In the heart of Brazil’s Atlantic Forest, an ancient ecosystem with a unique biodiversity, climate change is causing a subtle yet significant shifts. Species of trees are moving around, searching for more suitable conditions.

When climate change starts to hit and temperatures get hotter year after year, individual trees can’t move around. It’s different with tree populations, however. These populations have been shown to “migrate” over time. And this is exactly what’s happening now in the Amazon.

The Earth’s lungs are grasping for air

The Brazilian Atlantic Forest (or Mata Atlântica) stretches along the eastern coast of Brazil. It is one of the world’s most diverse ecosystems, home to thousands of plant and animal species, many of which are found nowhere else on Earth. However, this biodiversity hotspot is under threat from deforestation, urbanization, and now, climate change.

The study, conducted by an international team of researchers, analyzed the distribution of 627 tree species from 96 different plots. They calculated the Community Temperature Score (CTS) for different life-history stages of trees, juveniles and adults. From this data, they were able to infer migration trends.

The CTS is a measure of the optimal temperature niche for each species based on their abundance across different sites. The study’s findings show that around half of tree species are migrating, but not always in the way you’d expect. Some species are moving upward in search of cooler climates, while others show unexpected downward movements.

“We found that different species are moving in different directions—in lower forests, trees are moving downhill more often than uphill, probably due to factors besides temperature, like competition between species,” says lead author Dr. Rodrigo Bergamin, from the University of Birmingham.

This is particularly troublesome for trees that need colder temperatures and trees that have nowhere to go. These groups face dire risks, the scientist adds.

However, in the higher altitude forests, most species are moving uphill as temperatures rise and the undergrowth becomes more suited to those trees favoring warm temperatures. This could mean that species needing colder temperatures are at risk of dying out as the world continues to warm.

“Species from higher altitudes are generally more sensitive to temperature and those that need cold are more likely to lose out in competition under warmer temperatures to species that prefer hotter temperatures,” adds Sandra Müller from the Federal University of Rio Grande do Sul, senior author of the study.

A dynamic forest

The results show that 27% of tree communities exhibited upward migration, with younger trees establishing themselves at higher elevations than their adult counterparts. This trend was most pronounced in montane forests, where temperatures are cooler. Conversely, 15% of communities showed downward migration, primarily in lowland forests.

These migration patterns have profound implications for the forests. In higher-altitude forests, the upward shift could lead to a dominance of warm-adapted species, potentially displacing those adapted to colder climates. In lowland forests, the observed downward migration could be a response to competitive interactions rather than climate alone. Species from higher elevations might be moving downwards due to changes in the local biotic environment, such as reduced competition or altered resource availability.

The type of soil could also be playing an imporant role. The region with the right temperature may not have the best type of soil for a species. This scenario would put trees in a lose-lose situation, where they either face difficult temperatures or difficult soils. While the study did not analyze this directly, animals are likely also affected by these changes, as their ecosystem shifts within a few generations.

Probably a widespread problem

The study also highlights the importance of ecotones — transition zones between different forest types. These areas are particularly sensitive to climate change and can act as barriers to species migration. In the Atlantic Forest, a sharp transition occurs around 400 meters above sea level. Below this elevation, tropical species dominate, while above it, cooler-climate species are more prevalent. The rise in temperatures is likely making these transition zones more permeable, facilitating the movement of warm-adapted species upwards.

The study only looked at one part of the Amazon, but there’s a good chance some tree migration is happening in most if not all parts of the Amazon rainforest.

“This study showed what is happening in the South of the Atlantic Forest, but different regions might show other trends. We are now bringing together researchers from across the whole biome to create a big picture of how these forests are responding to global change,” said Dr. Adriane Esquivel Muelbert, associate professor from the University of Birmingham, co-author of the study.

As climate change continues to alter temperature and precipitation patterns, ecosystems around the world are experiencing similar shifts. Understanding these changes is crucial for conservation efforts, as it allows scientists and policymakers to predict which species are most at risk and to develop strategies to protect them.

Journal Reference: Rodrigo Scarton Bergamin et al, Elevational shifts in tree community composition in the Brazilian Atlantic Forest related to climate change, Journal of Vegetation Science (2024). DOI: 10.1111/jvs.13289