A tropical forest can change into a savanna under certain circumstances. PhD candidate Arie Staal shows that fires are the driving force behind this sudden change.
Tropical areas greatly vary in the degree of tree coverage. This degree of coverage can be high (tropical forest) or low (savannas), but is rarely found to be anything in between, says Staal. Coverage ratios of 30 to 60 percent rarely occur around the planet. Nature does not like to be average, it seems.
This pattern cannot be adequately explained using environmental factors such as temperature and rainfall. Staal: ‘This means something else is going on: fire.’ The Wageningen researcher from the Marten Scheffer school obtained his PhD on 22 June and is not the first to suggest this approach. But he has actually provided evidence for it using models and a lot of satellite data. Fire is the driving force behind the transition from tropical forest to savanna.
The mechanism is simple, Staal explains. ‘Fires are mainly fed by grass but can also burn trees. The latter lowers the forest cover, which in turn leads to more grass and thus to more fires.’ This positive feedback can have a snowball effect and push a tropical forest over the edge to become a savanna. Data on fires endorse the strong relationship between fires and forest cover. The number of fires strongly increases when forest cover falls below 40 percent.
As such, fires can be disastrous for a tropical forest (on a local scale). That forest becomes more flammable due to climate change and the associated periods of drought. But there is also some positive news. An analysis of rainfall in the Amazon shows that the area takes care of itself to some degree. One fifth of all rainfall in the Amazon comes from the area itself. It is water that the trees ‘pump up’ from the deep ground waters and which they subsequently evaporate.
This pump action also works during dry spells. Staal: ‘In this way, trees dampen the effect of drought and climate change. Our analyses show that mainly the southern part of the Amazon is important in this respect. The rain from that part keeps the Amazon elsewhere away from the tipping point. But unfortunately, the south is also the place where most deforestation takes place.’