STUDIES IN MONODOMINANT FORESTS IN EASTERN MATO GROSSO, BRAZIL: I. A FOREST OF BROSIMUM RUBESCENS TAUB

Abstract

The occurrence of a monodominant tropical forest dominated by the tree species Brosimum rubescens Taub. (Moraceae) in the transition zone between the cerrado region and the Amazonian rain forests along the Araguaia valley in the state of Mato Grosso in Brazil is reported. A 6000m² (40×150m) area located in the central portion of a forest (14°50′47″S, 52°08′37″W) on the Eldorado Farm in Nova Xavantina was sampled to determine the structure, phytosociology and soil properties. A total of 44 tree species was found in the forest, with a Shannon diversity index (H′) of 2.37 and the Pielou evenness index (J′) of 0.63. These values were similar to those reported by other workers in an Amazonian monodominant forest dominated by Peltogyne gracilipes. Average tree density equal or greater than (≥) 5cm dbh was 1066ha⁻¹ and basal area 37.46m² ha⁻¹. Dead standing individuals represented 7.03% of the total density and lianas 8.12%. The main families were Moraceae, Rubiaceae, Burseraceae, Hippocrateaceae and Leguminosae–Caesalpinioideae. The species with the highest importance value indices were B. rubescens (124.55), Amaioua intermedia var. brasiliana (32.97), Cheiloclinium cognatum (22.41), Tetragastris balsamifera (19.58) and Protium pilosissimum (16.64). Brosimum rubescens alone accounted for 85.5% of the individuals ≥30cm dbh, 80.35% of those ≥20cm dbh, 57.33% of those ≥10cm dbh, and 35.85% of those ≥5cm dbh. The forest was composed of upper, middle and lower storeys. Diameter distribution was unbalanced for the community and for the main species, suggesting that establishment has been episodic. The surface soil was acid, with high exchangeable Al levels and low fertility, as is the case for most cerrados and Amazonian forests. The availability of K was in the range of adequacy, as compared with most native soils. However, concentrations of available Mg in the soil was higher than that of available Ca, with Mg/Ca ratios above three, unlike most cerrados and Amazonian forests, but similar to the soils of many monodominant tropical forests reported in the literature. Soils were characterized by high concentrations of available Fe. Besides the high Mg/Ca ratios, a high proportion of K as compared with the bivalent cations Ca and Mg, and toxicities of Fe and Mn due to strong acidity could be factors influencing the occurrence of species in this forest.