A multi-proxy palaeoecological record of late-Holocene forest expansion in lowland Bolivia

Pollen, charcoal, and bulk sediment stable carbon isotope analysis of a 2.4 m sediment core from Laguna Yaguarú in eastern Bolivia, located near the modern ecotone between the humid, evergreen Amazon rainforest and the seasonally-dry, semi-deciduous Chiquitano dry forest, provides a > 5600 year record of environmental change in an area sensitive to changes in the strength of the South American monsoon. The core consists of organic sediment interrupted by a 15 cm mineral facies deposited ∼ 5270 yr BP in a low-energy flood event in the Yaguarú basin. Pollen data indicate that the basin contained a cattail (Typha) marsh prior to the flood. Some dry forest elements were present, probably growing within small patches of forest within a matrix of generally open vegetation including marsh, savanna, and savanna woodland. Above the flood-layer, Typha pollen is absent, indicating open-water conditions, and pollen assemblages are similar to modern pollen spectra from Bolivian dry forests. Around 1200 BP charcoal concentrations decrease, carbon isotope ratios become more negative, and Celtis pollen rises dramatically, signaling forest expansion and the development of the modern closed-canopy dry forest at Laguna Yaguarú. The wetter conditions implied by this forest expansion suggest the region was affected by the strengthening South American monsoon at this time. The timing of the arrival of monsoon moisture at Yaguarú is consistent with nearby records, allowing a ∼ 700 year delay for the migration of the monsoon to reach the more southerly location of Yaguarú.