Neglected CO2-source detected

Sebastian Wolf looks back on some phases of the project begrudgingly – like the run-up to Christmas, 2008: “I had to fly to Panama at a couple of days’ notice to repair a measuring tower.” It was down completely and there was nobody on site who knew how to deal with it. Sebastian Wolf from the Institute of Agricultural Sciences at ETH Zurich wrote his doctoral thesis on a topic on which there are very few studies to date: how much CO₂ do tropical ecosystems store or emit, especially if the land use changes, in other words if rain forests are cut down and the areas initially used as farmland, then pastures? And what happens if the pastures are afforested with indigenous wood types again?

“We know that afforestations sequester carbon, i.e. act as so-called carbon wells; but just how much CO₂ they absorb is still largely unknown,” says Wolf. Hardly any data exists for Central America, either. After all, the tropical studies available were almost all conducted in the Amazon area and primarily stem from the “Large Scale Biosphere-Atmosphere Programme”.

Chance project

Fortunately, ETH-Zurich professor Nina Buchmann, Sebastian Wolf’s doctorate supervisor, got talking to a colleague from McGill University in Canada who had already conducted research in Panama and told her that they were on the lookout for experts for micrometeorological carbon measurements there. And so the project “Sustainable agroforestry for carbon sequestration to improve the livelihood of small farmers in the tropics” was born. It was funded by ETH Zurich’s North-South Centre and conducted together with colleagues from McGill University and the Smithsonian Tropical Research Institute in Panama. The results are now out.

For the study area, Sebastian Wolf and his colleagues used pastureland in Sardinilla in Central Panama, thirty kilometres northeast of the Panama Canal, which was over fifty years old. There was nothing but rain forest here until 1953, after which it was deforested completely and the area used a farmland for two years until the soil was no longer able to provide enough nutrients and could only be used as pastureland.

Part of the area was afforested with a mixture of indigenous trees in an experiment in 2001. Small farmers continue to use the remaining section as grazing land for cattle – a form of alternative land use that is really on the increase in the tropics and that thus far was assumed not to have a negative impact on the carbon footprint. Admittedly, any deforestation of the rain forest and the subsequent use of the area as farm- and pastureland throws a system off balance in that, initially, much of the carbon stored in the topsoil is released. “After a while, however, the system returns to a kind of equilibrium.” Tropical pastureland thus neither constitutes a major source of nor reduces CO₂ after longer use.

Pastureland significant source

However, to the astonishment of the researchers, the tropical pastureland turned out to be a “striking” source: with 260 grams of CO₂ per square metre, almost as much carbon escaped – mainly from the soil – as an afforestation area at medium latitudes “swallows”, for instance – for Sebastian Wolf, an “impressive amount” that also shows, “It’s short-term, intensive overgrazing during the rainy season, not continuous grazing, that can nullify months of carbon sequestration in a very short space of time.” The grasses cannot recover sufficiently and sequester carbon during the dry season as, unlike the natural vegetation, the rainforest, they only have low root depths and therefore wither.

But it is not just the data itself that is new for the tropics; the method with which the researchers determined it has also only been used in a very limited scope in the tropics thus far – the so-called Eddy Covariance Method, or flux measurement. It involves placing – in this case two – three and fifteen-metre-high “FLUXNET towers in the area to be studied. They are reminiscent of an aluminium ladder, at the top of which diverse measuring instruments are attached. This enables the scientists to record the wind speed and direction, and the CO₂ and water vapour concentration at twenty-hertz intervals. “Based on this data, we can then calculate whether and how much carbon the soil and plants “breathe” into or absorb from the atmosphere,” explains Wolf.

In parallel, Wolf and his colleagues also measured in the biomass and determined how much carbon is emitted or absorbed by plants and soil. The results of the two measuring methods matched each other extensively, adding additional clout to the results of the flux measurements. “There are very few studies in the Amazon that measure fluxes and at the same time have been compared with biomass measurements.”

Spiders interfered with sensors

However, before Sebastian Wolf could collect all of his data, he had to deal with some unwanted guests, which were also the reason for the emergency Christmas mission. “We’ve particularly had a lot of problems with animals – like ants, which built a nest in our case of instruments; or spiders, which obviously found the optic sensors particularly appealing.” And so the instruments kept malfunctioning. “Reliable people on the ground are thus central to whether a project succeeds or not in developing countries.” Apparently, many projects have foundered due to such breakdowns.

However, the ETH-Zurich researchers found reliable partners and the project drew to a successful conclusion in the spring of 2010. And now? “Unfortunately, the project will not be continued. We would now need to act in accordance with the results.” Something when it comes to successfully implementing the “Clean Development Mechanism” agreed in the Kyoto Protocol. For instance, companies from industrialised countries can fund an afforestation project in the tropics and receive certificates for the amount of CO₂ saved as a result, which can then be credited to their emissions account or resold. But that only makes sense if measures are also implemented in the tropics that are as environmentally friendly as possible but also adapted to the individual social conditions. After all, many small farmers rely on the pastures, says Sebastian Wolf: “Whereas we put money in the bank for our old age, a farmer in Panama puts a cow out to pasture.”

According to Wolf, a mixture, a so-called silvopastoral system, would thus be a suitable compromise, where the pastureland is only partly afforested with trees to enable CO₂ to be sequestered and the storage function of the soil preserved. “The remaining area would then continue to be available to the small farmers as pastures.”

Further reading:

Wolf S, Eugster W, Potvin C, Turner BL, Buchmann N (2011) Carbon sequestration potential of tropical pasture compared with afforestation in Panama. Global Change Biology 17(9): 2763-2780,http://onlinelibrary.wiley.com. DOI: 10.1111/j.1365-2486.2011.02460.x