Site: Luquillo LTER
Tropical forest at El Verde, Puerto Rico, site of the Luquillo Long-Term Ecological Research Program.

Tropical forests (Fig. 1) absorb more atmospheric carbon dioxide (CO2) than any other terrestrial biome globally.

They also account for about 30% of global net primary production (i.e., carbon uptake) in about 17% of the land area. These ecosystems play a key role in the global carbon cycle, but there is growing concern that global warming could decrease the ability of tropical forests to remove atmospheric CO2, thus accelerating climate change. Humid tropical forests are not generally thought to be sensitive to minor warming because they experience warm and wet conditions throughout the year, with little seasonality in rainfall and temperature compared to higher latitudes. However, long-term research at the Luquillo (LUQ) LTER program suggests that these ecosystems may be adapted to near constant and favorable climatic conditions and that even relatively small changes in climate could have large consequences for atmospheric CO2 and carbon cycling.

Using a long-term data set from LUQ, scientists found strong seasonality in carbon inputs (litter production) and carbon exports (soil respiration) in a humid tropical forest (Fig. 2). Carbon fluxes doubled over the course of the year, and the temporal patterns differed between inputs and exports. Using long-term environmental monitoring it became clear that these two large carbon fluxes were responding to different environmental drivers. Plant litterfall, an index of plant productivity, responded most strongly to small changes in light availability over the year. These small changes in light were due in part to patterns in cloudiness, which slows photosynthesis and carbon uptake. By contrast, soil respiration, an index of root and microbial carbon respiration resulting in soil CO2 emissions, was most sensitive to small changes in temperature. Potential increases in microbial respiration with temperature are of particular concern. Although microbial activity is an essential component of nutrient and energy cycling in ecosystems, microbes consume stored carbon and have the potential to release considerable CO2 to the atmosphere while depleting soil carbon stocks.

There are several implications of this research. These data show that the carbon cycle in tropical forests is sensitive to climate change and that the potential exists for tropical forests to release stored carbon with global warming. If carbon fluxes become decoupled due to climate change – i.e., if carbon uptake slows, while microbial respiration increases — the carbon balance of these ecosystems could shift, resulting in more rapid accumulation of heat trapping CO2 in the atmosphere. The LUQ findings suggest that we may not be able to rely on tropical forests to continue to remove excess CO2 released to the atmosphere from human activities such as fossil fuel combustion.

Graph for
Carbon flux in terms of litterfall and soil respiration in tropical forests at El Verde, Puerto Rico. Both measures are higher in the slightly warmer (2° C) time of year, showing that tropical forest carbon emissions may increase with slight warming.