ABSTRACT We characterized taxonomic and functional differences between two vegetation physiognomies in the Brazilian Chaco, namely chaco woodland (CW) and chaco forest (CF), in order to understand which abiotic and biotic mechanisms underlie the establishment of different physiognomies. We characterized the vegetation physiognomies by comparing woody species composition, richness and diversity and functional diversity between CW and CF plots. We also measured soil variables to characterize abiotic factors related to the different physiognomies. Species richness and diversity and soil nutrient values were higher in CF. Leaf succulence and nitrogen content were higher in CW, while height, leaf area, and specific leaf area were higher in CF. The standardized effect size (SES) of functional richness was higher in CW, but the SES of functional dispersion did not differ between CF and CW. We observed a diversity gradient related to soil fertility. Traits of species in CW were related to adaptations to poor soils, while in CF floristic composition showed a prevalence of species with more attributes related to competition for light. Thus, the structure of the floristic community in CW is likely related to an environmental filter, while competition for resources prevails in CF.

Differences in plant species richness and composition are associated with soil properties and disturbances such as fire, which can therefore be key determinants of species occurrence in savanna plant communities. We measured species richness, using nine plant functional traits and abundance to calculate three functional diversity indices. We then used model selection analyses to select the best model for predicting functional diversity and richness based on soil variables at sites with three different fire frequencies. We also calculated the community-weighted mean of each trait and used ordination to examine how traits changed across fire frequencies. We found higher species richness and functional dispersion at sites that were more fertile and where fire was frequent, and the opposite at such sites where fire was infrequent. However, soil properties influenced functional evenness and divergence only where fire was infrequent, with higher values where soils were poorer. Fire can change functional traits directly by hindering development of plants and indirectly by altering competition. Different fire frequencies lead to different plant-soil relationships, which can affect the functioning of tropical savanna communities. Functional diversity components and functional identity of the communities are both affected by fire frequency and soil conditions.