Tropical forests have long been considered vital allies in the fight against climate change. Their dense canopies absorb vast amounts of carbon dioxide, cool the atmosphere, and support extraordinary biodiversity. For years, scientists assumed these ecosystems were resilient to rising global temperatures. However, new research challenges that belief and suggests that the soils beneath these forests may actually accelerate global warming rather than slow it.

New Research Points to a Climate Feedback Loop

A groundbreaking study by the U.S. Forest Service, with support from Chapman University, reveals that tropical forest soils are more active participants in climate dynamics than previously understood. Conducted in Puerto Rico’s Luquillo Experimental Forest, the research involved heating the soil by four degrees Celsius using infrared lamps and measuring the resulting carbon dioxide emissions every 30 minutes over a full year. The results were startling. Warmed soils released between 42 and 204 percent more carbon dioxide than the unheated control plots. Rather than absorbing more carbon as temperatures rose, these soils began to emit it at alarming rates.

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Microbes, Not Roots, Are Driving the Emissions

Contrary to expectations, it was not the tree roots that fueled the increased emissions. In fact, root biomass declined by over 30 percent in the heated soils. Instead, it was the microbial communities that surged in response to the warmth. Microbial biomass rose by more than 50 percent, and with it came a spike in decomposition activity. These microbes, feeding on organic matter in the soil, released carbon dioxide far more aggressively under warmer conditions. This microbial-driven feedback loop appears stronger than anything seen in previous tropical warming studies.

Moisture Levels Complicate the Warming Response

The role of water added another layer of complexity. Warming did not affect every part of the forest in the same way. On lower slopes, soils dried out, reducing aeration and limiting microbial respiration after rainstorms. Mid-slope areas, though also drier, still saw emissions rise. Meanwhile, upper slopes grew wetter under warming, leading to explosive increases in carbon release. These varied responses show how topography and moisture interact with temperature, creating unpredictable “hotspots” of soil carbon loss within a single ecosystem.

Microbial Adaptation Could Lock In High Emissions

Another key finding emerged from the measurement of a factor known as Q10, which gauges how sensitive respiration is to rising temperatures. In the warmed plots, Q10 fell by more than 70 percent. This means the soil microbes adjusted their metabolism, maintaining high emission levels even without further temperature increases. Essentially, the soils adapted to a new climate state, one where carbon release is locked in regardless of future warming. This behavior is not accounted for in many global climate models and represents a potential blind spot in our climate projections.

The Numbers Match Entire Forest Emissions

The scale of the emissions shocked researchers. On one of the forest slopes, the amount of carbon released by warmed soil alone each year equaled the annual productivity of entire temperate forests. Extrapolated across the tropics, such emissions could push global warming beyond what is currently predicted. Rather than being passive carbon sinks, tropical soils might act as active carbon sources in a warming world.

Previous Theories May Have Underestimated the Risk

Older models assumed tropical soils were relatively insensitive to temperature changes, based largely on laboratory experiments or elevation gradients. However, those methods failed to replicate the complex interactions of roots, microbes, and water in real forest conditions. This new experiment, the first of its kind in a tropical rainforest setting, exposes the urgent need to reassess soil feedbacks in climate models.

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From Carbon Sink to Carbon Source

Tropical forests are spread across more than 30 ecological life zones, each likely to react differently to climate stress. While the TRACE project focused on one site in Puerto Rico, the implications are global. Scientists caution that without field experiments across a wide range of tropical forests, we cannot fully understand the risks of these ecosystems tipping from carbon sinks to carbon sources.

The Soil Beneath Forests Deserves Attention

So far, much of the climate debate has focused on preventing deforestation and reducing emissions from aboveground biomass. This study shifts attention to what lies below. The soil, alive with microbial activity, holds the potential to either help stabilize the climate or rapidly worsen it. If trends observed in Puerto Rico spread across the tropical belt, the pace of global warming could intensify in ways that current strategies are unprepared to handle.

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