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ABSTRACT Background: Factors determining the coexistence of species have been recognized since Darwin, but empirical studies have brought the relationship between niche similarity and competition into question. Knowing the mechanisms of coexistence makes it possible to predict biological invasions and determine better species combinations for the restoration and recovery of degraded areas. The aim of this study was to test the competition-relatedness hypothesis for semiarid tree species. Thus, an experiment was carried out with a phylogenetic gradient of tree species, planted in pairs. The phylogenetically closest pair consisted of congeneric species of the same clade, the intermediate pair comprised congeneric species of distinct clades, and the more distant pair were species of the same family. Results: Our results show that the phylogenetically closest and the more distant pairs corroborated the competition-relatedness hypothesis, but the presence of one species with high competitive ability in the intermediate pair caused an unexpected pattern. Conclusion: Niche differences are important for coexistence, but it is necessary to know the competitive ability of each species and the specific associations that are more productive to improve the efficiency of restoration programs and to reveal the degree of aggressiveness in the case of invasive species.

Abstract Although the conservation of tropical biodiversity depends on protected areas, there is still a very large ‘gap’ of knowledge on the flora of Brazilian reserves, especially in the Northeast region of Brazil. Field and herbarium surveys of the phanerogamic flora of the Ubajara National Park, located on the Brazilian Northeast, were made and analyses on phenology and dispersal syndromes were performed. 418 taxa (213 trees and shrubs, 100 terrestrial herbs, 68 climbing plants, 33 sub-shrubs, two epiphytes, one hemiparasite and one aquatic herb) were recorded. The most representative families were: Fabaceae, Malvaceae, Asteraceae, Rubiaceae and Euphorbiaceae. The annual flowering / fruiting peak hypothesis was not fully confirmed, therefore, the forest may be an important food resource for the fauna all year long (especially in the moister region). Zoochory was the predominant dispersal syndrome in the moister area, whereas, autochory and anemochory together, predominated in the drier area.

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