Holocene increases in palm abundances in northwestern Amazonia

Aim: In Amazonia, 227 of ca. 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 northwestern 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 northwestern 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 northwestern Amazonia when human influence was minimal.