Anthropogenic Soils and Tree Distributions in a Lowland Forest in Bolivia

The distributions of 17 tree species with seeds or fruits that are commonly eaten or otherwise used by humans were studied in reference to anthropogenic soils in a seasonally dry lowland tropical forest in Bolivia. Two types of anthropogenic dark earths (ADEs) were identified: terra preta (TP: darkened with charcoal fragments and with abundant pottery shards) and terra mulata (TM: somewhat darkened but with little or no pottery). In 216 ha, we compared the densities of trees > 10 cm dbh of useful species in nine TP patches, three TM patches, and six areas with nonanthropogenic soils (N-ADE). The TP, which covers approximately 20 percent of the sample area, has higher contents of organic matter, extractable P, and extractable Ca than N-ADE soils, as well as higher pH and extractable P than TM. TM has organic matter and Ca contents similar to N-ADE soils but significantly less extractable K. In general, TP has higher nutrient content than surrounding soils, both at the surface (0-10 cm) and deeper in the profile (40-50 cm). Despite these differences in soil fertility and contrary to our expectations, none of the 17 tree species were concentrated on TP perhaps because interactions between local dispersal agents and natural disturbances have masked historical patterns. Alternatively, past inhabitants of the study area may have enriched forests with useful tree species in areas where they were not practicing the intensive soil husbandry that results in ADEs.