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CENTRES OF PLANT DIVERSITY AND ENDEMISM.
V. Interior Dry and Mesic Forests
The dry-forest landscapes of the interior of the South American continent are traditionally divided into three major regions:
1. the caatinga of north-eastern Brazil, east of c. 45°W longitude;
2. the cerrado, covering the bulk of the Brazilian Shield, generally west of 45°W and/or south of 15°S; and
3. the chaco, west of the Paraguay River and south of c. 17°S.
At its southern margin in São Paulo State to the south of c. 20°S latitude, the "cerrado" vegetation becomes restricted to interrupted patches; west of the Paraná River, nearly continuous cerrado reaches the Tropic of Capricorn in the Sierra de Amambay of central-eastern Paraguay; and at its western margin well-developed cerrado occurs on Bolivia's Huanchaco Plateau with scattered patches on rocky outcrops farther south and west in Santa Cruz Department.
To these three main dry-forest units might appropriately be added a fourth: (4) the Bolivian Chiquitania, which constitutes what has been called the transition zone between the chaco and Amazonia, and covers most of the northern half of Santa Cruz Department as well as some adjacent areas (Gentry 1994).
Together the vegetations of the interior dry-forest general regions cover over 4 million km²:
Of these, the driest formations are the "caatinga" and the "chaco", mostly with below 800 mm of annual rainfall and a strong dry season of 5-8 months; both are characterized by irregular year-to-year fluctuations in precipitation (Gómes 1981; Ramella and Spichiger 1989). The cerrado, climatically intermediate between wetter tropical forest and the scrubbier chaco and caatinga formations, mostly has c. 800-1800 mm of annual rainfall with a strong dry season of 3-4 months (Eiten 1972; Gentry 1995). However, there is some overlap in precipitation, with parts of the chaco and caatinga having up to 1000 (-1300) mm of annual precipitation and the wettest part of the cerrado with up to 2000 mm (Spichiger et al. 1991; Eiten 1972), whereas parts of the adjacent Mata Atlântica with less than 1500 mm, but relatively evenly distributed precipitation, have well-developed semi-evergreen or even evergreen tropical forest.
Although there are few climatological records, the Chiquitania dry forest evidently occurs in areas with between 1000-1500 mm of rainfall and a strong dry season. Apparently its differentiation from the cerrado is based on soils rather than climate, as indicated by the presence of outliers of Chiquitania-type tall deciduous forest scattered through the Brazilian cerrado.
As a whole the South American interior dry forests are surprisingly diverse botanically. Together the caatinga-cerrado-chaco region accounts for an estimated 11% of the neotropical flora with perhaps c. 9300 angiosperm species.
Smaller areas of these dry forests can also be extremely diverse. A recent compilation by Filgueiras and Pereira (1990) of the plant species in Brazil's Distrito Federal included c. 2500 species of vascular plants, almost exactly the same number as in a roughly comparable Florula for three sites in the Iquitos area of Amazonian Peru (R. Vásquez, in prep.). The Serra do Cipó Florula (Giulietti et al. 1987), for a cerrado variant known as "campo rupestre", includes almost 1600 species. At a second campo-rupestre site in Bahia, Harley and Simmons (1986) found 663 vascular plant species. Emperaire's (1987) caatinga relevés may be taken as the equivalent of a local Florula in south-east Piaui of this vegetation type; again the flora is relatively diverse with 615 species recorded. On the other hand the more subtropical entire chaco has a relatively low floristic diversity, with only 1000-1200 species estimated to occur.
At yet smaller scales, these dry forests also can be strikingly diverse. The cerrado typically has 230-250 vascular plant species (occasionally as many as 300-350 species) in 0.1-ha samples (Eiten 1984), making this one of the most species-rich vegetations on Earth at this scale (Gentry and Dodson 1987), being exceeded only by some wet tropical forests. The caatinga and chaco, which have a much poorer ground cover of grasses and herbs, are much less diverse at the 0.1-ha scale. The Chiquitania dry forests are intermediate, being unusually diverse for dry-forest (87 species with dbh 2.5 cm or more in 0.1 ha), a distinction shared with the dry forests of western Mexico, but lacking the rich layer of herbs and subshrubs that makes the cerrado so species-rich.
These dry-forest regions are characterized by a number of floristic peculiarities, some shared and some not. In the cerrado (e.g. both campo-rupestre Florula sites as well as the Distrito Federal as a whole), Leguminosae, Compositae and Gramineae are the most speciose families, whereas only the Leguminosae is speciose in Amazonia. At all three of the sites Melastomataceae, Myrtaceae and Malpighiaceae are far more prevalent in the woody flora than in Amazonia and among the most speciose 6-12 families. Even more distinctive floristically are the monocot families Eriocaulaceae, Xyridaceae and Velloziaceae, which are among the most speciose families in the campo-rupestre sites and are hardly present at all beyond the Brazilian and Guayanan shields. In the caatinga, Leguminosae are even more prevalent than in cerrado or Amazonia, constituting a fifth of the entire south-east Piaui Florula (Emperaire 1987). Other families conspicuously more prevalent in caatinga than cerrado are the second and third most species-rich families - Euphorbiaceae and Bignoniaceae - as well as Cactaceae, Turneraceae, Boraginaceae and Combretaceae. The Leguminosae are also very prevalent in the chaco, but less speciose and less dominant. More striking is the conspicuous representation of Cactaceae, which is as speciose as Leguminosae in 0.1-ha samples of plants with dbh 2.5 cm or more in the chaco vegetation (Gentry 1993a). Other families generally better represented in chaco than in cerrado or caatinga include Capparidaceae, Nyctaginaceae and Zygophyllaceae. The Chiquitania dry forest may have some typical neotropical dry-forest familial compositions, with Leguminoae the most speciose woody family and Bignoniaceae the predominate family of lianas.
The interior dry areas of South America are outstanding in their high levels of endemism. Gentry's (1982a) estimate for regional endemism is 73%, as high as that of the Mata Atlântica and exceeded only by Amazonia among the major neotropical phytogeographic regions. Moreover there is remarkably little floristic overlap between the major dry areas at the specific level, although their floras share most of the same families and genera. Extrapolating from Emperaire (1987), the majority of the caatinga species is endemic to caatinga and almost two-thirds are restricted to caatinga plus cerrado. In a series of 0.1-ha samples of chaco vegetation, 59%, 77% and 85% of the completely identified species of 2.5 cm dbh or more are chaco endemics (Gentry 1995); these figures should prove higher for the still uncensused herbaceous taxa. Joly (1970) suggested that the campos rupestres of the cerrado region have the highest levels of endemism of any Brazilian vegetation type. In families like Velloziaceae, Eriocaulaceae and Xyridaceae, 60-70% of the world's species occur in the campos rupestres, mostly as local endemics (Giulietti and Pirani 1988). As an extreme case, 91.5% of the 59 species of Velloziaceae of the Serra do Cipó are locally endemic. Preliminary observations indicate that the virtually unstudied Chiquitania dry forests are also floristically distinctive, with a significant level of endemism (Gentry 1994, 1995).
In summary, all four of the main interior South American dry-forest regions have significant complements of endemic plant species and a concordant conservation importance. This contrasts with conservation priorities that might be drawn from other groups of organisms. Especially noteworthy is the high level of plant endemism in the caatinga (Sampaio 1995) as compared with very low levels of endemism (and presumably conservation importance) for better known taxa like vertebrates.
In contrast, the swampy areas associated with the dry-forest formations, including the pantanal of Mato Grosso and adjacent easternmost Bolivia and the llanos of Bolivian Beni, have floras consisting mostly of widespread species and very little plant endemism (Prance and Schaller 1982; Haase and Beck 1989), despite their importance for conservation from a zoological perspective on account of their high concentrations of large vertebrates. Prance and Schaller (1982) listed only four plant species endemic to the pantanal. The main conservation significance to these swampy regions from the botanical viewpoint relates to their interesting hydrological mosaics.
Major regional timber trees in the dry-forest areas include Schinopsis in the chaco, and Tabebuia, Aspidosperma, Amburana, Swietenia and Cedrela in the cerrado, Chiquitania and adjacent areas. In the cerrado and related vegetation types, the authors of the CPD Data Sheets have emphasized native fruits, as well as timber products. Some of these dry-area fruits are from well known edible-fruited genera like Caryocar (C. brasiliensis), Byrsonima and Anacardium. However, some of the fruitproducing genera are widespread but rarely reported to have edible fruit, here including Salacia elliptica, Mouriri glazioviana and several Myrtaceae, whose use may be relatively particular to dryarea species. Liqueurs prepared from these fruits are reported to be especially popular. In contrast, in the chaco region natural fibres are noted along with timber as economically important, rather than fruits. The main chaco fibreproducing plants are Bromeliaceae and Trithrinax palms. Easily the most interesting and region-specific economic use for native plants, which constitutes a major cottage industry, is the collection of everlasting plants for dried floral arrangements from the campo-rupestre region of the cerrado. Some 300 tons a year of some 40 species are collected, mostly Eriocaulaceae and Xyridaceae, including some rare endemics.
The conservation status of the South American dry-forest regions only can be described as critical. Tropical dry forest is now widely regarded as one of the most threatened - perhaps the most threatened - tropical vegetation type (Janzen 1988; Lerdau, Whitbeck and Holbrook 1991; Bullock, Mooney and Medina 1995). This perception contrasts vividly with suggestions by conservationists a few years ago that an appropriate strategy for preserving the Amazonian rain forest might be to develop the cerrado instead!
The caatinga, where much of Brazil's population of rural poor is concentrated, has been over-exploited for centuries; now desertification is a serious threat. Huge areas of the cerrado have been devastated in the last decade by cattle-ranching and mechanized agriculture on a large scale. As of 1990, it was estimated that 56% of the original 2 million km² was in managed use and 37% under cultivation (Dias 1990). Similarly much of the chaco has been ravaged by over-grazing, and increasingly by mechanized agriculture.
There is no reserved area for the semi-deciduous dry forest here designated the Chiquitania formation, and the largest area of intact tall dry forest, the Tucuvaca Valley of Bolivia, is in the process of being subdivided for soybean farming (Gentry 1994). In Brazil these forests have been reduced to isolated stands, and this has been called the probably most endangered forest biome in Brazil (Ratter 1991).
About 7% of the cerrado remained in pristine condition in 1990, including 1.5% in reserves (Dias 1990). Perhaps the only large pristine area of chaco left is in south-eastern Bolivia near the Paraguay border (Parker et al. 1993).
Fairly large areas of cerrado and chaco have been designated as reserves, but most are new, and only in part effective. The Brazilian cerrado has 132,994 km² or 6.6% of its area in National Parks or other conservation units falling in UNESCO categories I-IV (Dias 1990); Noel Kempf Mercado NP in Bolivia includes another 9000 km², a quarter in cerrado (T. Killeen, pers. comm.). In Paraguay 11,000 km² of chaco have been designated as National Parks and in Argentina an additional 950 km² as National Parks or Natural Reserves; in Bolivia, ironically the best preserved chaco has been totally unprotected.
Centres of Plant Diversity and Endemism
SA19. Caatinga of north-eastern Brazil
SA20. Espinhaço Range region
SA21. Distrito Federal
Argentina, Paraguay, Brazil, Bolivia
SA22. Gran Chaco
SA23. South-eastern Santa Cruz
SA24. Llanos de Mojos region
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