Once degraded, Cerrado does not regenerate naturally

Abandoned areas of Brazilian savanna converted into pasture become closed forest with poor biodiversity, according to Brazilian researchers.

By José Tadeu Arantes  |  Agência FAPESP – Some of Brazil’s most important rivers, including the Xingu, Tocantins, Araguaia, São Francisco, Parnaíba, Gurupi, Jequitinhonha, Paraná and Paraguai, rise in the Cerrado, the only savanna in the world with perennial rivers. This biome is at risk owing to rapid conversion to pasture and cropland, in conjunction with inadequate management of preserved areas, despite its tremendous importance as a natural resource to a country with the world’s third-largest technically usable hydroelectric potential and where hydropower accounts for 77.2% of the total supply of electricity.

Moreover, destruction of the Cerrado represents an inestimable loss in terms of biodiversity. At the microscale, this biome, containing areas with 35 different plant species per square meter, is richer in flora and fauna than the tropical forest biome (read more at agencia.fapesp.br/26064).

The Cerrado is known to have significant natural regeneration potential. However, how far does its resilience go? What is necessary for the Cerrado to recoup its natural configuration once it has been converted into pasture? How long does restoration take?

A new study, conducted at São Paulo State University (UNESP) in Brazil and with results published in Journal of Applied Ecology, set out to answer these questions.

“Our initial effort was to locate old pasture areas of São Paulo State that are naturally regenerating thanks to their status as ‘legal reserves’,” said Giselda Durigan, principal investigator for the study. Durigan teaches graduate studies in forestry science at UNESP and is a researcher at the São Paulo State Forestry Institute.

The study was conducted as part of the PhD research of Mário Guilherme de Biagi Cava, who had a scholarship from FAPESP, with Durigan as supervisor. It was also supported by a research grant awarded to Professor Milton Cezar Ribeiro and a doctoral scholarship awarded to Natashi Aparecida Lima Pilon.

“More than 80 areas were found. At first this seemed promising, but my PhD student’s initial enthusiasm was cooled by many landowners’ reluctance to allow access to the areas for sampling purposes,” Durigan said. “This led us to conclude that the rigor of Brazil’s laws on preservation has not been accompanied by the necessary assistance, which should be provided by the government to private owners so they can restore the vegetation.”

Despite the social importance of this research project, opposition from landowners reduced the sampling opportunities to 29 areas of Cerrado that had been converted to pasture and later reconverted to conservation units or legal reserves by reforestation companies, sugar mills and farms.

The researchers surveyed the vegetation in these areas, counting both trees and the small plants (shrubs, grasses, etc.) that make up the ground layer and constitute the greatest treasure of the Cerrado’s flora. Although these 29 areas, with ages ranging from four to 25 years, were located in different regions, they could be ranked in chronological order in terms of their stage of regeneration.

“To sum up our findings in a very simplified way, we discovered that the tree layer recovers, and does so quite easily, whereas the ground layer, which contains most endemic species, doesn’t regenerate once it’s been destroyed. Therefore, when pasture is simply abandoned, after a time it becomes cerradão, high-canopy closed forest with poor biodiversity,” Durigan said. 

The trees recover because they have very deep roots and have evolved over millions of years, developing the capacity to resprout countless times. 

“Anyone who tries to establish pasture in the Cerrado knows the main maintenance cost is mowing and tree cutting. If you don’t do that at least every two years, the trees take over again. You can’t get rid of them even with herbicide,” Durigan said.

The shrubs and other low plant cover in the ground layer, however, will have been removed to establish the pasture in the first place and will not grow back owing to invasion by brachiaria, a highly resistant and aggressive exotic signalgrass.

“This grass only disappears when the tree cover is dense enough to provide shade. But when the exotic grass disappears, there can be no regeneration of the original shrubs and bushes, which were completely eradicated by the use of herbicide, as well as by mowing and competition from brachiaria, and don’t tolerate shade,” Durigan said.

To return an area to typical Cerrado, the exotic grass would have to be eliminated by burning combined with herbicide, after which native species could be reintroduced. However, this process is costly and operationally difficult, and it would not be feasible on a large scale with existing resources.

“We’ve researched different techniques to promote regeneration. With seeds, you need a huge amount, so many there wouldn’t be anywhere to extract them from. What worked very well on an experimental scale was transplanting shrubs and herbaceous plants to recreate the ground layer, accompanied by native grasses and bushes,” Durigan said.

“The main problem is that there are no more source areas for this in São Paulo State. The only open Cerrado remnants left have been invaded by exotic grasses, so when you transplant the ground layer you get the brachiaria too. This happens even in protected areas.”

Degraded forest

The study conducted at UNESP led to a diagnosis and predictions. Typical Cerrado, once degraded, does not regenerate spontaneously. Pasture can be returned to typical Cerrado with rich biodiversity, characteristic flora and habitats for wild animals that specialize in savanna only by human management: the tree density cannot be allowed to exceed 15 square meters per hectare, exotic grasses must be eradicated, and native plants must be reintroduced to form a ground layer.

Evolving spontaneously without management, tree cover in abandoned pasture will become cerradão in an estimated 49 years. The sparse ground cover typical of cerradão is completed in four years, and the poor biodiversity of the herbaceous layer develops in 19 years. “The process is rapid, but the results aren’t what we want. Cerradão is no different from degraded forest,” Durigan said.

Two years after the original study, the group will return to the same areas during the second stage of Cava’s PhD research and measure everything again in order to determine how much the plant cover, density and biodiversity have increased.

“These precise values will tell us exactly what the regeneration potential is in the different areas and help us identify favorable factors, such as soil type or distance from seed sources and water. All these parameters will be considered,” Durigan said.

The article published by her group, she added, is highly innovative because no one in any other country is addressing the issue of savanna regeneration.

“Neither in Africa nor in Australia has there been a process similar to the one we’re experiencing here, with savanna being converted into extensive pasture and large plantations of soybeans, sugarcane or corn,” Durigan said. “In Africa, the savannas are very degraded, but this is due to overgrazing, firewood extraction and other activities with less visible impacts in the short run. In Brazil, we’re seeing changes that occur overnight.”

The article “Abandoned pastures cannot spontaneously recover the attributes of old-growth savannas” (doi: 10.1111/1365-2664.13046) by Mário G. B. Cava, Natashi A. L. Pilon, Milton Cezar Ribeiro and Giselda Durigan can be retrieved from onlinelibrary.wiley.com/doi/10.1111/1365-2664.13046/full.