Geographical zonation in the Neotropics of tree species characteristic of the Paraguay-Parana Basin

Aim This paper uses data from the literature (monographs and taxonomic reviews) to investigate the geographical zonation in South America of 32 common tree species encountered in Paraguay. The actual spatial distribution of the species is then used to provide clues on the plant communities present in the past, and especially during the Wisconsinan age (Last major ice age: 80,000-10,000 BP). The floristic relatedness between the Paraguayan flora and the neighbouring floras is also investigated. Methods The main vegetation patterns were highlighted using a Discriminant Analysis of the Eigenvectors of Neighbourhood Operator to emphasize the geographical zonation in South America of 32 tree species predominant in the Paraguay-Parana Basin. Results Three main vegetation patterns were emphasized: the Chaco, the Parana-Sao Francisco gradient and the Peri-Amazonian gradient. The Chaco is well defined. The Peri-Amazonian gradient is characterized by a continuous change in species from the Colombian pole towards the Sao Francisco pole. The Sao Francisco gradient shows a continuous change from the Sao Francisco pole towards the Parana pole. Some of the species are monocentric (related to only one dispersal centre), whereas others are polycentric (distributed in several poles). Main conclusions The Chaquean xeromorphic forests are considered as a climax, with a stable composition resulting of the saline soil conditions. The Chaco acts as an edaphic barrier to many species, although the migration of some non-Chaquean species is made possible by the net of gallery-forests and low montane forests between the Rio Paraguay and the Andean Piedmont. The Cerrado and the Chaco can be considered as a barrier limiting the expansion of the Paranean forest. This latter formation is a semi-deciduous forest made of a mix of monocentric and polycentric elements. The numerous species found both in Colombia and in Parana indicate that bridges exist, or have existed, between these two poles. The persistence in the Paraguay-Parana Basin of a moist forest made of the Paranean assemblages highlighted in this study confirms the hypothesis of moister and cooler phases than today.