A 5,000-year vegetation and fire history for tierra firme forests in the Medio Putumayo-Algodón watersheds,northeastern Peru

This paper addresses an important debate in Amazonian studies; namely, the scale, intensity, and nature of human modification of the forests in prehistory. Phytolith and charcoal analysis of terrestrial soils underneath mature tierra firme (nonflooded, nonriverine) forests in the remote Medio Putumayo-Algodón watersheds, northeastern Peru, provide a vegetation and fire history spanning at least the past 5,000 y. A tree inventory carried out in the region enables calibration of ancient phytolith records with standing vegetation and estimates of palm species densities on the landscape through time. Phytolith records show no evidence for forest clearing or agriculture with major annual seed and root crops. Frequencies of important economic palms such as Oenocarpus, Euterpe, Bactris, and Astrocaryum spp., some of which contain hyperdominant species in the modern flora, do not increase through prehistoric time. This indicates pre-Columbian occupations, if documented in the region with future research, did not significantly increase the abundance of those species through management or cultivation. Phytoliths from other arboreal and woody species similarly reflect a stable forest structure and diversity throughout the records. Charcoal ¹⁴C dates evidence local forest burning between ca. 2,800 and 1,400 y ago. Our data support previous research indicating that considerable areas of some Amazonian tierra firme forests were not significantly impacted by human activities during the prehistoric era. Rather, it appears that over the last 5,000 y, indigenous populations in this region coexisted with, and helped maintain, large expanses of relatively unmodified forest, as they continue to do today.

More than 50 y ago, prominent scholars argued that due to severe environmental constraints (e.g., poor natural resources), prehistoric cultures in the Amazon Basin were mainly small and mobile with little cultural complexity, and exerted low environmental impacts (1, 2). Contentious debates ensued and have been ongoing ever since. Empirical data accumulated during the past 10 to 20 y have made it clear that during the late Holocene beginning about 3,000 y ago dense, permanent settlements with considerable cultural complexity had developed along major watercourses and some of their tributaries, in seasonal savannas/areas of poor drainage, and in seasonally dry forest. These populations exerted significant, sometimes profound, regional-scale impacts on landscapes, including with raised agricultural fields, fish weirs, mound settlements, roads, geometric earthworks called geoglyphs, and the presence of highly modified anthropic soils, called terra pretas or “Amazonian Dark Earths” (Fig. 1) (e.g., refs. 3–15).Open in a separate windowFig. 1.Location of study region (MP-A) and other Amazonian sites discussed in the text. River names are in blue. The black numbers represent major pre-Columbian archaeological sites with extensive human alterations (1, Marajó Island; 2, Santarém; 3, Upper Xingu; 4, Central Amazon Project; 5, Bolivian sites) (3, 5–10, 14, 15). ADE, terra preta locations (e.g., refs. 19 and 20); triangles are geoglyph sites (6, 8). The white circles are terrestrial soil locations previously studied by Piperno and McMichael (29, 31–33, 54) (Ac, Acre; Am, Amacayacu; Ay, Lake Ayauchi; B, Barcelos; GP, lakes Gentry-Parker; Iq, Iquitos to Nauta; LA, Los Amigos; PVM, Porto Velho to Manaus; T, Tefe).An important, current debate that frames this paper centers not on whether some regions of the pre-Columbian Amazon supported large and complex human societies, but rather on the spatial scales, degrees, and types of cultural impacts across this continental-size landscape. Some investigators drawing largely on available archaeological data and studies of modern floristic composition of selected forests, argue that heavily modified “domesticated” landscapes were widespread across Amazonia at the end of prehistory, and these impacts significantly structure the vegetation today, even promoting higher diversity than before (e.g., refs. 14–21). It is believed that widespread forms of agroforestry with planted, orchard-like formations or other forest management strategies involving the care and possible enrichment of several dozens of economically important native species have resulted in long-term legacies left on forest composition (e.g., refs. 14–22). Some (20) propose that human influences played strong roles in the enrichment of “hyperdominant” trees, which are disproportionately common elements in the modern flora (sensu ref. 23). Some even argue that prehistoric fires and forest clearance were so spatially extensive that post-Columbian reforestation upon the tragic consequences of European contact was a principal contributor to decreasing atmospheric CO₂ levels and the onset of the “Little Ice Age” (24, 25).However, modern floristic studies are often located in the vicinity of known archaeological sites and/or near watercourses (26). Many edible trees in these studies are early successional and would not be expected to remain as significant forest elements for hundreds of years after abandonment. Historic-period impacts well-known in some regions to have been profound have been paid little attention and may be mistaken for prehistoric legacies (26–28). Moreover, existing phytolith and charcoal data from terrestrial soils underneath standing tierra firme forest in some areas of the central and western Amazon with no known archaeological occupations nearby exhibit little to no evidence for long-term human occupation, anthropic soils, agriculture, forest clearing or other significant vegetation change, or recurrent/extensive fires during the past several thousand years (Fig. 1) (29–33). Even such analyses of terrestrial soils of lake watersheds in western Amazonia known to have been occupied and farmed in prehistory revealed no spatially extensive deforestation of the watersheds, as significant human impacts most often occurred in areas closest to the lakes (Fig. 1) (34). Furthermore, vast areas have yet to be studied by archaeologists and paleoecologists, particularly the tierra firme forests that account for 95% of the land area of Amazonia.To further inform these issues, we report here a vegetation and fire history spanning 5,000 y derived from phytolith and charcoal studies of terrestrial soils underneath mature tierra firme forest in northeastern Peru. Phytoliths, the silica bodies produced by many Neotropical plants, are well preserved in terrestrial soils unlike pollen, and are deposited locally. They can be used to identify different tropical vegetational formations, such as old-growth forest, early successional vegetation typical of human disturbances including forest clearings, a number of annual seed and root crops, and trees thought to have been cultivated or managed in prehistory (e.g., refs. 29–33 and 35).