000 trees of more than 2,500 species spanning an 11,500-foot elevation gradient in Peru and Bolivia. Findings were published this past Wednesday in the Proceedings of the National Academy of Sciences of the United States of America (PNAS), a multidisciplinary, peer-reviewed scientific journal.

The climatic strain on the tree species affects the rest of their biomes: all the pollinators that feed on and propagate the species, as well as the creatures that rely on the trees for food and shelter. These changes are particularly critical in the tropical forests of the Andes and Amazon, which have some of the highest levels of biodiversity in the world. These forests are also among the largest and most important components of “Earth’s lungs,” which sequester carbon, release oxygen, and regulate the Earth’s climate.

In 2023, the temperature average for the month of October in the Amazon river basin broke all previous records, rising 5.4 degrees Fahrenheit above normal.

Unlike temperate forests, tropical species often cannot tolerate wide temperature variations. And they often lack access to places to which they might migrate. Deforestation also remains a problem in the Amazon and is surging in some places like Colombia.

A similar study from two years ago, also published in PNAS, found comparable climate-driven problems for western forests in the United States.

The authors of the present study state that species’ lag in climate adaptation creates the climate debt Dr. Farfan-Rios mentioned. If the debt accumulates, it will push ecosystems past climate-change tipping points. This means the Amazon rainforest will lose some or most of its ability to store carbon. As a further result of climate debt, plant and animal species will lose habitat, increasing the risk of ecosystem collapse, especially in mid-elevation cloud forests where mortality is high.

More than 20 scientists from institutions across the Americas and Europe contributed to this study, including Wake Forest University, the University of Miami, Oxford University, Washington University in St. Louis, the University of Leeds and the Missouri Botanical Garden. Also contributing to this work were individuals from the Andes Biodiversity and Ecosystem Research Group (ABERG), the Amazon Forest Inventory Network (RAINFOR) and ForestPlots.net. Funding was provided by the National Science Foundation Long-Term Research in Environmental Biology program, the Gordon and Betty Moore Foundation’s Andes to Amazon Initiative, and other organizations.

This present study is one of the most comprehensive, long-term monitoring efforts in the tropics. “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 went on to say that while tree communities can adapt to changing conditions, taking thousands of years to do so, individual trees die quickly and new seedlings 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.”