Experiment Aims to Steep Rainforest in Carbon Dioxide Sensor-Studded Plots in the Amazon Forest Will Measure the Fertilizing Effect of the Gas

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The ways in which rising carbon dioxide levels will affect the Amazon rainforest are still highly uncertain.

CLIMATE CHANGE Experiment aims to steep rainforest in carbon dioxide Sensor-studded plots in the Amazon forest will measure the fertilizing effect of the gas.

BY JEFF TOLLEFSON Climate modellers trying to build carbon the effect in the real world. Hosted in Washing- fertilization into their forecasts have had ton DC by the Inter-American Development ne of the wild cards in climate change precious few data to go on. “The number one Bank (IADB), a group of some 30 scientists is the fate of the Amazon rainforest. question is, how will tropical forests react if met this month to flesh out the details of a Will it shrivel as the region dries in a we put more CO2 into the atmosphere?” says project that would bathe a patch of rainforest Owarming climate? Or will it grow even faster Carlos Nobre, a climate scientist who heads in extra CO2 and, over the course of a decade as the added carbon dioxide in the atmos- research programmes at the Brazilian Minis- or more, measure how the plants respond. The phere spurs photosynthesis and allows plants try of Science, Technology and Innovation in experiment, the first of its kind in the tropics, to use water more efficiently? A dying rain- Brasilia. “We don’t know.” would be modelled on free-air CO2 enrichment forest could release gigatonnes of carbon into Now an international NATURE.COM (FACE) experiments conducted over the past the atmosphere, accelerating warming; a team of scientists is For more on couple of decades in the young and biologi- CO2-fertilized forest could have the opposite developing an ambitious Amazon ecology, cally simpler temperate forests of the Northern effect, sucking up carbon and putting the brakes experiment in the central see: Hemisphere. on climate change. Amazon that could study go.nature.com/xfgmba The experiment’s results could foretell the

at the University of Illinois in Urbana-Cham- GAS RING paign and one of the principal investigators in Scientists are planning an experiment in the NATURE Amazon rainforest that would measure how a FACE experiment on young pines in South elevated carbon dioxide levels enhance plant growth. Wind Carolina.

CO2 injection direction Another challenge is developing tools to track nutrient cycles in the soil and to monitor the peculiar growth dynamics of tropi- JASIEK KRZYSZTOFIAK/ JASIEK Scientists cal forests. For instance, most of the trees in would measure a mature tropical forest are hardly growing, changes in photosynthesis with a minority quickly filling in gaps created within the by the death of old trees, says Jeff Chambers, canopy. an ecologist at Lawrence Berkeley National Sensors would Laboratory in Berkeley, California. measure wind, After two days of discussion, the group temperature, and CO levels. was able to converge on a basic design (see 2 ‘Gas ring’). A pilot project, north of Manaus CO2 injection in the central Amazon, would consist of a ring of about 16 towers circling an area 30 metres in diameter. Sensors would monitor background CO2 levels and

CO2 tank winds, and CO2 would be injected from the towers as needed, to boost levels within the circle by 200 parts per million. Extra rings would be added in subsequent Scientists phases to allow replication and provide would measure improved statistics. tree growth and analyse soils. The team is exploring different options for Toroidal tube acquiring the CO2, which could cost several million dollars annually for the full experiment. Options include buying the gas from future of the Amazon. In 2000, a team at the much larger than had been thought. “This is a local beer and soft-drinks factory, and pro- UK Met Office’s Hadley Centre in Bracknell why we have been really keen for people to go ducing it independently, along with meth- proposed that drought caused by global warm- out and study the Amazon forest,” says Betts. ane, from a local landfill. The team must also 1 ing could devastate the rainforest , although “Our model indicates CO2 enrichment, and we decide whether to build a pipeline for the CO2 other climate modellers disagreed. Since then, need to know how realistic it is.” or to maintain a road for the large trucks that the Hadley team has lowered its estimates of The experiments in temperate for- would deliver the gas. So daunting are the the likelihood of drying and the resulting forest ests — rings of towers that inject CO2 into challenges that the team plans to ask the engi- dieback2. But the Hadley Centre’s simulations, circular plots — showed an initial fertilization neering arm of the Brazilian military for help. like all climate models, assume a substantial effect, although the long-term response varied The pilot project would cost about CO2-fertilization effect in the tropics. depending on the availability of nutrients in the US$10 million for the first few years, and scien- 4 In an atmosphere of elevated CO2, not only soil, such as nitrogen . In theory, the fertiliza- tists are looking to the IADB and the Brazilian do plants grow faster, but also their stomata tion effect should be Ministry of Science, Technology and Innova- (tiny openings on their leaves) do not need to stronger in the tropics, “Let’s do it tion for seed money. Nobre has encouraged the open as widely or for as long. This means that where warmer tem- right. We team to apply for a large grant from the Amazon less water escapes through transpiration, which peratures work in con- only get one Fund, a pot of money that Brazil uses to combat makes plants better able to withstand heat and cert with higher CO2 chance.” deforestation and promote sustainable develop- drought. The net result is that, at least in climate levels to increase the ment. The tentative goal is to begin fieldwork models, the extent of CO2 fertilization largely rate of photosynthesis (but plants shut down next year and to bring the pilot facility online determines the Amazon’s resilience to global altogether if the temperature gets too high). in 2015. warming. Nitrogen is also more plentiful in the tropics, Some scientists wonder whether the project Because of the sheer volume of carbon although other nutrients, such as phosphorus, will provide the answers they need at a price cycling through the tropics, the fertilization could be limiting factors. that politicians are willing to pay. But so far, its effect has a massive impact on the amount of The idea of conducting a FACE experi- planners are finding themselves in the enviable carbon that forests take up globally — and on ment in the tropics has been around for years, position of being pushed to think big by poten- how much remains in the atmosphere. Using but proposals have tended to fizzle out amid tial funders. “Let’s do it right,” said Jerry Melillo, the Hadley Centre climate model, UK mod- concerns about the feasibility of working in a a senior scientist at the Marine Biological Lab- 3 ellers showed last year that atmospheric CO2 mature tropical forest. First among them is the oratory in Woods Hole, Massachusetts, who levels in 2100 depended largely on the magni- forest’s diversity: how could an experiment be proposed a FACE experiment in Brazil more tude of the fertilization effect, and could vary large enough to be representative of a forest than a decade ago. “We only get one chance.” ■ from 669 to 1,130 parts per million (CO2 levels that has thousands of species of canopy trees 1. Cox, P. M. et al. Nature 408, 184–187 (2000). today stand at 395 parts per million). That range and a cascade of plants beneath? On this point, 2. Cox, P. M. et al. Nature 494, 341–344 (2013). corresponds to a 2.4 °C rise in global tempera- the scientists meeting in Washington simply 3. Booth, B. B. B. et al. Environ. Res. Lett. 7, 024002 tures. Richard Betts, a member of the Hadley threw up their hands. “At the end of the day, (2012). 4. Norby, R. J., Warren, J. M., Iversen, C. M., Medlyn, B. E. team, says the paper showed that the effect of no experiment is representative of the totality & McMurtrie, R. E. Proc. Natl Acad. Sci. USA 107, the CO2-fertilization feedback was potentially of the biome,” says Evan DeLucia, an ecologist 19368–19373 (2010).