By José Tadeu Arantes | Agência FAPESP – Fire is almost always presented as an enemy of ecosystems, yet scientists who study the subject say fire is indispensable to the conservation of savannas. Brazil’s Cerrado biome, the most biodiverse savanna in the world, is severely endangered by a combination of two factors: expansion of the agricultural frontier and prohibition of the use of fire as a land management method. This is the argument put forward in the article “The need for a consistent fire policy for Cerrado conservation” by Giselda Durigan, a researcher at the São Paulo State Forestry Institute, and James Ratter of the Royal Botanic Garden Edinburgh, Scotland, published in the Journal of Applied Ecology.
Durigan, who also teaches graduate studies in forestry science at São Paulo State University (UNESP) and in ecology at the University of Campinas (UNICAMP), has studied the Cerrado for over 30 years. She recently took part in the project “Impact of human drivers (fire, agriculture and grazing) on biodiversity in savannas”, supported by FAPESP under the auspices of the Belmont Forum. Among several other research initiatives in progress, she is currently participating in the project “Effects of fire and fire suppression on ecosystem structure, composition and biodiversity in the physiognomic gradient of the Cerrado at the Santa Bárbara Ecological Station”, partially supported by the United States National Science Foundation.
“The world’s savannas are all undergoing a process of vegetation densification and biodiversity loss,” Durigan told Agência FAPESP. “In Brazil, the main cause is fire suppression. The Cerrado is gradually filling up with trees and starting to become forest. Four-fifths of the biome’s plant biodiversity is in the herbaceous stratum, so the emergence of forest means a huge loss of biodiversity. Most plants in the Cerrado don’t tolerate shade. When the canopy formed by tree crowns closes and casts the ground into shadow, hundreds of species of endemic plants disappear.”
“Our study at the Santa Bárbara Ecological Station in the west of São Paulo State showed that once a certain threshold is reached in the densification process, transformation of the Cerrado into forest becomes irreversible. So we mustn’t let the biomass cross that threshold. We need a fire management program. Everyone thinks fire is ‘evil’ as far as ecosystems are concerned, but among savanna researchers, there’s a consensus that fire is necessary as long as it’s properly managed. We have to learn how to manage fire as the indigenous inhabitants did thousands of years ago.”
It is important to note that Durigan was not referring to uncontrolled wildfires but to a carefully planned land management method that includes total area zoning and a rotating fire schedule. Zoning schemes define a mosaic structure. Scheduling establishes the right time of year to burn each patch. Thus, one patch is burned at a particular time, another some months later, yet another the following year, and so on. The areas are burned in rotation, but the scheme maintains a mosaic made up of recently burned patches, areas burned some time ago, and areas not burned for a long time. This ensures that vegetation will grow back while leaving animal escape routes and habitats intact. “At the Santa Bárbara Ecological Station, we’re burning continuous areas of 20 to 30 hectares, with no risk to the flora, no loss to wildlife, and many benefits,” Durigan said.
“Savannas burn spontaneously. C4 grasses, which are fundamental to the existence of savannas, evolved some 8 million years ago in the presence of fire, long before humans appeared. A huge amount of inflammable material accumulated as a result. The most disastrous example of a major wildfire occurred in Yellowstone National Park in the US, which also had a fire-prevention policy at the time. The upshot was that the entire park burned, and it was a calamity because the animals were left without a habitat and without food.”
According to Durigan, savannas are tropical climate biomes comprising scattered trees and soil covered by grasses, herbaceous plants and shrubs. These formations emerge in response to two main factors: a characteristic rainfall regime with clearly demarcated wet and dry seasons, and generally associated soil properties.
Rain on sandy soil
When rain falls on argillaceous soil, the clay retains water for a long time. When rain falls on sandy soil, however, two days of dry weather suffice for the soil to become dry again. Where the soil is mostly sandy, the three-month dry season common in the region is sufficient to prevent forest cover from becoming generalized, and in this case, the result is Cerrado. In open savanna, trees have deep roots capable of finding water retained in the subsoil from rainfall months prior. The key is the availability of water in the soil for plants, which depends on how much it rains and how much water the soil can store up.
All open savannas in the world have two fundamental traits: an extended dry season and fire as a factor in natural selection and evolutionary pressure. The plants of the Cerrado evolved in the presence of fire and adapted to fire. Many rustic trees of the Cerrado have thick, corky bark that resembles a mantle. This suberized phellem is made of dead cells and protects trunk and branches in a fire, acting as a layer of heat insulation and preventing the living inner tissue from being scorched. The grasses burn but soon resprout. Two months is enough time for a burned area of Cerrado to grow back into an exuberant garden.
“The Cerrado’s extraordinary resilience, its ability to react to disturbances, is due above all to the underground structure of its plants, which resprout countless times,” Durigan explained. “Therefore, the main threat to the Cerrado’s survival today is agricultural expansion. When cattle ranching came to the Cerrado, the vegetation was cleared, and the landscape changed to become predominantly open meadow with sparse vegetation and few trees. However, the underground structure of the plants was mostly left intact, so total biodiversity loss didn’t occur. Agriculture is an entirely different story. The underground structures are deliberately destroyed, because all the existing vegetation and its capacity to grow back must be eliminated to make the land arable. Implements that cut the roots at great depth are used, together with powerful herbicides that leave the soil completely without plant cover. Nothing of the Cerrado proper is left.”
The result is an extremely damaging effect on the Cerrado’s water. “The most valuable dimension of the Cerrado, compared both with all other Brazilian biomes and with the world’s other savannas, is the production of water,” Durigan said. “Some of Brazil’s most important rivers – the Xingu, Tocantins, Araguaia, São Francisco, Parnaíba, Gurupi, Jequitinhonha, Paraná and Paraguay, among others – rise in the Cerrado. Allowing the Cerrado to die means jeopardizing the survival of these rivers, not only as a source of fresh water but also as a source of hydropower. This is alarming in a country where hydroelectricity accounts for 77.2% of all the power produced. Brazil’s technically usable hydroelectric potential is the third-largest in the world, and it’s putting this precious resource in jeopardy.”
The Cerrado is the only savanna in the world with perennial rivers. In the savannas of Africa, Asia and Oceania, most rivers are seasonal, disappearing in the dry season and causing calamitous floods in the rainy season. This biome, which is still predominant in Central Brazil but is also found in patches from Maranhão State (Northeast Brazil) to Paraguay, originally covered more than 2 million square kilometers, or about 25% of Brazil’s land mass. Its harsh landscapes, so often underestimated in the past and poorly understood even today, hide fabulous biodiversity.
“We’re only now inventorying all the species of the Cerrado, including those in the herbaceous stratum, thanks to the large-scale study under way for the past three years at the Santa Bárbara Ecological Station,” Durigan said. “There are areas in which we found 35 different plant species per square meter. So far, we’ve sampled almost 500 different plant species in the entire station. And colleagues are studying the fauna: snakes, lizards, toads, ants, etc.”
To understand the significance of 35 different plant species per square meter, suffice it to note that this biodiversity is greater at the micro scale than that of tropical forests. “Tropical forests have incredible biodiversity at the macro scale but aren’t as diverse at the micro scale,” Durigan said.
“We have records of the use of fire by indigenous people thousands of years ago,” Durigan said. “They used fire for different purposes and hence with varying frequency. They used it to facilitate hunting or to increase the yield of plant species they harvested for food, for example. We need to combine this ancestral wisdom with advanced scientific knowledge. Our aim is to provide input for a responsible and consistent fire-management policy.”