Natural disturbances and fire suppression effects on cerrado ground-layer diversity and structure

Fire, frost and also fire suppression are important drivers of plant communities in tropical grasslands and savannas. However, plant community responses to these disturbances are largely unknown in savanna ecology, especially for plant species from the ground-layer. Aiming to advance in this research area, this thesis had as main objectives to elucidate the Cerrado ground-layer responses to both: a prescribed fire and a severe frost and, additionally, to understand vegetation changes due to fire suppression. We conducted this study in a savanna-forest mosaic in Santa Bárbara Ecological Station (EEcSB), Águas de Santa Bárbara, SP, Brazil. The experimental units were settled through the canopy cover gradient (from campo cerrado to cerradão). In the first chapter, we monitored the vegetation dynamics for four years in experimental plots where fire was suppressed. Despite the short time interval, we observed significant changes in the ground-layer. Tree species categorized as generalists, increased both in richness and density. In addition, grass cover and density of savanna specialists decreased in the same period. In the second chapter, we investigated how light availability filters grass species distribution and functional traits in a landscape under woody encroachment. As canopy cover increased, grass richness and grass total cover decreased substantially. The probability of occurrence of grasses with C4 photosynthetic pathway gradually decreased as shade conditions increased, and opposite pattern was found for C3 grasses. In the third chapter, analyzing fire effects on the ground-layer plant community, we have recorded increases in plant density, richness and bare soil, as well as reductions in grass cover and non-grass cover, compared with unburned community and the pre-fire condition. Eight months after fire, species abundance did not differ from pre-disturbance values for 86% of the species. Rapid recovery of soil cover by native vegetation in burned areas was driven by species with high capacity to resprout and spread vegetatively. However, resilience of the savanna ground-layer community, as a whole, resulted from a combination of different species traits. Finally, in the fourth chapter, addressing the frost effects on the ground layer, species from open vegetation types were more severely impacted than those under closed canopy, regarding to the intensity of damage. In general, frost effects were opposite to the fire effects in the cerrado ground-layer as described in the chapter 3. The increase in litter and reduction in bare soil were the main evidence for this conclusion. Although the literature pointed out fire and frost as similar disturbances, our results showed limitations to this comparison when savanna components other than the tree layer are taken into consideration. The information presented in this thesis represents an advance in savanna ecology, being a relevant contribution to support Cerrado conservation, management and restoration actions (AU)