Observations on the vegetation of northeastern Mato Grosso: I. The woody vegetation types of the Xavantina-Cachimbo Expedition Area

Abstract

The Xavantina-Cachimbo Expedition worked during 1967-9 in a 20 km square around a base camp (12° 49' N, 51° 46' W), ca . 260 km north of Xavantina (NE Mato Grosso) and near Xavantina itself. The vegetation is of special interest because the base camp is situated near the junction of the savanna region of Central Brazil and the Amazonian forest. It is a pattern of savanna (cerrado), savanna woodland (cerradão), forest and treeless grassland (campo) with often remarkably abrupt boundaries between the different communities. Until 1967 the area had been very little affected by man. The climate is characterized by high temperature throughout the year, an annual rainfall of about 1200 to 1400 mm, and a more or less rainless dry season from June to September inclusive. During the dry season the cerrado, campo and some forms of cerradao vegetation are subject to fire, but are not burned every year. The forest, except the Deciduous Seasonal forest, is not normally burned. The rocks consist of sandstones overlying shale and mudstones. The sandstone weathers to form widespread dystrophic soils of low nutrient content, whilst the finer textured rocks, exposed in some deeper valleys, produce somewhat richer mesotrophic soils. The woody vegetation types of dystrophic soils are classified into three types of Evergreen Seasonal forest (‘Swampy Gallery' forest,‘Valley' forest and ‘Dry' forest), cerradão and cerrado. The Swampy Gallery forest is found along streams where the water table is close to the surface even in the dry season and is often bordered on one or both sides by strips of campo. In composition it resembles an impoverished Amazonian rain forest. The top-storey is dominated by Qualea ingens and Q. wittrockii , growing sometimes to 40 m, and the undergrowth includes numerous dicotyledons, Scitamineae, grasses and other monocotyledons. At a slightly higher level in stream valleys there is another type of tall forest, Valley forest, in which characteristic trees (all growing to about 40 m) are Apuleia molaris, Copaifera langsdorfii, Hymenaea stilbocarpa and Ormosia sp. (Tento). Much the most extensive type of Evergreen Seasonal forest is the Dry forest which represents the southern fringe of the Amazonian forest and covers a vast area stretching away northwards from the base camp area. This is a mixed community in which the trees seldom grow to more than 20 m. The most abundant species of the upper storey in the area studied are C haetocarpus echinocarpus, Licania blackii, L. kunthiana, Saco glottis guianensis and Xylopia amazonica . The transition from Dry forest to cerrado is sometimes abrupt, but elsewhere there is an ecotone in which Hirtella glandulosa cerradão forms a recognizable nodum, occupying a zone up to 4 km wide. Characteristic species in this are Emmotum nitens, Sclerolobium paniculatum and Vochysia haenkeana , as well as H. glandulosa . The boundary between cerrado and Dry forest appears to be dynamic and there are some indications that the forest has recently invaded the cerrado. The present boundary does not seem to be primarily dependent on climate or burning but shows some relation to soil conditions, though apart from a higher clay content in the latter the cerrado and forest soils are much alike. Cerrado has a lower degree of crown cover than cerradão; it is a type of open savanna with grassy undergrowth and is extremely variable in floristic composition and no clearly defined associations could be recognized. The boundary between cerrado and campo in valleys is sharp and appears to be determined by the height of the water table in the wet season. The mesotrophic soils are occupied by Deciduous Seasonal forest, the only woody community in the area in which the top storey becomes leafless in the dry season. The fioristic composition of this community is very different from that of the other forest types and characteristic top-storey species include Cedrela fissilis, Piptadenia macrocarpa, Platypodium elegans and Sterculia striata , with Acacia polyphylla and Bauhinia cupulata as a second storey. Bamboos and the palm Acrocomia sp. are features of the undergrowth. Floristically this community is similar to forest types found on calcareous rocks in Goiás and Minas Gerais. It is fringed by a characteristic cerradão, termed Magonia pubescens / Callisthene fasciculaia cerradão.