Historical Human Footprint on Modern Tree Species Composition in the Purus-Madeira Interfluve, Central Amazonia

Background: Native Amazonian populations managed forest resources in numerous ways, often creating oligarchic forests dominated by useful trees. The scale and spatial distribution of forest modification beyond pre-Columbian settlements is still unknown, although recent studies propose that human impact away from rivers was minimal. We tested the hypothesis that past human management of the useful tree community decreases with distance from rivers. Methodology/Principal Findings: In six sites, we inventoried trees and palms with DBH≥10 cm and collected soil for charcoal analysis; we also mapped archaeological evidence around the sites. To quantify forest manipulation, we measured the relative abundance, richness and basal area of useful trees and palms. We found a strong negative exponential relationship between forest manipulation and distance to large rivers. Plots located from 10 to 20 km from a main river had 20-40% useful arboreal species, plots between 20 and 40 km had 12-23%, plots more than 40 km had less than 15%. Soil charcoal abundance was high in the two sites closest to secondary rivers, suggesting past agricultural practices. The shortest distance between archaeological evidence and plots was found in sites near rivers. Conclusions/Significance: These results strongly suggest that past forest manipulation was not limited to the pre-Columbian settlements along major rivers, but extended over interfluvial areas considered to be primary forest today. The sustainable use of Amazonian forests will be most effective if it considers the degree of past landscape domestication, as human-modified landscapes concentrate useful plants for human sustainable use and management today. © 2012 Levis et al

Holocene increases in palm abundances in north-western Amazonia

Aim: In Amazonia, 227 of c. 16,000 tree species account for half the individual trees (termed ‘hyperdominant’ species), and a disproportionate number of these species are palms. Our objectives are to show how and whether palm abundance has changed through the Holocene. Here, we reconstruct a detailed fire and vegetation history from north-western Amazonia, with a focus on changes in palm abundances, and compare our results with regional data. Location: Amacayacu, Colombia. Taxon: Amazonian palms. Methods: We performed charcoal and phytolith analysis on soil cores, and obtained ages of past fires using 14C dating. We measured charcoal abundances and the relative abundances of phytoliths (silica-based microfossils) for all samples. We used these data to reconstruct changes in fire and vegetation, and compared these data with the species composition of palms in the modern forest. Results: Seven 14C dates from charcoal in three cores provided fire ages ranging from 1630 to 2450 calibrated years before present. Charcoal was absent from one-third of the cores. Palm phytoliths from genera such as Iriartea, Socratea, and Astrocaryum have increased through time, while genera such as Euterpe, Hyospathe, and Oenocarpus have remained relatively stable and similar to modern levels. Overall, palm abundances were negatively correlated with charcoal measurements. Decorated sphere phytoliths, produced from unknown arboreal taxa were positively correlated with charcoal presence and abundance. Main conclusions: Palms have increased at Amacayacu and other forest plots through time, but the increases are largest in north-western Amazonia. The presence of fire, however, dampens the increase in palms through time. When compared with reconstructions from other Amazon regions, our results suggest that increases in palm abundances in the late Holocene occurred both in the presence and absence of direct pre-Columbian human influence, and that response was strongest in north-western Amazonia when human influence was minimal

Cyclic sediment deposition by orbital forcing in the Miocene wetland of western Amazonia? New insights from a multidisciplinary approach

In the Miocene, a large wetland system extended from the Andean foothills into western Amazonia. This system has no modern analogue and the driving mechanisms are not yet fully understood. Dynamic topography and Andean uplift are thought to have controlled deposition, with allocyclic base level changes driven by eustasy and orbital forcing also playing a role. In this study we investigate the presumed orbital cyclicity that controlled sediment deposition, while also assessing sediment source and biomes in the Miocene wetland. We do this by integrating lithological, palynological, malacological and geochemical data from the Los Chorros site (Amazon River, Colombia), and by placing our data in a sequence stratigraphic framework. In this sequence biostratigraphic evaluation, the Los Chorros succession is visualized to be composed of a series of flood-fill packages, with a rapid initial flood, marine-influenced conditions at the time of maximum flood, followed by a longer regressive infill phase. Based on the palynology we could differentiate local vegetation, such as palm swamps, from regional origin such as terra firme vegetation (non-flooded Amazonian forest) and Andean montane forest, while from sediment geochemistry we could separate local and regional sediment sources. At the times of flooding, oligotrophic and eutrophic aquatic conditions alternatively characterized the wetland, as is shown by the presence of algae, floating ferns, and mollusc assemblages, while intervening subaquatic debris points to proximal submerged lowlands. In the lower 20 m of the section, marine influences are intermittently evident and shown by short-lived maxima of mangrove pollen, foraminiferal test linings, dinoflagellate cysts, coastal mollusc species, and an episodic decline in terrestrial biomarkers. The upper 5 m of the section is characterized by floodplain forest taxa with a diversity in tropical rain forest taxa and relatively few lacustrine indicators. These marine, mangrove, and lacustrine indicators suggest that the outcrops at Los Chorros represent predominant marine-influenced lacustrine conditions during periods of sea level highstand. The sequence biostratigraphic evaluation further points to eight 41 kyr obliquity-driven depositional cycles, with rapid phases of transgression. Mangrove elements would have colonised within the timeframe of each sea level rise. Based on this relative age constraint and comparison to regional records, deposition likely took place prior to the 13.8 Myr global sea level fall, and most likely during the period just after 14.5 Ma, between Middle Miocene Climatic Optimum (MMCO; 17–14 Ma) and Middle Miocene Climate Transition (MMCT; 14.7–13.8 Ma). Palynological evidence further suggests that to the west, surface elevation ranged from ~1000 up to ~3500 m and hosted protoparamo vegetation, the oldest yet reported and in agreement with predictions from molecular studies. In contrast, contemporaneous sites to the northeast of the wetland consisted of fluvial and cratonic formations, as shown by their Nd and Sr isotopic sediment signature. In summary, our data lead to an improved understanding of how geological and astronomical mechanisms controlled the floral and faunal distribution and controlled sediment deposition in western Amazonia during the middle Miocene. As Miocene conditions strongly contrast with modern western Amazonia, our data provide an important context for the deep time history and evolution of the modern western Amazon rainforest