Diversity, composition and structure of a tropical semideciduous forest in the Chiquitanía region of Santa Cruz, Bolivia

Abstract

An exhaustive floristic inventory was conducted in a 400-ha block of tropical semideciduous forest in the Chiquitanía region of Santa Cruz, Bolivia. A total of 501 species were collected using both quantitative and traditional plant collecting methods. Trees represented the most diverse life form (124 spp), followed by herbs (101 spp), lianas (85), shrubs (66), herbaceous climbers epiphytes (15) and parasites (two). Floristic diversity was greatest for the forest floor community, when compared to either the understorey or canopy tree communities. Quantitative data were obtained for 336 species in 100 plots which contained a series of nested subplots to sample smaller-stature plants. Stem density for trees (dbh≥5 cm) was 914 trees ha⁻¹, with a total density for all life forms estimated to be 135,000 plants ha⁻¹; the total basal area for the forest was estimated to be 27.6 m² ha⁻¹. Species richness when measured by standard methods was found to be one of the highest for a dry forest region reported for the Neotropics with a mean of 70.8 spp 0.1 ha⁻¹ (dbh ≥ 2.5 cm) and 50 spp ha⁻¹ (dbh ≥ 10 cm). Comparison of life forms and vertical strata showed that the flora on the forest floor was more diverse than all other vertical strata combined. Habitat heterogeneity was studied using ordination procedures based on floristic data and to identify the characteristic species of three plant communities: granite outcrops, valley forest and upland forest. The most abundant tree species in the study area were Acosmium cardenasii, Neea hermaphrodita, Aspidosperma tomentosa and Galipea trifoliata, while the species with the greatest basal area were Anadenanthera colubrina, Acosmium cardenasii, Caesalpinia floribunda, Aspidosperma tomentosa, Piptadenia viridiflora, Chorisia speciosa, Tabebuia impetiginosa, Centrolobium microchaete, and Machaerium scleroxylon. Most canopy and understorey tree species had a population structure characterized by numerous juveniles and relatively few large trees, while emergent species tended to have a size-class distribution with relatively few juvenile individuals. Lianas and canopy trees were predominantly anemochorous, while understorey trees and shrubs were predominantly zoochorous; herbaceous species were largely autochorous, with only fern species relying on wind dispersal (sporochory). A comparison of structural attributes with other dry forest areas in the Neotropics, demonstrates the heterogeneous nature of vegetation types that are commonly assigned to this ecosystem. The Chiquitano dry forest shares many floristic elements with the semideciduous forests of the Andean piedmont of northwestern Argentina, the Misiones region of eastern Paraguay and northeastern Argentina, as well as the Caatinga region of northeastern Brazil.