New Zealand-led study shows slower advance of forests into savannas

The world's savanna grasslands are being invaded by trees, but this environmental transformation might be occurring at a slower pace than previously thought, according to New Zealand-led research out Wednesday.

Lead researcher Professor Steven Higgins of Otago University's Department of Botany said several lines of evidence suggested that many savanna ecosystems around the world were becoming increasingly dominated by trees.

"It is thought that the advance of trees in these eco-systems is fuelled by increases in atmospheric carbon dioxide. Due to their different lifestyles and photosynthesis systems, trees are better positioned to take advantage of higher CO2 (carbon dioxide) levels than grasses are," Higgins said in a statement.

Alongside CO2, another key factor controlling the growth of plants was the rate at which nitrogen was made available to them in soils, a process known as nitrogen cycling.

Higgins and his team undertook a field experiment in Kruger National Park, South Africa, to test whether nitrogen cycling in savannas would change as trees came to dominate.

Their findings suggested that nitrogen cycling would significantly slow as trees advanced and that this might in turn limit the rate at which trees encroached into savannas.

In earlier research, Higgins and colleagues in Germany had produced a model indicating that large swathes of Africa's savannas might be forests by 2100.

"Our latest research illustrates that puzzling out just what limits plant growth is far from trivial if it is not atmospheric carbon dioxide, then perhaps it is nitrogen, or phosphorous," he said.

The problem had challenged plant scientists for a good part of the almost two centuries.

"It's a challenge we have to master for a wide range of reasons, from using fertilizers more efficiently, to anticipating the impacts of global change," he said. Endi