Indiscriminate tree planting destroys grasslands and savannas instead of restoring them, scientists warn

In a special issue of the journal Science, some of the leading experts on the subject argue that rehabilitating degraded areas requires more complex solutions that take the biome’s specificities into account.

By José Tadeu Arantes  |  Agência FAPESP – Grasslands and savannas are not degraded forests and cannot be restored by planting trees. This is often pointed out by leading researchers in the field. The warning is reiterated in a review article published in a special issue of Science in August 2022. It is particularly relevant during the United Nations Decade on Ecosystem Restoration, which runs until 2030, as ecosystem restoration projects regularly ignore the specific features of biomes as complex as grasslands, planting trees indiscriminately as a panacea.

“There are many degraded forests in the world, but the concept doesn’t apply to grasslands or savannas. In the case of deforested areas of Amazonia, for example, the term ‘savannization’ is inappropriate and hinders instead of helping, because savannas, unlike degraded forests, are ancient and complex biomes with enormous biodiversity,” said ecologist Alessandra Fidelis, a professor at São Paulo State University (UNESP) and a co-author of the article.

“We’re in a decade dedicated to ecosystem restoration. We urgently need to conserve and restore savanna-type ecosystems,” she continued. “But how to restore them? Planting trees isn’t a solution. Many people think they’re recent and simple environments, but tropical savannas, for example, have existed for millions of years. They’re highly complex, with an aerial component made up of a continuous herbaceous stratum rich in grasses and herbs as well as sparse bushes and trees, and above all with great functional diversity underground thanks to all the roots and reserve organs beneath the surface. These give the system resilience. They contain not just reserves but also gemmae, budlike masses of tissue that develop into new branches after a fire, for example. We don’t yet know how to restore this.”

The herbaceous stratum includes C4 grasses, which emerged 25 million years ago. Grasslands covered vast swathes of the planet 10 million years ago. “Most of the species now found in grasslands were selected and evolved by disturbances such as fire and herbivory [feeding on plants]. None of this can be easily restored. If this vegetation burns, for example, it quickly resprouts from its roots, but if the underground organs and roots are pulled up by farm machinery, there’s nothing left to resprout. There are grasslands and savannas that were destroyed more than a century ago and haven’t grown back,” she said.

Cerrado endangered

This consideration is especially important in Brazil, because the Cerrado, the world’s most biodiverse savanna-like biome, is disappearing day by day owing to pressure from large-scale agriculture. Its survival is even more precarious than that of the Amazon Rainforest.

According to Fidelis, the “old-growth grasslands” and similar formations discussed in the Science article cover no less than 40% of Earth’s surface. They are ecosystems that form open landscapes and consist mainly of grasses, herbs, tussocks, bushes, and small or medium trees. In Brazil, they account for 27% of the territory and predominate in four of its six biomes: the Cerrado, Caatinga (a semi-arid portion of the Northeast region), Pampa (in the South) and Pantanal (wetlands in the Center-West). They also appear in the other two biomes:  as campinaranas (scrubby forest) in the Amazon, and campos de cima da serra (mountain grasslands) in the Atlantic Rainforest.

Grasslands and savannas with their rich biodiversity provide ecosystem services directly to more than 1 billion of the planet’s inhabitants, but their importance is even greater in Brazil, because the Cerrado is the only savanna-like biome in the world full of perennial rivers. Indeed, it is the birthplace of such mighty rivers as the Xingu, Tocantins, Araguaia, São Francisco, Parnaíba, Gurupi, Jequitinhonha, Paraná and Paraguay, among others.

It is worth recalling that hydroelectricity accounts for 77.2% of the energy mix in Brazil, which is the world’s third-largest producer of hydropower (after China and Canada). Degradation of the Cerrado threatens this vast source of energy as well as the supply of freshwater for consumption by the population and for farming, at a time when water is becoming one of the most precious goods on Earth because of the climate crisis.

“In the context of growing destruction, people naturally bet on restoration, but we must be very careful about this because projects often confuse restoration with tree planting. Badly managed tree planting is actually another threat because it creates artificial forests in savanna-like ecosystems. For example, almost 1 million square kilometers of grasslands and savannas in Africa have been targeted for tree planting until 2030, ignoring their specificity and value in terms of biodiversity and ecosystem services,” Fidelis said.

“The unique characteristics of these ecosystems, which have been around for millions of years, combine complexity, diversity of both aerial vegetation and underground components, and resilience to disturbances such as fire and herbivory. All this makes restoration very difficult. Degradation that destroys gemmae and other underground structures can lead to irreversible damage.”

Three important points must be taken into account in any long-term grassland restoration project, she added. The first is that many species resprout and hence depend on the presence of underground structures with reserves and viable gemmae. They do not regenerate by germinating. The importance of gemmae and underground structures cannot be overemphasized.

The second point is that these ecosystems cannot be restored quickly. Restoration projects must be closely monitored to prevent biological invasion by exotic grasses like signalgrass (brachiaria), for example, and overdense tree cover, both of which can ruin restoration completely.

The third point is that these ecosystems have highly important feedback loops involving the soil, vegetation, fire and herbivory. The relations among these factors change over time, and project administrators need to understand them, how to maintain them and when to promote them, by carefully managed use of fire, for example. 

“An important message is that we must guide restoration of these ecosystems on the basis of the characteristics of what we call old-growth grasslands or primary savannas. Functionally speaking, these complex systems were produced by nature over millions of years. We have to learn from nature,” Fidelis said.

She has led several projects focusing on management of the Cerrado with funding from FAPESP and is currently principal investigator for the project “Optimizing long-term management of invasive species that affect biodiversity and the rural economy using adaptive management”. 

The article “Ancient grasslands guide ambitious goals in grassland restoration” is at: www.science.org/doi/10.1126/science.abo4605.