Differing local-scale responses of Bolivian Amazon forest ecotones to middle Holocene drought based upon multiproxy soil data

Uncertainty remains over local-scale responses of ecotonal Amazonian forests to middle Holocene drying due to the scarcity, and coarse spatial resolution, of lacustrine pollen records. This paper examines the palaeoecological potential of soil phytoliths, stable carbon isotopes and charcoal for capturing local-scale ecotonal responses of different types of Bolivian Amazonian forest to middle Holocene climate change. Soil pits 1 m deep were dug at ecotones between rainforest, dry forest, Chaco woodland and savannah, and sampled at 5-10 cm resolution. Both phytolith and stable carbon isotope records indicate stability of dry forest-savannah ecotones over the last ca. 6000 years, despite middle Holocene drought, revealing the dominance of edaphic factors over climate in controlling this type of ecotone. In contrast, d(13)C data reveal that rainforest-savannah ecotones were more responsive to climate change, with rainforest likely replaced by drought-tolerant dry forest or savannah vegetation during the mid-Holocene, consistent with regional-scale lacustrine pollen records. However, such shifts are not apparent in most of our phytolith records due to insufficient taxonomic resolution in differentiating rainforest from dry forest. Charcoal data show that ecotonal dry forests experienced greater fire activity than rainforests and that recent high fire activity at all forest sites is unprecedented since at least the middle Holocene.