Past research implied that positive feedback might exist between climate change and greenhouse-gas emissions from soil. A study finds that drought-induced declines in such emissions from tropical forests could counter climate change.
Tropical forests have been aptly described as the planet's lungs. Tropical vegetation inhales vast amounts of carbon dioxide and exhales oxygen to the atmosphere, and in so doing, provides a vital climate benefit by absorbing some of the nearly 9 petagrams (1 petagram is 1015 g) of carbon that human activities release into the atmosphere each year1. What's more, tropical soils influence climate by releasing large amounts of CO2 as a by-product of the decomposition of organic matter, and through several other soil microbial processes that both emit and consume greenhouse gases, including nitrous oxide and methane2.
Important questions remain about the effect of climate change on greenhouse-gas fluxes from soil, and hence to what extent the beneficial climate services provided by tropical forests will persist in the future. Writing in Global Biogeochemical Cycles, Wood and Silver3 report that an experimentally induced drought had reduced greenhouse-gas emissions from tropical soil to the atmosphere, an effect that represents a negative feedback to climate change.
Many of the scenarios used to predict future climate focus on rising temperature, but changes in the water balance of tropical forests might have more immediate and pronounced effects. Although substantial uncertainty remains, most climate models predict significant declines in rainfall across large areas of the tropics4,5. So how might such declines alter the net balance of the gases that contribute to climate change?
To address this question, Wood and Silver manipulated rainfall in a Puerto Rican tropical forest (Fig. 1) to test how drought affects soil emissions of CO2, methane and nitrous oxide. Such experimental manipulations remain the gold standard for directly assessing the potential effects of environmental change on ecosystem processes, and can provide critical input to the development of better Earth-system models. The authors simulated the effects of a prolonged dry season by preventing rainfall from reaching the forest floor for three months, and measured greenhouse-gas emissions from soil before, during and after the experiment.
After the simulated drought, Wood and Silver documented profound declines in the efflux of CO2 from soil and increases in soil consumption of methane. In addition, the drought elicited surprising reductions in denitrification (the production of nitrous oxide), a process that is carried out by a tiny fraction of the soil's microbial community. Together, these effects lowered the global-warming potential of the soil emissions. When the heat-trapping ability of nitrous oxide is taken into account, a substantial reduction in total global-warming potential from all three gases was accounted for by the decline of denitrification. Because tropical forest soils are a major source of nitrous oxide emissions to the atmosphere6, this result implies that a drier climate could cause substantial reductions in global emissions of nitrous oxide from soil.
However, extrapolating results from experiments such as these is particularly challenging for tropical forests. In Wood and Silver's experiment, drought effects varied depending on landscape position: ridge and valley plots had larger responses to experimental drought than did those on slopes. This variation has considerable implications for models used to simulate biogeochemical responses to climate change, none of which is able to capture the complex interplay of factors that can create substantial biogeochemical heterogeneity within and across tropical forests7. The authors' results are a prime example of the challenge facing scientists — many tropical regions have highly complex landscapes with varying nutrient availability, which in turn can regulate biological processes that influence greenhouse-gas production8,9.
The possibility of positive feedback between climate change and greenhouse-gas emissions from soil has been recognized for decades. Wood and Silver's results suggest the opposite: declines in greenhouse-gas emissions following drought would reduce climate forcing. But as the authors highlight, the handful of studies in which rainfall has been excluded from tropical forests have shown positive, negative or no net effects of drought on soil greenhouse-gas emissions. Moreover, although it is crucial to consider the effects of drought alone, real-world emissions will hinge on the combined effects of changing precipitation and temperature, along with chronic shifts in atmospheric CO2 levels and nutrient deposition — factors that were not manipulated in the authors' experiment. Finally, studies10,11 have shown that the growth and carbon uptake of trees in the tropics are highly sensitive to climate, but the experimental plots used in Wood and Silver's study (1.54 square metres) were not large enough to simulate the potentially negative effects of drought on carbon uptake through tree growth. The overall effects of drought on the greenhouse-gas balance of the sites therefore remain unknown.
In recent decades, tropical forests have given us a discount on anthropogenic CO2 emissions by absorbing more greenhouse gases than human activity produces. Wood and Silver's findings suggest that tropical soils may continue to offset greenhouse-gas emissions during drought. Perhaps more notably, their study highlights the need for additional large-scale experiments that can more completely resolve the potential effects of climate change on trace gas emissions in tropical forests. The authors' data are critical for improving and validating models that predict ecosystem and climate responses over large spatial and temporal scales. Considering the pivotal role of tropical ecosystems in basic human health and welfare, our understanding of fundamental ecosystem processes, and their potential response to climate change, remains woefully incomplete in complex and diverse tropical forests.
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Journal of Geophysical Research: Biogeosciences (2013)