World’s most diverse forests are failing to adapt to climate change

Forests from the Amazon lowlands to the high Andes are failing to adjust quickly enough to rising temperatures. Populations are not moving upslope at the pace of climate change, and entire communities are not reorganizing to match the new normal.

The gap threatens tropical biodiversity and the ecosystem services people rely on – from climate regulation to pollination.

“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.”

Four decades of first monitoring

The study draws on more than four decades of forest monitoring in Peru and Bolivia. Researchers tracked over 66,000 trees spanning about 2,500 species across 66 permanent plots.

Those plots run from a few hundred feet above sea level in the Amazon to more than 12,000 feet in the Andes.

The team expected to see warm-adapted species becoming more common over time. Ecologists call that shift thermophilization. Instead, they found little to no evidence that communities are warming in step with the climate along the elevation gradient.

A growing climatic debt

The analysis shows that the rate at which warm-adapted forests replace cool-adapted ones is far slower than the regional rate of warming.

That mismatch is the “climatic debt” – the amount of change that should have occurred to keep pace with rising temperatures but has not.

If the debt keeps growing, forests will edge closer to thresholds from which they may not recover. The risk is highest where species live near their thermal limits and have limited options for movement.

Cloud forests show most change

Mid-elevation forests near the cloud base at roughly 3,900 to 6,600 feet showed the strongest signs of reshuffling. The signal there is driven largely by higher death rates among cool-adapted trees. Mortality is doing more of the work than the arrival of new warm-adapted species.

By contrast, the lowland Amazon plots showed no consistent directional change. That stability may appear resilient in the short term – but it also signals vulnerability. As heat and drought intensify, communities with few hotter-adapted species waiting in the wings may struggle to cope.

Higher in the Andes, many communities remain effectively stuck. Populations are not moving uphill quickly enough to keep within their historical climate envelopes.

Tropical forests in the Andes-Amazon harbor the planet’s highest concentration of life. They store vast amounts of carbon and generate rainfall. They support pollinators and seed dispersers that keep the system running.

If tree communities fail to adjust, the carbon sink will weaken. Habitats shrink or vanish. The odds of ecosystem collapse rise, especially in cloud forests where drought and heat push trees past their limits.

Time reveals forest shifts

“You have to be there for long periods of time to understand how these forests change,” said co-author Miles Silman, 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.”

Trees can adapt over millennia. But individuals die quickly, and new trees recruit and grow slowly. Dispersal adds another constraint.

“They also need the full complement of animal dispersers and pollinators to help expand their range – and loss of habitat is shrinking their ranks,” Silman explained.

“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 tropical constraint

The results align with a growing pattern: tropical species often have narrow heat tolerances and already live close to their maximum temperatures.

Unlike in temperate systems, there may be few hotter-adapted taxa ready to move in as heat rises. In the lowlands, there is literally nowhere cooler upslope to go.

In the Andes, topography offers ladders for movement. But climbing those ladders requires time, intact habitat, and healthy animal partners to carry seeds. Fragmentation and defaunation remove those rungs.

Forests can’t keep up

Protecting continuous elevational forests gives species a path to track their climates. Safeguarding pollinators and seed dispersers keeps the engine of range expansion running. Reducing local stressors – selective logging, fire, and overhunting – buys time.

Still, cutting greenhouse gas emissions remains the decisive lever. Slower warming reduces the debt and gives forest communities a chance to catch up.

The study’s core message is simple yet sobering: the world’s richest forests are changing, but not quickly enough in the directions that would make them safer. Thermophilization is lagging far behind warming, and mortality – rather than recruitment – is driving most of the turnover.

Cloud forests are showing strain, and although the Amazon lowlands are quiet for now, that quiet may not last. “These forests are simply not keeping up with climate change,” Farfan-Rios concluded.

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