Using data from plant communities from different biogeographic regions, we show that the taxonomic trees of the species belonging to these ecosystems have universal statistical properties. The study of these properties is a mean to understand the basic rules of taxonomic diversity in ecosystems. We find a power law prbability distribution of the number of subtaxa belonging to a higher taxa that is different from the case of randomized assemblages of species. Further we find the same difference in the relationship between the number of species and the number of higher taxa. These properties are common to very different ecosistems characterized by a wide range in the number of species and in the ecological properties of the different communities, not only through space but also through time (we considered a certain number of ancestral communities). The results of our analyses indicate that these properties are linked to evolutionary and ecological processes.