The Dry Plant Formations of South America and Their Floristic Connections

Plant formations adapted to dry climates extend in South America from tropical to temperate latitudes and from sea level to the highest mountain ranges, to constitute a significant proportion of the vegetation in this continent. Thirteen different dry formations are considered in this paper and their overall floristic similarities are analysed on the basis of their woody flora at the generic level. Through the use of Sorensen's coefficient of similarity, the matrix of interformation floristic similarities was obtained, and these results then analysed by means of constellation diagrams and two-axis ordination. The same procedure was repeated utilizing only the genera of the single family Cactaceae. The comparisons show the various degrees of floristic affinity between the dry floras considered, with some formations scarcely related to the others. The data show the existence of different South American aridfloras, pointing to a complex history of contacts and divergences. The major foristic discontinuity is located in the Peruvian Andes, and a possible explanation for this discontinuity is suggested.