Aim. In savannas, a grass dominated ground layer is key to ecosystem function via grass-fire feedbacks that maintain open ecosystems. With woody encroachment, tree density increases, thereby decreasing light in the ground layer and potentially altering ecosystem function. We investigated how light availability can filter individual grass species distributions and whether different functional traits are associated with response to a shade gradient in a landscape experiencing woody encroachment. Location. Savanna-forest mosaic in the Cerrado domain, southeastern Brazil. Methods. Along an encroachment gradient of increasing tree Leaf Area Index (LAI) and shade, we determined how changing light availability alters grass diversity and ground layer structure relative to grass cover and grass functional traits (photosynthetic pathway, underground storage organs, bud protection and traits related to grass shape, size and leaf dimensions). Results. Increasing shade led to a decrease in grass cover and grass species richness, and also compositional and functional changes. We found that where tree LAI reached 1, grass cover was reduced by 50% and species richness by 30%. While C4 grass species abundances decreased with increasing shade, the opposite pattern was true for C3 grasses. There were only small differences in light preferences among C4 subtypes, with PCK species tolerating slightly more shaded conditions. Persistence of some C4 species under more shaded conditions was possible likely due to an ability to store starch reserves via underground storage organs. Conclusions. Woody encroachment changes diversity and structure of the grassy layer that is critical to the functioning of savanna ecosystems, highlighting the dependence of the diverse grass layer on open and sunny conditions. Our results suggest a threshold of tree cover close to LAI ≈ 1 as being critical to cerrado grassy-layer conservation.
1. Disentangling species strategies that confer resilience to natural disturbances is key to conserving and restoring savanna ecosystems. Fire is a recurrent disturbance in savannas, and savanna vegetation is highly adapted to and often dependent on fire. However, although the woody component of tropical savannas is well studied, we still do not understand how ground‐layer plant communities respond to fire, limiting conservation and management actions. 2. We investigated the effects of prescribed fire on community structure and composition, and evaluated which traits are involved in plant community regeneration after fire in the cerrado ground layer. We assessed traits related to species persistence and colonization capacity after fire, including resprouter type, underground structure, fire‐induced flowering, regeneration strategy and growth form. We searched for functional groups related to response to fire, to shed light on the main strategies of post‐fire recovery among species in the ground layer. 3. Fire changed ground‐layer community structure and composition in the short term, leading to greater plant species richness, population densities, and increasing bare soil, compared with unburned communities. Eight months after fire, species abundance did not differ from pre‐disturbance values for 86% of the species, demonstrating the resilience of this layer to fire. Only one ruderal species was disadvantaged by fire and 13% of the species benefited. Rapid recovery of soil cover by native vegetation in burned areas was driven by species with high capacity to resprout and spread vegetatively. Recovery of the savanna ground‐layer community, as a whole, resulted from a combination of different species traits. We summarized these traits into five large groups, encompassing key strategies involved in ground layer regeneration after fire. Synthesis: Fire dramatically changes the ground layer of savanna vegetation in the short term, but the system is highly resilient, quickly recovering the pre‐fire state. Recovery involves different strategies, which we categorized into five functional groups of plant species: grasses, seeders, bloomers, undergrounders, and resprouters. Knowledge of these diverse strategies should be used as a tool to assess conservation and restoration status of fire‐resilient ecosystems in the cerrado.
Fire has been a natural force modulating the vegetation of the Brazilian Cerrado since long before the first humans arrived in this region about 12 thousand years BP. Cerrado plants are, therefore, adapted to fire and some of them depend on fire to maintain their reproduction and survival. However, after the use of fire as a tool to destroy vast extents of forests in the last centuries, Brazilians in general formed a very negative opinion about the use of fire and burning native vegetation was prohibited in the country. Although this decision was effective in protecting tropical forests, the zero-fire policy has been quite negative in the Cerrado. Fire suppression in Cerrado reserves has caused a dramatic loss of biodiversity at landscape level (homogenizing the vegetation structure), at species level (leading non-tree species to local extinction) and likely at population level (sexual reproduction impaired). Furthermore, fuel accumulation over several years without fire increases the risks of catastrophic firestorms that will inevitably occur. Such negative consequences of fire suppression in the Cerrado have not been noticed even among ecologists, including myself in early career and until recently. The biased tree-and-carbon perspective of environmental campaigns and ecological studies have reinforced the misunderstanding of fire effects in Cerrado vegetation. Both neglect that the absolute majority of Cerrado plant species, as well as associated fauna and ecosystem services, depend upon open vegetation-structures which are mostly maintained by fire.
Although savannas are fire-adapted ecosystems, prescribing fire for biodiversity conservation remains controversial at least in some regions where savannas occur. Faced with uncertainty, many decision makers and even scientists are still reluctant to prescribe fire for conservation purposes in fire-prone ecosystems, invoking the precautionary principle. Knowledge gaps on the ideal fire regime, such as how and when to burn, and especially the fear of biodiversity losses, are among the main arguments against fire management applied to remnants of native savanna vegetation. To inform this debate, we assessed the impact of prescribed fires on diversity of plants (different growth forms), ants, frogs, lizards, birds, and small mammals, in savannas and grasslands of the Brazilian Cerrado. We assessed the existing species richness, composition, and abundance in areas subjected to long periods of fire suppression and compared to that observed over a short period after prescribed dry-season fires, within each group of plants and animals. Whenever possible, we carried out separate analyses for grassland and savanna. Burning did not significantly reduce species richness of any of the groups analyzed, but had a positive effect on richness of graminoids in grassland. When analyzed at the species level, abundance of most animal groups did not show consistent responses to fire, except for a decrease in some frog populations in grasslands. Forbs, graminoids, and subshrubs increased in abundance after fire in grassland areas, though in savanna areas, abundance of forbs, and subshrubs tended to decline after fire. Species composition changed little in response to fire as indicated by low levels of dissimilarity between burned and unburned areas. These results confirm the high resilience of Cerrado biota to fire, as expected for savanna ecosystems in general. Besides, we demonstrate here that the risk of biodiversity losses cannot justify the objections to the use of prescribed fire for conservation purposes in the Cerrado.
Background: Fire has been reported to trigger the production of flowers and fruits in many fire-prone ecosystems around the world. However, for tropical savannas, little is known about the effects of fire on flower production at community and species scale, especially for the ground-layer. Aims: We assessed the role of fire as a trigger to short-term flowering in a preserved cerrado grassland, compared with unburnt vegetation. Methods: We recorded the presence of flowers or fruits in 2441 individuals from 47 plant species (grasses, forbs and subshrubs), during six months after fire, in burnt and unburnt areas of cerrado grasslands, in southeastern Brazil. Results: In the burnt areas, 63% of individuals sampled flowered, in contrast to 19% in unburnt areas, demonstrating a strong and positive effect of fire on plant communities of cerrado grasslands. Fire significantly induced flowering in 79% of the studied species, of which 20 species flowered only after fire (nine grasses, seven subshrubs and four forbs). Conclusions: These results highlight the role of fire triggering important ecological processes in the cerrado grasslands, potentially benefiting seed production and genetic diversity of many species. Fire is a crucial factor to be maintained for conservation of these ecosystems and their biodiversity.
Tropical savannas have been increasingly viewed as an opportunity for carbon sequestration through fire suppression and afforestation, but insufficient attention has been given to the consequences for biodiversity. To evaluate the biodiversity costs of increasing carbon sequestration, we quantified changes in ecosystem carbon stocks and the associated changes in communities of plants and ants resulting from fire suppression in savannas of the Brazilian Cerrado, a global biodiversity hotspot. Fire suppression resulted in increased carbon stocks of 1.2 Mg ha⁻¹ year⁻¹ since 1986 but was associated with acute species loss. In sites fully encroached by forest, plant species richness declined by 27%, and ant richness declined by 35%. Richness of savanna specialists, the species most at risk of local extinction due to forest encroachment, declined by 67% for plants and 86% for ants. This loss highlights the important role of fire in maintaining biodiversity in tropical savannas, a role that is not reflected in current policies of fire suppression throughout the Brazilian Cerrado. In tropical grasslands and savannas throughout the tropics, carbon mitigation programs that promote forest cover cannot be assumed to provide net benefits for conservation.
The Cerrado is a fire-dependent savanna requiring a clear and urgent fire management policy. The extensive misuse of fire for deforestation or pasture management in Brazil has created an overall perception that its use is always deleterious. This view, reinforced by threats of global warming and climatic change, has lead to current policies of fire suppression. Cerrado ecosystems depend on the historical fire regime to maintain their structure, biodiversity and functioning. The suppression of fire has transformed savanna vegetation into forests, causing biodiversity losses and drastic changes in ecological processes. Policy implications. The National Fire Policy required by law must be urgently implemented in Brazil, including use of fire for Cerrado conservation in public and private lands on the basis of existing knowledge of indigenous people and scientists. Objective regulations on prescribed burning, land manager training, incentives for fire research and experimentation and a broad campaign to disseminate the benefits of fire for Cerrado conservation should be the cornerstones of the policy. If implemented, the policy can give the biodiversity of the Cerrado a future that has previously been severely threatened by fire suppression.