A RE-ASSESSMENT OF THE 'PLEISTOCENE DRY FOREST ARC' HYPOTHESIS: PRELIMINARY EVIDENCE FROM THE BOLIVIAN CHIQUITANO DRY FOREST

The Chiquitano Dry Forest in eastern Bolivia constitutes the largest remaining intact block of seasonal dry forest in the world, but is critically endangered, experiencing among the highest rates of deforestation on Earth. Understanding the origin and development of these forests over sufficiently long, millennial-scale time series is an important pre-requisite to predicting likely responses to future climate change and habitat degradation.

The aim of our research is to test the hypothesis that the Chiquitano Dry Forest constitutes an ancient disjunct relic or refugium of a formerly far more extensive forest formation at the last glacial maximum (LGM), termed the ‘Pleistocene Dry Forest Arc’ (Prado & Gibbs, 1993). Our approach is to produce palaeovegetation reconstructions from lake sediment cores constraining the current ecotonal boundaries of this ecosystem.

Our preliminary 60,000 yr palaeovegetation record from Laguna La Gaiba (17°47’S, 57°43’W) suggests that seasonally dry forests may have existed in easternmost Chiquitanía since the LGM (consistent with the hypothesis). By contrast, the more detailed 50,000 yr pollen record from Laguna Chaplin (14°28’S, 61°04’W), located just beyond the northern ecotone, suggests that these forests were no more widespread at the LGM than today, raising the possibility that the ‘Pleistocene Dry Forest Arc’ hypothesis is false. Pollen data from both sites show that the key dry forest indicator taxon (Anadenanthera colubrina), which is today common throughout Chiquitanía, is a relatively recent arrival, first appearing only 8,000 yr BP (in contrast to inferences based on biogeographic studies by Prado & Gibbs, 1993). Even if dry forests have existed in parts of eastern Bolivia since the LGM, they should not be considered as ancient ice-age relics or refugia, since they appear to be highly dynamic ecosystems which have undergone considerable species turnover and re-assortment over this period.

References Prado, D.E., & Gibbs, P.E. 1993. Patterns of species distributions in the dry seasonal forests of South America. Annals of the Missouri Botanical Garden 80, 902-927.