Since cerrado fires may impede the growth of seedlings into trees, they may shape the population of woody species. In this study, we assessed the effects of a severe fire on the population structure and spatial distribution of Zanthoxylum rhoifolium, a widespread cerrado tree. We were interested to know the importance of the resprouting and sexual reproduction in regenerating the population. The study area had been for about six years protected from fire, before a severe fire at the end of the dry season in 2006. We sampled and measured all individuals of Z. rhoifolium found in 80 plots of 25 m 2 . We found 149 individuals before the fire and 112 after the fire, of which 77 were resprouts from burned seedlings and saplings. We did not find significant differences between the population structure before and after the fire. The spatial distribution of the population remained clumped after the fire. Thus, the Z. rhoifolium population was very resilient to a severe fire. We did not find any new seedlings. As a consequence, resprouting seems to be more important than sexual reproduction in promptly regenerating the Z. rhoifolium populations.Keywords: cerrado, resilience, resprouting, sexual reproduction, spatial distribution. Efeitos do fogo na estrutura da população de Zanthoxylum rhoifolium Lam (Rutaceae) em uma savana brasileira ResumoUma vez que as queimadas no cerrado podem impedir o crescimento de plântulas, elas podem moldar a população de espécies arbóreas. Neste estudo, avaliamos os efeitos de uma queimada severa na estrutura e na distribuição espacial da população de Zanthoxylum rhoifolium, uma espécie de árvore comum no cerrado. O objetivo era saber a importância relativa do rebrotamento e da reprodução sexuada na regeneração da população. A área de estudo, há aproximadamente seis anos sem queimar, sofreu uma queimada severa no final da estação seca de 2006. Amostramos todos os indivíduos de Z. rhoifolium encontrados em 80 parcelas de 25 m 2 . Encontramos 149 indivíduos antes do fogo e 112 depois dele, dos quais 77 eram rebrotas de plântulas e jovens queimados. Não encontramos diferenças significativas entre a estrutura da população antes e após o fogo. A distribuição espacial da população permaneceu agregada após a queimada. Portanto, a população de Z. rhoifolium foi resiliente ao fogo. Não foi encontrada nenhuma nova plântula. Consequentemente, o rebrotamento parece ser mais importante que a reprodução sexuada na regeneração imediata das populações de Z. rhoifolium. Palavras-chave: cerrado, distribuição espacial, rebrotamento, reprodução sexual, resiliência.

Competition and facilitation are expected to leave different signatures in the pattern of species co-occurrence. Competition may result in a given species pair occurring less often than expected by chance, whereas facilitation may result in a given species pair occurring more often than expected by chance. We assessed the co-occurrence of pairs of herbaceous and shrubby species in Brazilian savannas, asking (1) whether a given species pair occurs more often than expected by chance, (2) whether the number of species pairs in sites with frequent fires is higher than expected by chance, (3) whether the difference in the functional traits of heterospecific pairs is lower in sites with frequent fires, and (4) whether small environmental variations in each site instead of species interactions could explain the co-occurrence of species. We used null models to answer the first two questions, analyses of variance to answer the third question, and detrended correspondence analyses to answer the fourth question. In all studied sites, approximately half of the heterospecific pairs occurred more often than expected by chance. So, facilitation seems to be important in determining the co-occurrence of some species in Brazilian savannas. However, high fire frequencies changed the pattern of occurrence of the species pairs, resulting in a spatial signature indistinguishable from random. Frequent fires also promoted phenotypic clustering of species. Nevertheless, wherever fire frequency is reduced, competition may lead to phenotypic overdispersion of plant species. Thus, less harsh environmental conditions in savannas may increase the competition among plant species.