Lurking 11km to the south west of Skukuza is an unlikely structure - a 22 metre high steel tower, bristling at the top with three arms, and equipped with high-tech computer equipment. One might be forgiven for thinking it was a cell phone or radio mast, but in fact it is a flux tower, known to its friends as the "Kruger Park Eddy Covariance System".
It is the only one of its kind in South Africa and one of a handful in the whole of Africa. "A flux tower?" "Eddy Covariance?" I hear you say – read on and find out how the tower is helping scientists understand the workings of the savannah landscape and help reveal how the plants growing there form part of the global carbon equation.
ERECTED BY NASA
Surprisingly, the flux tower was initially erected by NASA. It was part of an experiment that looked at the effect of fires on the atmosphere over southern Africa. Since that experiment came to an end, the tower has been used by researchers from Colorado State University and the CSIR (Council for Scientific and Industrial Research). Drs Niall Hanan, Bob Scholes and Mary Scholes are responsible for drawing conclusions from this long-term experiment.
The tower is loaded with equipment that measures the changes in concentrations of carbon dioxide, water and energy. It also measures solar radiation, reflectance, temperature, humidity and wind speed. The tower stands in the savannah with sister towers to the north and the south, which measure the carbon dioxide in the air at two metre intervals. These smaller towers, essentially converted gumpoles, also record soil moisture and temperatures. These two towers are located in two different types of savannah – Acacia (knobthorn) savannah and Combretum (bush willow) savannah.
When the wind blows from the south, the instruments take readings from the Acacia savannah, and when it blows from the north, the flux tower takes readings from the Combretum savannah. However, the instruments cannot be left to their own devices for too long, and so Walter Kubheka entered the picture two years ago. Leaving the chemical laboratories of Wits University behind, he now walks through the bushveld to check and pamper the instruments every Monday and Friday.
Armed with a memory stick and a laptop, he drains the dataloggers of all their vital information, and scales the heights of the flux tower to gain a magnificent view of the surrounding area. While up at the top of the tower, he checks the instruments and empties and cleans the rain collector. The information he collects is then put onto CDs and whisked off to Johannesburg and America once a month. This data is now providing invaluable long-term information on how carbon is used in natural systems – plants essentially swap carbon dioxide for water, ‘breathing' the two substances in and out.
Scientists are well aware that carbon dioxide is important in global warming, but not as sure exactly how different ecosystems use carbon dioxide. They have looked at all the things that produce carbon dioxide, and all the things that use it back up again (basically plants), but their equations don't always add up – something must be absorbing more carbon dioxide than it gives off. And now the Skukuza flux tower enters the equation – measuring all the carbon dioxide that passes it by, along with the water changes. It is ideally located to sit in one place and monitor two systems – the Acacias and the Combretums.
Having a data set that covers almost five years, the researchers can see how carbon dioxide and water balances change during the day, the season, the year, after a fire…. And so on. This will help provide a better understanding of how the savannah ecosystem functions, and so better manage it in the long term, as well as help provide insight into the role of savannahs in global carbon and water cycles. Other flux towers in Africa are located in a mopane woodland in Botswana, semi-arid grasslands and savannahs in West Africa, and in a eucalyptus plantation in Congo.
The data from these different places can be used to compare how the ecosystems work and how Africa contributes to regional and global processes. While the number-crunching is still ongoing, Dr Hanan said that the tower has already taught them something about Acacia and Combretum savannahs. Acacias grow in soils with more clay, while Combretums prefer sandier soils.
The Acacia was thought to be at a disadvantage because it is much harder to suck water out of clay than sand. But the Acacias seem to be gaining on two sides – they have found that because the water is harder to get out of the finer soils, it stays there for longer, and the Acacia trees take advantage of this with a longer growth period than the Combretums. The Acacias also have the benefit of a better nutrient supply in the more clay soils.
The flux tower is now getting another interesting piece of equipment – Dr Hanan arrived in Skukuza on March 19 to install a gizmo that measures the precise levels of carbon dioxide in the air above the vegetation. This new instrument will give an indication of carbon dioxide over a large region, and so help reveal the role of the African savannah as a whole in global carbon dynamics. And so a 22 metre high tower in the Kruger National Park will help the world to better understand carbon dioxide and global warming.