Interpreting landscapes of pre-Columbian raised-field agriculture using high-resolution LiDAR topography

Abstract

Extensive areas of seasonally flooded savannas in South America are covered with vestiges of pre-Columbian raised fields. The present-day state of these vestiges depends on both initial human agency and the balance between erosion and accumulation by soil organisms. We investigated the taphonomic determinants of mound-shaped raised fields in the Llanos de Mojos, Bolivia, using high-resolution digital terrain models produced by LiDAR mounted on an unmanned aerial vehicle, allowing fine-scale characterization of topographic gradients and of shapes of hundreds of mounds. We found that pre-Columbian farmers built larger and more widely spaced mounds at lower elevation, where seasonal flood level is higher. However, mound size depended also on natural forces that are still in play. As for earthworks everywhere, these forces include the landscape-smoothing process of erosion. But in seasonally flooded savannas, positive feedbacks between topography and activities of soil engineer organisms can drive accumulation of material on mounds. The erosion/accumulation balance hypothesis posits that the balance between erosion and accumulation depends on whether conditions favor local accumulation of soil by engineer organisms. These conditions vary along elevational gradients. At the lowest end of the gradient, deep flooding prevents activity of soil engineers and erosion is unopposed. In intermediate positions, where mounds constitute islands of well-drained soil, soil engineers concentrate their activities on mounds, leading to accumulation that may counter erosion. At the top of the gradient, where all soil is well-drained, conditions for soil engineers are favorable everywhere, and thus no force drives local accumulation in mounds. Erosion is thus again unopposed. Patterns of variation in mound size and shape revealed by analysis of LiDAR imagery were consistent with the erosion/accumulation balance hypothesis. Mounds were largest at intermediate positions along topographic gradients and smaller towards both extremes, until they finally became undetectable. Relative height of mounds (maximal height/ radius at base) decreased as mound size decreased, consistent with the expectation that as mounds erode, they also become flatter. Finally, trees occurred preferentially on mounds, and mounds bearing a tree were less eroded (higher and less flat) than mounds without trees. Data generated by LiDAR thus contributed to archaeological interpretation, supporting inferences about how raised fields were initially constructed and how natural processes have modified their size, shape, and distribution in the centuries since.