Location and General Description
Roughly in the center of the South American continent, most of the Chiquitano forest lies within the eastern lowlands of Santa Cruz, Bolivia, with smaller patches extending into western Mato Grosso, Brazil. Situated at the southern limit of Amazonian forests, this forest marks a transition to drier thorny scrub forests further south in the Chaco. Characterized by a strong dry season during the austral winter, the region experiences an annual water deficit of 500 mm, notwithstanding a mean annual rainfall of 936 mm (Montes de Oca 1997). The ancient Brazilian shield underlying the forest presents a gently undulating plain framed by scenic quartzitic ridges and mountains of Precambrian origin aligned in a generally NW to SE direction (ENTRIX 1999). Tertiary soils in the peneplain are derived from gneiss and granitic rocks, while Quaternary sediments overlaying the bottom of broad U-shaped valleys such as Tucavaca Valley, which supports the taller forest. Calcareous rocks are found in the Sunsas Ridge, as well as ultramafic soils around Rincon del Tigre, adding to the area´s unique diversity. Rich in hot water springs, some of these contain locally endemic fish like Bujurquina oenolaemus (Ergueta and Morales 1996). Hydrologically, the ecoregion straddles the watershed divide between two major basins in South America, Amazonas and La Plata.
The floristic associations in this forest are very strongly dependent on drainage patterns. The most abundant association "soto/curupaú" establishes a 20 m canopy on well drained soils where the dominant species is soto (Schinopsis brasiliensis). Soto is joined by curupaú (Anadenanthera macrocarpa), momoqui (Caesalpinia pluviosa), morado (Machaerium scleroxylon), roble (Amburana cearensis), cedro (Cedrela fissilis), and others where they provide 80% canopy closure. A 5-level strata develops, where emergents reach 30 m high, with an arboreal, shrubby and herbaceous understory complete the structure (ENTRIX, 1999).
A second association, "cuchi/curupaú" also grows on well-drained but poorer soils. Dependent on establishing on high (steep mountain slopes with rocky soils) or low (sandy piedmont) terrain, the dominant species is curupaú (Anadenthera macrocarpa) or cuchi (Astronium urundeuva), respectively. It forms a 10 – 15 m canopy with 65% coverage where emergents reach 25 m.
A third forest association develops on hygrophilous soils adjacent to creeks that experience shallow flooding during the rainy season. The "cuta/ajo-ajo" association has cuta (Phyllostyllon rhamnoides) as the dominant species, but ajo-ajo (Gallesia integrifolia) is a better indicator as it is highly restricted to these flood-prone sites.
A fourth common grouping is the "tajibo/tusequi" association formed on small isolated forest islands 0.5-1 m higher than the surrounding herbaceous savannas. Tajibo (Tabebuia heptaphylla) is dominant here, while tusequi (Machaerium hirtum) is very characteristic as it indicates soils alkalinized through inverse leaching (ENTRIX, 1999).
Still relatively unstudied, this ecoregion suggests several good candidates for endemic taxa. The ultramafic soils around Rincon del Tigre are conducive to endemism and interesting adaptations in sessile taxa (MHNNKM 1997), but have not yet received attention from botanists. The Sunsas Ridge is known to harbor numerous limestone caves rich in bat colonies that have never been studied, for bats or invertebrates. Finally, the difficult access to the isolated grasslands on flat table-top mountains in the area has delayed scientific work on them. It was in this forest that Gentry (1995) measured plant species richness in the Tucavaca Valley and found the second highest alpha diversity in the world for dry forests.
Surprisingly, the Chiquitano forest is floristically more similar to the geographically distant Caatinga, Misiones and Tucumano forests than its neighbors in the Chaco and Cerrado ecoregions (Prado and Gibbs 1993; FAN et al. 1999). It appears that the Chiquitano forest functioned as a central node to these currently fragmented dry forest holocene refugia, making it a very promising area for adaptive radiation research.
As headwaters for the Pantanal, the forest is strongly linked to both the flooding and fire regimes typical in grasslands. Many species withstand temporary flooding such as "ajo-ajo", "alcornoque" (Tabebuia aurea) and develop fire-resistant bark "soto", "verdolago" (Calycophyllum multiflorum). The flooding pulse causes large annual displacements from open grasslands into higher forested land for marsh deer (Blastocerus dichotomus) and white-lipped peccary (Tayassu pecari) (pers. obs.). One very large block of intact habitat (40,000 km2) in the SE reaches of the ecoregion remains as the sole guardian of Chiquitano biodiversity. Jaguars, smaller cats and maned wolves (Chrysocyon brachyurus) maintain prey populations in check within an intact biota which boasts healthy populations of cracids, tapirs, and giant armadillo (Priodontes maximus)(ENTRIX 1999; MHNNKM 1997).
This unique forest shelters a critically endangered reptile (Caiman latirostris); one endangered bird (Sporophila nigrofufa) and at least three mammals (Priodontes maximus, Chrysocyon brachyurus and Pteronura brasiliensis) and three birds, one reptile and 12 mammals in a vulnerable status (Groombridge 1994; Ergueta & Morales 1996; IUCN 1996; Tarifa 1996). Chiquitano forest contained the highest mammlian species richness of several biomes in the ecoregion, with 42 species accounted for in less than two weeks of surveys. In addition, 11 or 26% of those species were threatened or endangered, suggesting that this area is extremely important for harboring rare mammals (Brooks et al. 2001).
This is the largest patch of healthy dry forest ecosystem alive today (Bryant et al. 1997), and one of the most biologically diverse dry forests in the world (Parker et al. 1993). Two large blocks of forest in outstanding conservation condition, comprising 20% of the original ecoregion, remain. Both blocks lie east of San Jose de Chiquitos, split into a north and a south block by a road with ranches along it. Two protected areas Otuquis and San Matias, include important parts of this remnant forest, but linkages between them and effective management are urgent. The Tucavaca Valley is the middle area which needs to be put under protection to provide long term ecological viability.
Types and Severity of Threats
Dry forests are the most endangered tropical forest in the world (Janzen 1988) and the Chiquitano forest is no exception. Habitat conversion due to agricultural expansion and unplanned colonization is the major threat. Habitat degradation comes next in the form of the Paraguay-Parana Hidrovia Dam project and uncontrolled logging. The third major threat is habitat fragmentation with improved and new access ways promoted by multinational energy companies (pipelines, powerlines and electricity generation) and the transportation sector (roads, ports). Considered globally outstanding for its biological distinctiveness and critically threatened the ecoregion still faces an uncertain future (Dinerstein et al. 1995).
Justification of Ecoregion Delineation
The dry forests of the Chiquitano region were deliniated according to national vegetation cover maps. In Brazil, linework follows IBGE (1993) classifications of "semideciduous seasonal submontane forest" and the surrounding "secondary vegetation and agricultural activities" within this broader classification. In Bolivia our linework follows Ribera et al. (1994), who classify this region as "region of precambian semideciduous forest (Brazilian shield)".
Brooks, D.M., J.M. Rojas, H. Aranibar, R.J. Vargas and T. Tarifa. 2001. A preliminary assessment of the mammalian fauna of the Eastern Bolivian Panhandle. Mammalia 65(3).
Bryant, D., D. Nielsen, & L. Tangley. 1997. The Last Frontier Forests: Ecosystems and Economies on the Edge. WRI Publications, Baltimore, MD.
Dinerstein, E., D.M. Olson, D.J. Graham, A.L. Webster, S.A. Primm, M.P. Bookbinder, and G. Ledec. 1995. A conservation assessment of the terrestrial ecoregions of Latin America and the Caribbean. World Wildlife Fund and World Bank. Washington, DC.
ENTRIX. 1999. Supplemental Environmental Assessment, Cuiabá Pipeline – Bolivian Portion, Phase I Direct Impacts. ENTRIX, INC. Houston, Texas.
Ergueta, P., and C. Morales, editors. 1996. Libro Rojo de los Vertebrados de Bolivia. CDC, La Paz, Bolivia.
Fundação Instituto Brasilero de Geografia Estatástica-IBGE. 1993. Mapa de vegetação do Brasil. Map 1:5,000,000. Rio de Janeiro, Brazil.
FAN-MBG-MHNNKM-WCS-WWF, 1999. The San Miguel-Cuiabá Pipeline Project. An Independent Supplemental Environmental Assessment. Santa Cruz, Bolivia.
Gentry, A. 1995. Diversity and floristic composition of neotropical dry forests. Pp. 146-194 in S.H. Bullock, H.A. Mooney, and E. Medina, editors, Seasonally dry tropical forests. Cambridge University Press, Cambridge.
Groombridge, B., editor. 1993, 1994. IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland.
IUCN, 1996. 1996 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland. 368 pp. Janzen, D. 1988. Tropical dry forests, The most endangered major tropical ecosystem. In E.O. Wilson, Biodiversity.
Killeen, T., E. Garcia, and S. Beck, editors. 1993. Guia de Arboles de Bolivia. Quipus S.R.L., La Paz, Bolivia.
Montes de Oca, I. 1997. Geografia y Recursos Naturales de Bolivia. La Paz, Bolivia.
Museo de Historia Natural Noel Kempff Mercado. 1997. Propuesta Técnica para la creación de las áreas protegidas Pantanal de Otuquis y San Matías. MHNNKM, Santa Cruz, Bolivia.
Parker, T.A., A.H. Gentry, R.B. Foster, L.H. Emmons, and J.V. Remsen, Jr. 1993. The lowland dry rorests of Santa Cruz, Bolivia: A global conservation priority. Conservation International, Washington, DC.
Prado, D.E., and P.E. Gibbs. 1993. Patterns of species distributions in the dry seasonal forests of South America. Ann. Missouri Botanical Garden. 80: 902-927.
Ribera, M.O., M. Libermann, S. Beck, and M. Moraes. 1994. Mapa de la vegetacion y areas protegidea de Bolivia. 1:1,500,000. Centro de Investigaciones y Manejo de Recursos Naturales (CIMAR) and Universidad Autónoma Gabriel Rene Moreno (UAGRM), La Paz, Bolivia.
Tarifa, T., 1996. Mamíferos, Pages 165-264 en P. Ergueta -S. y C. de Morales, editors, Libro Rojo de los Vertebrados de Bolivia CDC-Bolivia.
Prepared by: Roger Landivar
Reviewed by: Dr. Dan Brooks