Included here is the Chaco or Gran Chaco of South America. Chacoan vegetation covers the vast plains of north-central Argentina, western Paraguay, southeast Bolivia and extends into Brazil along the a narrow strip parallel to the River Paraquay in Mato Grosso do Sul.

Humid Chaco Woodland

Situated in waterlogged areas where soils are often slightly saline these forests are typically dominated by endemic Schinopsis balansae (Anacardiaceae), while other common canopy trees include the endemic or near endemic Aspidospermum quebracho-blanco (Apocynaceae)and Caesalpinia paraquariensis (Fabaceae). A second story usually includes Acacia praecox, Sideroxylon obtusifolium and the endemic or near endemic Geoffroea decorticans (Fabaceae) and Prosopis nigra (Fabaceae).Common under storey species include the endemic or near endemic Achatocarpus praecox (Achatocarpaceae), Castella coccinea (family?), Shinus fasciculatus (Anacardiaceae)and the cactus Opuntia retrorsa. Surprisingly, cacti are an important feature of these woodlands. At field level common herbaceous species are Aechmea distachantha, Dyckia ferox and various grasses. Lianas and vines are very rare but epiphytes are quite common, and epiphytic Tillandsia species are well represented. On the high ground of this eastern zone, there are woodlands transitional to the wetter forests of southern Brazil. Here notable species are Tecoma ipe (Tabebuia impetiginosa), Gleditsia amorphoides, Patagonula americana and several members of the Mytaceae (such as Eugenia uniflora and Myrcianthes cisplatensis). Others include Astronium balansae, Ceiba speciosa, Diplokeleba floribunda and Pithecellobium scalare, and in contrast to the previous woodlands lianas and epiphytes are abundant.

Chacoan Palm Groves

Stands of the endemic, fan-leaved palm tree Copernicia alba (Arecaceae) are features of marshes in seasonally flooded, alkaline soils. It can reach heights of 8-10 m but stands are usually open with variable levels of density. Other arboreal species may include various endemic or near endemic species such as Acacia caven (Fabaceae), Geoffroea decorticans (Fabaceae), Sapium haematospermum (Euphorbiaceae) and Tabebuia nodosa (Bignoniaceae). The field layer is typically dominated by the endemic or near endemic grass Spartina argentinensis (Poaceae) although species of Paspalum may replace it in places. In seasonally flooded parts of northeastern Paraguay and southeastern Bolivia, Copernicia alba is often co dominant with the shrub or small tree Triplaris gardneriana (Polygonaceae) which is endemic to the upper basin of the Paraguay River. Other characteristic species include Albizia inundata and Parkinsonia aculeata. 

References

Anon. 1996. Habitats of South America. Institute of Terrestrial Ecology and Intitut Royal Des Sciences Naturelles De Belgique.

Bucher, E. H. 1982. Chaca and Caatinga - South American Arid Savannas, Woodlands and Thickets. In: Ecology of Tropical Savannas. Eds. B. J. Huntley and B. H. Walker. Springer-Verlag.

Daly, D. C. & Mitchell, J. D. 2000. Lowland vegetation of tropical South America – an overview. In: Imperfect Balance: Landscape Transformations in the pre-Columbian Americas. Ed. D. Lentz. Columbia University Press, New York.

Junk, W. J. 1993. Wetlands of tropical South America. In: Wetlands of the World: Inventory, ecology and management. Vol. 1. Eds. D. Whigham, D. Dykyjova and S. Hejny. Kluwer Academic Press.

Lewis, J. P., Pire, E. F., Prado, D. E., Stofella, S. L., Franceschi, E. A. & Nélida, J. 1990. Plant communities and phytogeographical position of a large depression in the Great Chaco, Argentina. Vegetatio, 86: 25-38.

Navarro, G. Cochabamba (Bolivia), Molina, J. M. Madrid (Spain) and Pérez de Molas, L. Asunción (Paraquay). 2006. Classification of the forests of the northern Paraguayan Chaco. Phytocoenologia, 36: 473-508.

Prado, D. E. 1993. What is Gran Chaco vegetation in South America? I. A review. Contribution to the study of flora and vegetation of Chaco. V. Candollea, 48: 145-172.

Prado, D. E., Gibbs, P. E., Pott, A. & Pott, V. J. 1992. The Chaco-Pantanal transition in southern Mato Grosso, Brazil. In: Nature and Dynamics of Forest-Savanna Boundaries. Eds. P. A. Furley, J. Procter and J. A  Ratter.  Chapman & Hall.

Zak, M. R., Cabido, M. & Hodgson, J. G. 2004. Do subtropical seasonal forests in the Gran Chaco, Argentina have a future. Biological Conservation, 120: 589-598.