Plant functional types in Brazilian savannas: The niche partitioning between herbaceous and woody species

An essential characteristic of savannas is the presence of two main plant functional types: grasses and trees. The tree-grass coexistence in savannas is expected to be stabilised either deterministically by water availability or by occurrences of fire and herbivory. However, under a neutral macroevolutionary perspective, functional types could arise purely as a consequence of branching processes in plant lineages. We sought plant functional types in cerrado by assessing 360 species and 11 functional traits that correspond to important dimensions of the niche of plants. Then, we tested whether random branching processes in plant lineages could generate the observed plant functional types. We sought functional types with the k-means partitioning analysis. We constructed a phylogenetic tree for all species sampled and tested whether the phylogeny could be generated by one of the branching processes that can randomly produce functional types: (1) pure birth, (2) rate-constant birth-death, and (3) rate-variable birth-death models. Then, we compared the functional types we observed with a multivariate analysis of variance in a phylogenetic context. Two major functional types characterised the cerrado plant community - one type dominated by grasses and herbaceous shrubs and other by woody shrubs and trees. These functional types were different from the functional types simulated under Brownian motion of evolution of traits. Thus, underlying adaptive processes rather than random evolutionary processes accounted for the plant functional types. The herbaceous and woody functional types seem to have appeared as a consequence of adaptive processes of plants to cope mainly with water availability, fire, and herbivory. However, the niche partition between herbaceous and woody species supports that the reduction in the competitive interactions may be also important to the stable coexistence between trees and grass in savannas.