Included here is the Chaco or Gran Chaco. 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. This is one of the few areas in the World where the transition zone between tropic and temperate regions supports semiarid forest and woodland rather than desert.

Subhumid (or Eastern) 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.

Semi-Arid (or Western) Chaco Woodland

The most extensive vegetation here are medium-stature forests (16-18 m) dominated by the endemic or near endemic Aspidospermum quebracho-blanco (Apocynaceae) and Schinopsis quebracho (Anacardiaceae). A second story up to 12 m high typically consists of Ceiba chodatii, Cercidium praecox, P. kuntzei, and the endemic or near endemic Caesalpinia paraquariensis (Fabaceae), Geoffroea decorticans (Fabaceae), Prosopis alba, P. nigra (Fabaceae), Ziziphus mistol (Rhamnaceae)and a variety of arboreal cacti. The later are a particular feature of these woodlands and include endemic or near endemic species like Cereus validus, Opuntia quimilo, Quiabentia pflanzii and Stetsonia coryne (Cactaceae). The shrub layer may include Cercidium australis, Jodina rhombifolia, Maytenus spinosa, Ximeria americana and the endemic or near endemic Castella coccinea (family?)and Ruprechtia triflora (Polygonaceae).Typical herbaceous species are Bromelia hieronymi, B. serra and Deinacanthon urbanianum. On limey-clayish soils often with impeded drainage other trees such as the endemic or near endemic Aspidosperma triternatum (Apocynaceae), Bulnesia sarmientoi (Zygophyllaceae)and Tabebuia nodosa (Bignoniaceae)may become dominant, while along riverbanks, Calycophyllum multiflorum, Enterolobium contortisiliquum and the endemic or near endemic Acacia caven (Fabaceae) usually become the main species. In places, however, especially on sandy soils, these woodlands become more savanna-like with widely spaced trees of Jacaranda mimosifolia and the endemic Schinus heterophylla and S. quabrancho-colorada (Anacardiaceae), and where saline condition prevail the endemic giant cactus Stetsonia coryne (Cactaceae) becomes more conspicuous. Other endemic or near endemic species include Jatropha matacensis (Euphorbiaceae), Lophocarpinia aculeatifolia (Fabaceae), Mimoziganthus carinatus (Fabaceae), Setiechinopsis mirabilis (Cactaceae), Stenodrepanum bergi (Fabaceae) and many others.

Sierra Chaco Woodland

In the Argentinean uplands two woodland zones are recognized. On dry, sunny slopes up to about 1800 m the woodlands are mainly dominated by the endemic Schinopsis haenkeana (Anacardiaceae), while on the cooler slopes Lithrea ternifolia and Fagara coco are the main species. Other species generally associated with these woodlands include Bougainvillea stipitata, Kageneckia lanceolata, Prosopis chilensis, P. torquata, Ruprechtia apetala and the endemic or near endemic Aspidosperma quebracho-blanco (Apocynaceae), Geoffroea decorticans (Fabaceae) and Prosopis nigra (Fabaceae). In the Paraguayan uplands (Cerros León and Cabrera) other woodland types occur. On well-watered slopes, for example, there are deciduous woodlands dominated by Anadenanthera colubrina, while other species include Amburana cearensis, Aspidosperma pyriformis and Pterogyne nitens.

Thorn Chaco Woodland

Dense thorn woodland reaching not much more than 3 m in height occurs throughout the Chaco in low lying poorly drained areas. In many of these, especially where the water table is high, the arboreal stratum is reduced to species Prosopis. In eastern areas the endemic or near endemic Prosopis alba and P. nigra (Fabaceae) are often the main species. These stands are common, for example, around swamps and adjacent to streams. In less flooded areas other arboreal species such as the endemic or near endemic Acacia cavens (Fabaceae), Aspidosperma quebracho-blancho (Apocynaceae) and Geoffroea decorticans (Fabaceae) occur, but there is rarely any well developed shrub layer, and ground layer herbaceous species are limited. In the northern Paraguayan Chaco some of the more typical species include Cordia bordasii, Prosopis elata and the endemic or near endemic Bulnesia sarmientoi (Zygophyllaceae), Capparis retusa (Capparaceae), Prosopis rojasiana (Fabaceae), Ruprechtia triflora (Polygonaceae) and Tabebuia nodosa (Bignoniaceae). Other endemic or near endemic species characteristic of these miniature woodlands include Acacia praecox (Fabaceae), Bougainvillea praecox (Nyctaginaceae), Capparis speciosa, C. salicifolia (Capparaceae), Castella coccinea (family?), Jacaratia corumbensis (Caricaceae), Jatropha grossidenta (Euphorbiaceae), Prosopis ruscifolia and P. sericantha (Fabaceae).

Calycophylla multiflora (palo blanco) Transitional Forest

Forest dominated by the magnificent Calycophylla multiflora, are situated in the sub Andean piedmont regions of southwest Bolivia and northwest Argentina, which is zone of transition between the Chaco and Yungas ecozones. The rainfall in this zone is intermediate between these two regions and has more of a monsoon character. The forest canopy can reach heights of 30 m and interestingly these forests have a higher number of deciduous species (79%) than any other subtropical seasonal forest. Certain trees, such as Calycophylla multiflora and Phyllostylon rhamnoides, have straight, white boles up to one meter in diameter. Other trees include Amburana cearensis, Anadenanthera colubrina, Astronium urundeuva, Corda trichotoma, Enterolobium contortisiliquum, Myroxylon balsamum, Patagonula americana, Ruprechtia laxiflora and Senna spectabilis. There is usually a dense shrub layer up to 2 m high making movement through the forest difficult, and there are abundant epiphytes, vines and lianas. Forests of Calycophylla multiflora together with Acosmum cardenasii can also be found in northern Paraguay. They also typically occur in transition zones usually in seasonally flooded areas such as riverbeds with seasonal drainage. Other trees include Casearia gossypiosperma and Phyllostylon rhamnoides, while common under storey species are Celtis iguanea, Fagara pterota, Helietta mollis and Lasiacis sorghoides.


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