Tropical forests are a key regulator of the global carbon cycle, exchanging more carbon dioxide (CO2) with the atmosphere than any other terrestrial biome. Climate models predict unprecedented climate warming in tropical regions. However, in situ field studies of warming in tropical forests are critically lacking. The prevailing view was that tropical ecosystems would be less sensitive to warming than ecosystems at higher latitudes, based largely on laboratory experiments and studies of altitudinal gradients. However, a new large-scale study conducted in Puerto Rico yields surprising results that challenge this paradigm.

The TRACE experiment in Puerto Rico

Researchers at the Tropical Responses to Altered Climate Experiment (TRACE) in the Luquillo Experimental Forest in Puerto Rico artificially raised soil and understory temperatures by 4°C above ambient to a depth of at least 50 cm. They used a hexagonal array of infrared heaters to achieve this warming. The study ran for a year (September 2016 to September 2017), with half-hourly soil respiration rates measured continuously, resulting in 57,450 measurements. The three heated plots were paired with three control plots with similar topographic locations (lower, middle, and upper slopes) to ensure comparability.

Dramatic increase in soil respiration

The results of the experiment showed that soil respiration rates were in the heated plots 42 % to 204 % higher compared to control plots. These rates represent some of the highest soil respiration rates recorded in any terrestrial ecosystem. Warming led to the release of significant amounts of additional CO2 into the atmosphere: 6.5, 9.7, and 81.7 Mg CO2-C per hectare per year more in the warmed plots on the lower, mid, and upper slopes, respectively. For example, the additional CO2 released from the mid slope was equivalent to the total annual net primary production (NPP) of the temperate grassland. The most dramatic increase (204 %) was observed on the upper slope, making it "hot spot" for CO2 releaseThese findings are significantly higher than the 95 % range observed in warming experiments in Northern Hemisphere forests (12–31 %).

Changes in mechanisms and soil moisture

Despite the significantly higher breathing rate, temperature sensitivity (Q10) of the soil decreased by 71.7 % in the heated plots (from 2.51 to 0.71). Although Q10 decreased, respiration rates themselves were higher, suggesting a shift towards overall higher basal metabolic rates. The study also found that fine root live biomass was 32 % lower in the heated plots after six months of warming. Conversely, The biomass of soil microorganisms increased significantly, by more than 50 % in heated plots. These results indicate that the increase in soil respiration was mainly due to increased CO2 release from microbial sources, while the contribution of CO2 coming from roots decreased.

The interactions with soil moisture are also interesting. While the soil on the lower and middle slopes was drier under the influence of warming (6.8 % and 16 % less water), the soil on the upper slope was surprisingly wetter (18 % more water). These data suggest that warming will partly regulate soil respiration through interactions with soil moisture.

Implications for future climate

These findings refute a long-standing assumptionthat tropical forests will be relatively insensitive to warming. Soil respiration rates were significantly higher (by 42–205 %) in heated plots in all topographic locations. This has significant implications for future climate projections on a global scale and highlights that tropical forests may lead to large-scale carbon losses in a warmer world. Study It also shows that the responses of tropical ecosystems are complex and may vary depending on the specific forest type and conditions, underscoring the need for further in situ research. JRi